I have been getting more and more photos sent to me from tomato growers showing symptoms of a foliar disease that never used to be present in Maine. Though the disease was only formally confirmed to be Stemphylium gray leaf spot in the 2024 growing season, I suspect that we have been seeing smaller outbreaks of it, with less progression of disease symptoms, for several years now.

Gray leaf spot can be caused by several Stemphylium species. Stemphylium solani and S. lycopersici are most common in North America, but S. botrysum also causes the disease. Gray leaf spot is typically considered to be most prevalent in humid tropical and subtropical regions. In fact, I was not even aware of the disease until several years ago when Andy Radin, the former research associate/ag extension agent for the University of Rhode Island, alerted a group of New England crop specialists that he was beginning to see the disease on farms in Connecticut and Rhode Island, with dramatic crop losses in some instances. Within a year or two of that warning, I started to receive photos from Maine farmers who were noticing symptoms similar to gray leaf spot.

The arrival of gray leaf spot is likely an aspect of the new climate reality that growers will continue to need to adapt to. Though it likely can’t be tested scientifically, it seems probable that recent summers’ prolonged bouts of higher heat and higher relative humidity — which can also cause elevated temperatures to persist overnight to a greater extent — have allowed this disease to expand the range in which it effectively infects tomato plants and spreads. While we’ve been having summers with these conditions more frequently, the disease can probably only spread so far in a given year, and we may now be seeing the result of overwintering populations having gradually established themselves further and further north over many seasons.

As gray leaf spot is a new concern for growers here in Maine, it is important to both recognize what the symptoms look like and to understand the disease organism’s lifecycle. In that way, we can best understand where we might be able to interfere with that lifecycle, using organic production methods. Because of the way gray leaf spot presents itself, I’ve started to think of it as “acting like early blight but infecting like botrytis.” One fortunate difference is that gray leaf spot does not affect tomato fruit the way that early blight or botrytis can. However, that’s likely to be cold comfort if your plant has lost all of its leaves.

Stemphylium infections can start from nearby infected plants (tomatoes, peppers, and other solanaceous weeds, though I’ve only heard of it confirmed on tomatoes in Maine so far); can be seedborne; or, more likely in its gradual spread into Maine, can start from infected crop debris from prior years. Fungal spores produced on living or dead plant tissues spread via splashing rain or irrigation water, or go airborne and travel on the wind in favorable conditions. The disease survives and infects most successfully in relatively hot and wet weather conditions: requiring liquid water or simply high humidity to infect leaf surfaces, and developing most successfully at 77 degrees Fahrenheit. Though petioles and stems can become infected, the disease is mostly noticed on leaves. Fruit do not become infected.

Initial symptoms show up as small dark brown specks, which turn lighter — eventually turning tan to gray in their centers as they enlarge. The centers of these lesions will commonly crack as they begin to dry out. As the disease progresses, the entire leaf may turn gray, dry, and crispy. This defoliation is the primary damage of this disease, reducing the plant’s photosynthetic potential and also diminishing the affected leaves’ ability to shade fruit and protect it from sun scalding.

The leaf symptoms of Stemphylium gray leaf spot can be easily confused for bacterial leaf spot or Septoria leaf spot. The disease development and symptoms of Stemphylium seem to me, however, to also have many similarities with Alternaria linariae, the disease which causes “early blight” (not to be confused with the more devastating “late blight” caused by Phytophthora infestans and responsible for the great potato famine). Both gray leaf spot and early blight overwinter on infected crop debris in the soil, spread via splashing water or from windborne spores from active infections, and tend to progress up an infected plant from the lower leaves towards the upper leaves. The big difference is that early blight requires liquid water to successfully infect leaves, while gray leaf spot can do the same simply with high humidity. Because of this, my first interactions with this disease were photos sent to me from Maine farmers noticing symptoms that looked like early blight but showed up in covered production (i.e., greenhouses or high tunnels) the way that botrytis might — but early blight typically does not.

The best strategies for management of gray leaf spot are likely to be multi-faceted for many growers. As with many tomato foliar diseases, any growing practices that facilitate rapid drying and reduce trapped air around the plants will be helpful in limiting initial infection and subsequent spread (like greater plant spacing, and sucker, stem, and leaf pruning). When conditions are still favorable for the disease (i.e., prolonged high humidity or leaf wetness during warmer periods) it will likely still be able to infect susceptible plants, unless a preventative fungicide has been applied. Copper is likely to be the most effective organic control for home and commercial growers, though I suspect commercial growers may also see some control with Oso (polyoxin D zinc). Similarly, other common best practices for organic methods of disease management are also important. Because the disease overwinters on infected crop debris, it will be important to practice good sanitation and crop rotation. Remove or destroy infected tomato crop debris, or at least incorporate it into the soil to speed its decomposition before tomatoes are grown again in the same area.

Luckily, resistance to gray leaf spot can be found in tomatoes that contain a single gene (Sm), which has been traditionally bred from a wild tomato relative into many modern varieties. This means that anyone willing to grow these varieties likely won’t see gray leaf spot infect their crop in the first place. I recommend growing one or more resistant varieties for at least an “insurance portion” of your overall tomato plants, especially if you have already struggled with gray leaf spot showing up in your growing area.

Cornell Cooperative Extension maintains a spreadsheet of crop varieties with disease resistances noted, available here: vegetables.cornell.edu/pest-management/disease-factsheets/disease-resistant-vegetable-varieties/disease-resistant-tomato-varieties. Additionally, I took a quick look at several commonly used seed companies and compiled the following list of currently available tomato varieties that have a resistance to gray leaf spot. This is likely not a complete list, and availabilities may change or improve for the coming growing seasons.

• From Fedco: Rutgers 250 Schermerhorn, Sweet Treats.
• From High Mowing Seeds: Enroza, Galahad, Mountain Vineyard.
• From Johnny’s Selected Seeds: Amai, Big Beef, Big Beef Plus, Celebrity Plus, Enroza, Galahad, Grand Marshall, Hot Streak, Lemon Boy Plus, Mountain Merit, PinkID, RuBee Dawn.
• From Seedway: Better Boy Plus, Big Beef Plus, Camaro, Celebrity Plus, Delicious, Jolene, Kuzco, Picus, Pink, Red Bounty, Roadster, Rubee Dawn, Ruby Crush, STM 2255, Sunfresh.

Please note: This information is for educational purposes. Any reference to commercial products and trade or brand names is for information only, and no endorsement or approval is intended. Pesticide registration status, approval for use in organic production, and other aspects of labeling may change after the date of this writing. It is always best practice to check on a pesticide’s registration status with your state’s board of pesticide control and for certified organic commercial producers to update their certification specialist if they are planning to use a material that is not already listed on their organic system plan. The use of any pesticide material, even those approved for use in organic production, carries risk — be sure to read and follow all label instructions. The label is the law. Pesticides labeled for home garden use are often not allowed for use in commercial production unless stated as such on the label.

This article originally appeared in the winter 2024-2025 issue of The Maine Organic Farmer & Gardener.

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Today there are many more plant disease management products available to organic growers than in decades past, but it continues to be difficult for me to confidently recommend some of the newer products as “worth it” — especially when I’m talking with commercial growers that are relying on a crop for their livelihood — as these new materials are often considerably more expensive than the oldest, and most tried and true materials, namely copper and sulfur. The efficacy data from research trials using these newer products is oftentimes limited and inconsistent. With this in mind, I conducted a trial of fungicides approved for organic use on winter squash at the MOFGA Common Ground Education Center in Unity, Maine, during the 2023 growing season.

Fungicides Trialed

While both copper and sulfur fungicides can easily be used very safely, there are some organic farmers and gardeners who want disease management options that are even lower risk, as copper can be harmful to aquatic wildlife, and while an essential trace mineral in the human diet it is toxic at very rare high levels. This trial included Disperss (sulfur) and Kocide 3000-O (copper hydroxide) as “standard practice” treatments, though there are several other brands of similar products.

Among the fungicides trialed was Regalia, one of the better known and perhaps “oldest of the newer” materials. It is made of an extract of giant knotweed (Reynoutria sachalinensis, a cousin to the Japanese knotweed often found in Maine) and works by stimulating plants’ innate defense systems — such as growing a thicker cuticle that will be harder for disease organisms to penetrate.

Another material I have seen in use on some farms, called Thyme Guard, has thyme oil as its active ingredient. Thyme oil is considered a “minimum risk pesticide” by the Environmental Protection Agency (EPA), which exempts it from normal EPA registration and regulations. As such, I have to admit that I held a bias that it likely was not very effective. That suspicion was part of my motivation for performing this trial — I wanted to know if I could confidently say “don’t bother” or if I had to reconsider its potential usefulness.

What seem to be the bulk of new disease management products, however, are bacterial-based bio-fungicides. There are many different strains of bacteria, and their extracts, that have been commercialized — and in this arena of bacterial-based bio-fungicides there are far too many new products to be able to trial them all, so I chose one to trial, named Stargus, and decided to give it its best chance for success by also including it in concert with other products.

We tested the fungicides on Honey Boat delicata squash, a crop and variety that is susceptible to foliar diseases (powdery mildew in particular), which we planted densely — two plants every 36 inches in-row — in order to best create the conditions needed to test the efficacy of the fungicides. Each fungicide treatment was randomly assigned to four identical plots — called replications — to allow for statistical analysis of the results. We applied the chosen fungicides (see Table 1) on July 21, August 5, August 10, and August 24.

The Experiment and Results

The overall experimental approach was to both maximize the potential for foliar disease development by means of cultural practices (i.e., variety selection, planting density) while simultaneously attempting to give the fungicide products their best opportunity at showing efficacy by applying them “early and often” relative to common farmer practices. Unfortunately, the unprecedented rainy conditions of 2023 may have increased environmental suitability for foliar disease proliferation in the trial, therefore limiting fungicide efficacy. Two foliar diseases of squash, squash anthracnose and cucurbit powdery mildew, were observed at the last spraying date, August 24, with anthracnose having killed enough foliage to initiate the trial’s harvest on September 7 (winter squash need to be harvested when there is no longer a leaf canopy to shade fruit from the sun). That being said, marketable yield from each plot (measuring 24 row feet in length) was 41.46 squash fruit weighing just over 1 pound each on average; this equated to 12,541 fruit on a per acre basis, with a total marketable yield of 12,867 pounds per acre, which is more than 50% greater than the New England five-year average of 7,940 pounds per acre. This gives us confidence that the results of the trial are applicable to the same real-world farming conditions of the farmers served by MOFGA’s farmer programs.

Marketable yields, and the number of fruit per plot, were not statistically different between fungicide treatments (Fig. 1). In other words, the yield differences between plots receiving different fungicides were not large enough in comparison to the differences between replicated plots receiving the same fungicide to indicate that those differences were due to anything other than random chance. The presence or absence of powdery mildew, however, was statistically different between the fungicide treatments (Fig. 2). Powdery mildew was not visible on any of the plots treated with sulfur alone or with a combination of sulfur and Stargus, though both of those treatments were only statistically different than Regalia- or Stargus-only plots, which were the only treatments to show powdery mildew in all four replications. This finding aligned with my standard recommendation of sulfur for control of powdery mildew in cucurbits.

The only other statistically significant difference between the treatments was a visual foliage health assessment, performed on September 2, five days before the harvest data was collected. This was driven by the contrast between the treatment with the healthiest foliage, copper, and the treatment with the poorest foliage health score, sulfur. While this may seem contradictory to the finding regarding powdery mildew presence, the historically wet growing season of 2023 (the second wettest June-July-August period on record for Maine) was not conducive to the progression of powdery mildew and was instead much more conducive to the spread and development of squash anthracnose. 

Squash was chosen for this trial because its odds of becoming infected with powdery mildew by harvest is typically considered “a sure bet,” and therefore these fungicides would be given at least some opportunity to show their efficacy against one or more diseases. In practice, many farms choose not to spray for squash foliage diseases, except perhaps in a year when powdery mildew presence begins early and is threatening to defoliate the protective crop canopy before fruit have reached full maturity. Our results, which show no statistically significant marketable yield differences between treatments, suggest that these fungicides may not make financial sense to apply to winter squash in many, if not most, years. While the design of the trial only allows for some conservative interpretations of the statistically significant differences, the overall direction of the results suggests that sulfur treatments continue to be the most useful in a “normal” growing season that favors powdery mildew, while copper continues to be the “gold standard” in terms of efficacy against other foliar diseases. Had the overall trial not needed to be harvested at the same time, copper plots, which showed healthier foliage, may have been able to produce a greater marketable yield or more fully mature fruit with greater long-term storage potential if harvested later. Anecdotally, thyme oil plots looked much better than had been anticipated. While we now have much better footing to urge grower caution when choosing the most expensive bio-fungicides, thyme oil products may warrant further investigation as potential options for growers that are looking for alternatives to copper and/or sulfur products, despite those products’ affordability and seemingly continued outperformance of many bio-fungicides in many situations.

It is important to mention the limitations of these results. This was only one small trial of some products, at single application rates, on one variety of one crop. Crop diseases vary in their infective potential every year, and it continues to be difficult to extrapolate findings from any one trial to other potential use cases. The variety selected for this trial exhibited significant genetic diversity — while this is often a good thing in the long term, it undoubtedly added variability to the trial’s yield results, making it more difficult to distinguish impacts of fungicide treatments. Additionally, we did not leave any untreated plots as a control for comparison, meaning that we cannot say that any of these treatments provided a better outcome than doing nothing at all (and the associated labor and material cost savings of doing nothing!).

Special thanks to The Peter Alfond Foundation, Foundation For Sustainability and Innovation, and the Evergreen Foundation for their support of this trial, and Jack Kertesz, Joshua Pavese, Jake Ressel, Marta Łaszkiewicz, and C.J. Walke for assistance in planting, maintaining, and harvesting the trial.

This article was originally published in the fall 2024 issue of The Maine Organic Farmer & Gardener. Browse the archives for free content on organic agriculture and sustainable living practices. Subscribe to the publication by becoming a member!

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In 1903, the Massachusetts State Board of Agriculture commissioned a report on the browntail moth following a substantial outbreak of the pest, which had been introduced to Cape Cod, Massachusetts, only a few years earlier. Among other things, in “A Report on the Life History and Habits of the Imported Brown-Tail Moth,” the authors wrote of another outbreak of the moth — in 1543, in its native Europe. People at that time believed that the inexplicable spread of insects, which were defoliating orchards and causing horrid skin rashes, were a punishment for human shortcomings. A member of the city council at Grenoble, France, introduced a resolution begging the local church official to “excommunicate these pests and censure them, in order to check the damage they were doing daily.”

During the decades following the report, the invasive moth (Euproctis chrysorrhoea) expanded its North American territory into coastal Maine and the Canadian Maritimes and south to Long Island. Efforts to mitigate browntail moth’s spread were made, including the spraying of DDT and a quarantine of nursery products grown within the moth’s expanding range. By mid-20th century the population had decreased to a relic population on Cape Cod and several islands off coastal Maine. The quarantine on nursery products was lifted and the afflicted humans relaxed for a couple generations. Although there were sporadic pockets of browntail moth population growth every 10-15 years, these were quickly eradicated through insecticides and winter web clipping. 


Browntail Moth Populations Increase in Maine

When increasingly large browntail moth populations began spreading more widely across Maine in 2015 and 2016, citizens, local government officials, scientists and even public health experts were caught off guard. Among the questions being asked were: Why now? Where did all these caterpillars come from all of a sudden?

Eleanor Groden and her colleagues at the University of Maine set out to determine if there was a relationship between the rapidly multiplying moth population, the moth’s expanding range, and Maine’s warming temperatures. Other insect populations had been affected by the changing climate, Groden, now retired, pointed out.

After assessing 23 years of weather patterns overlaid with browntail moth population expansion and contraction, Groden and her colleagues concluded that increasingly warmer fall temperatures were resulting in more older and larger browntail moth caterpillars emerging from winter hibernacula (protective silk webs) in the spring.

“Climate trends indicate continued increases in fall temperatures since browntail moth resurgence,” Groden et al. published in the October 2021 issue of Environmental Entomology.

Cool temperatures and adequate spring precipitation can drive outbreaks of Entomophaga aulicae, a fungus that can infect and kill browntail moth larvae, the entomologists pointed out. However, the healthy populations of larvae coming out of the warmer autumns will be more resilient and able to bounce back more quickly during dry springs.

One of the mitigation efforts taken during the outbreak of the early 20th century was to inoculate browntail moth caterpillars with Entomophaga aulicae. “Entomologists began rearing browntail moth larvae in outdoor insectaries in 1908 and infecting them with the naturally occurring entomopathogenic fungus, Entomophaga aulicae,” Groden and her colleagues wrote. “These infected larvae were then transported and released into local populations throughout Massachusetts over four years.” Groden reported that some data showed that the E. aulicae releases were successful.

In 2023, when browntail moth larvae infected with the fungus began dying on a 17-acre island in Casco Bay during the cool and wet spring, the Maine Forest Service offered a very cautious note of hope.

“In the past few weeks, we have been finding some evidence of pathogen-related death at a few of our monitoring sites,” the Forest Service reported in a browntail moth update on June 12. “We responded to a request from the manager of Eagle Island State Historic Site and confirmed high mortality of browntail caterpillars from the fungus Entomophaga aulicae. The recent cloudy and rainy days are helping contribute to the spread of fungal and viral pathogens that attack browntail caterpillars. Although pathogens can regulate some populations of browntail caterpillars, disease outbreaks can only happen with co-occurrence of rainy weather and the presence of the pathogen. Therefore, we should caution that pockets of disease like this epizootic event (disease outbreak) may be quite isolated.”

The Forest Service is monitoring 10 sites throughout the state in 2023 and reported more mortality on June 27. This time the reported cause of death at the monitoring sites was viral — likely Euproctis chrysorrhoea Nucleopolyhedrosis Virus or EcNPV, according to research done on browntail moth mortality by the University of Maine between 2016 and 2019. “We are seeing some mortality of caterpillars at our sites, although, there is just a handful of dead caterpillars at each site,” the agency reported. “When caterpillars die from a viral pathogen, they latch on to a twig with their first set of prolegs and hang in a ‘U’ shape. Eventually, these caterpillars will liquefy and their insides will drip onto foliage below, which may allow other caterpillars to encounter the viral pathogen. Interestingly, a large proportion of caterpillars look like they died as they were pupating.”

Monitoring Moth Populations

According to Allison Kanoti, director of forest health and monitoring at Maine Forest Service in Old Town, “These sites allow us to monitor phenology through the growing season across the region and communicate that information through our updates to provide information to those managing and living with browntail moth.”

Angela Mech, a forest entomologist, agrees that monitoring the browntail moth population across the state is an important part of understanding the insect. Mech leads research at the forest entomology lab in the School of Biology and Ecology at the University of Maine in an effort to better understand, monitor and control browntail moth populations. The lab had 35 browntail moth monitoring sites across the state in 2023, and had assistance observing those in Southern Maine through a collaboration with the University of New Hampshire.

Mech’s team put a lot of effort into developing a monitoring trap that would entice the male browntail moth. The main attractant is a female sex pheromone that was identified in the early 1990s. The Maine scientists worked with Trécé, Inc. of Adair, Oklahoma, to test different purity levels and concentrations of pheromone and have also been considering different trap styles and colors.

“White sticky traps work the best,” Mech said. “That makes sense because browntail moths are nocturnal.”

One reason Mech’s lab is conducting monitoring is to determine what she calls the “itch threshold” — the point at which the browntail moth population is on the cusp of a significant expansion.

Mech’s research team is working on the assumption that the current outbreak of browntail moth will behave like all of the other known outbreaks of the last 500 years. The population will eventually decline and be limited to a relic population once again. Then, likely years or even decades from now, the population will rapidly expand again. Knowing what the itch threshold is will provide a forecasting tool that wasn’t previously available.

“Male BTM catch data will be collected yearly and correlated to the number of winter webs in each trap area. The predictive model will help answer the question ‘how many males in a trap equates to a damaging population level?’ and can be used to develop risk assessments,” according to the lab’s 2022 report.

“We’re hoping we’ll have something that will alert us before the next outbreak gets to the point where we are now,” Mech said.

Where we are now is a serious public health problem combined with the defoliation of around a quarter million acres of forest land.

Browntail Moth Mitigation

The forest entomology lab at the University of Maine is juggling research projects that will respond to infestation in the future as well as the present. One of those projects involves taking a second look at the female pheromone that has been developed for monitoring traps.

The idea is to use the female pheromone to confuse males. Called mating disruption, it’s a technique that’s been used for decades to successfully keep a lid on spongy moth (Lymantria dispar). The process involves inundating a forested area with the female pheromone at a concentration that confuses the male to the extent that he can’t find a female.

“We are trying to mimic the spongy moth results with the female pheromone of the browntail moth,” Mech said.

The pheromone can be applied by airplane or from the ground. “This method can be used over large areas, does not directly affect natural enemies, is cheaper than using chemicals, has minimal non-target effects, is environmentally friendly, and can be highly effective,” the lab’s 2022 report stated.

Mech categorizes efforts at browntail moth mitigation into two levels: a landscape level involving thousands of acres, and a backyard or urban park level. Her interest in local-scale mitigation has more of a public health focus than a forest entomology focus.

If you live in Maine, it’s very hard to avoid the rash caused by browntail moth caterpillars, she said. “I’ve had it repeatedly over the last three years and each time it gets worse. We take lots of precautions at the lab and cover ourselves from head to toe when we’re working with it but we still get it. The hairs are very small.”

The poisonous hairs can go airborne and can cause a dermatitis similar to poison ivy even without direct contact with a caterpillar. For some, the response is systemic, and a reaction spreads throughout their body. Respiratory complications are also a concern.

To help people protect themselves in their backyards and neighborhoods, the lab researched porch and yard lighting. Eliminating all lighting results in attracting the fewest browntail moth during their short adult lifecycle.

“Beginning in late June and early July, adult browntail moths will become active, and you may start seeing them near light sources,” the Forest Service stated in a June newsletter, citing Mech’s research. “We recommend keeping unnecessary outside lights off between 9:00 PM and midnight until the beginning of August to avoid attracting adult browntail moths to your property. If this is not possible, consider switching to yellow-spectrum lighting to reduce property attractiveness to dispersing moths.”

“My husband and I have changed our outdoor lighting,” Mech said. Her lab found that the browntail moth particularly likes light from bulbs that emit UV and/or blue parts of the light spectrum. These include black lights as well as compact fluorescent, incandescent and cool white LED bulbs.

Another neighborhood-level control method is to be alert to hitchhiking larvae and cocoons. The Forest Service reports that browntail moth showed up in counties far outside its range in 2022. “We think these caterpillars more than likely hitched a ride to get there,” their May 19, 2023, update stated.

The Forest Service recommends not parking under or near infested trees. They also recommend checking vehicles, ATVs, boats and trailers before leaving home if in an infested area. Firewood from infested areas should never be transported out of the area it was harvested. They further caution if you find larvae or cocoons on any of these sources to remove them and dispose of them with the utmost care.

Winter Web Removal

With winter approaching, Mech recommends clipping webs out of trees. It’s a method that was widely used during the outbreak at the beginning of the early 20th century and probably 500 years ago.

“Clipping webs is still one of the most effective control methods,” she said.

Laura Sieger, an orchard specialist at MOFGA, says that the orchard crew manages browntail moth at the Common Ground Education Center in Unity, Maine, by clipping winter cocoons and spraying Bt (Bacillus thuringiensis) when the larvae are feeding.

“We prune out all of the nests we find and stick them in the wood stove,” they said. “That’s done during winter pruning from December through March or April. If we find any lingering, we typically add Bt to a spring spray.”

Sieger says that clipping nests out of tall shade trees, such as the oaks that browntail moth favor, is more challenging. “One year a few of us went out to the fairgrounds in early spring scouting and shooting nests out of the tall oaks and collecting the ones that were blasted out of the tree and drowning them in soapy water,” Sieger said.

Jack Kertesz, MOFGA’s landscape coordinator, has worked on nest removal on the tall trees at the fairgrounds for several years. He says that shooting the nests was supposed to break them up and decrease the survival of the larvae in them. 

“It appeared to work reasonably well,” he said. Though he cautioned that the shooters had sore shoulders, and there were lots of spent plastic shotgun shells littering the ground.

Sieger agreed that collecting debris afterward was more of a hassle than they would have liked it to be.


The following year, in 2022, MOFGA hired an experienced nest remover with a lift truck, which was expensive. In 2023, they rented an aerial lift that had a steep learning curve but ultimately worked well. “Pruning with a pole pruner and rented lift in 2023 seemed better since it was easier to keep track of all the nests that came out of the tree this way,” said Sieger.

The airborne nest clippers had the most success when supported by a spotter on the ground. Nests are easier to spot with the sun at your back, according to the Maine Forest Service.

In 2022 a Canadian company called DeLeaves began experimenting with aerial drones for nest removal. The drones cost around $30,000, making them competitive in price compared with bucket trucks and aerial lifts.

Like MOFGA, cities and towns across Maine face challenges in establishing a safe and effective response to the outbreak. The state of Maine requested proposals from Maine municipalities and nongovernmental organizations to develop mitigation programs. As of this writing, Jim Britt of the Maine Forest Service said nine proposals were submitted but no awards had been made yet.

“We didn’t apply because we felt the grants were for communities with fewer resources,” Bangor Public Works Director Aaron Huotari said.

Bangor has had a multi-pronged mitigation effort for several years that includes education, monitoring, clipping and, in 2023, some experimental chemical applications of Bt and an injected insecticide into a few trees in city parks.

“We also intend to have a lending library of pole trimmers that residents can borrow to clip winter nests on their own property,” Huotari said.

When it comes to managing this invasive pest, as history has demonstrated, a community-wide effort can make a difference.

About the author: Tim King is a produce and sheep farmer, a journalist and cofounder of a bilingual community newspaper. He lives near Long Prairie, Minnesota.

This article was originally published in the fall 2023 issue of The Maine Organic Farmer & Gardener.

The post Browntail Moth in Maine appeared first on Maine Organic Farmers and Gardeners.

European corn borer in peppers (among others)

“Never let ’em set seed”

By Andrew Radin, University of Rhode Island Extension

“Winter squash looks ready, should I harvest?”

By Eric Sideman

Winter squash look ready to harvest before they actually are mature. It is important to wait for maturity to have maximum storage and best eating quality. Most squash varieties reach full size by 20 days after fruit set. Accumulation of starch and other dry matter peaks at about 30-35 days after fruit set. But, the fruit is not fully mature until the seeds are fully developed, which occurs about 55 days for butternut. This is different for other varieties. If you would like to read more, take a look at this article by Brent Loy, the well known, and sadly, recently deceased plant breeder at the University of New Hampshire. Or, here is a summary put together by the folks at Johnny’s Selected Seeds.

Harvest and Storage

A critical factor in eating quality is maturity of the squash when it is harvested. Most small varieties reach full size by 20 days after fruit set. Accumulation of dry matter and starch content peaks at 30 to 35 days after pollination. However, the fruit is not mature until the seeds are fully developed, which occurs about 55 days after fruit set.

Dr. Loy stresses the importance of maintaining healthy plants until at least 50 days after fruit set because photosynthesis is essential to the development of sugars and dry matter. A squash that is picked too early will continue to develop seeds, but it does so by depleting dry matter of the fruit, thereby reducing eating quality.

Although fruit and seed maturity are similar across the three main species of edible winter squashes and pumpkins, harvest and storage recommendations vary by type.

Cucurbita maxima | Kabocha, hubbard, and buttercup squashes

Kabocha, hubbard, and buttercup (C. maxima) varieties should be harvested before complete seed maturation, at about 40 to 45 days after fruit set, when the fruit is still bright. That’s when the rind is hardest, so less likely to be damaged in storage. They also are susceptible to sunburn as the vines die down, so it’s best to get them harvested and out of direct sun before then to prevent the rind from turning brown or, with extreme sunburn, white. Kabocha squash have a high dry matter content and small seed cavity, so seed maturation off the vine is not a problem. However, once harvested, they should be stored at room temperature for 10 to 20 days to allow sugars to reach acceptable levels.

C. pepo | Acorn squash, most pie pumpkins, delicata squash

Acorn squash (C. pepo) are misleading because they reach full size and develop a dark green-to-black mature color about two weeks after fruit set – 40 to 50 days before they should be harvested. Dr. Loy says that a better way to judge maturity is to look at the rind where it touches the ground. Immature squash have a light green or light yellow ground color, whereas mature squash have a dark orange ground color. Immature acorn squash have low sugar levels and although they will develop sweetness after harvest, they do so by depleting the dry starchy matter to convert it to sugars. This means storage life is shortened and eating quality declines.

C. moschata | Butternut squash, some pie pumpkins

Butternut squash (C. moschata) are easier to judge by sight because they don’t acquire their characteristic tan color until late in development, 35 days or more after fruit set. If the weather stays frost-free, they should be allowed to remain on the plants until 55 days after fruit set. In short-season areas, they often are harvested soon after turning tan because of the risk of frost damage. At that point, however, the sugars have not elevated to the 11% required for good flavor, so butternut squash harvested at 55 days after fruit set should be stored for 60 days at 56-60°F/10-16°C, with relative humidity between 50 and 70%. Carotenoid content also increases in storage, making the butternut squash more nutritious after it’s been stored for a couple of months. To accelerate maturity and increase sweetness, Dr. Loy found that butternuts held at warm temperatures (up to 85°F/29°C) for two weeks develop acceptable levels of sugars.

This material is based upon work supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, through the Northeast Sustainable Agriculture Research and Education program under subaward number ONE19-334.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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Early Blight (Alternaria)

The pathogen overwinters on diseased plant residues in the soil. Initial infection is from splashing up from this overwintering site. However, by mid-summer, most of the infection is coming from spores blowing in the wind from the widespread infection in gardens and on farms across the region.

 

It is important to avoid early onset of the disease because the longer you can avoid the problem the more likely it is that you will harvest red tomatoes before the plants die.

Hornworms (Manduca sexta, M. quinquemaculata)

Hornworms are probably the most destructive insect attacking tomatoes, and they are showing up now. They are giant caterpillars that can do a vast amount of eating in a very short amount of time. Right now you may find young, tiny ones. Sometimes it seems that healthy-looking tomato or pepper plants are stripped of their leaves overnight, leaving bare stems. The hornworms will also attack fruit, eating gouges out so large that they look more like bites of a furry animal than an insect.

Look now for feeding damage and frass (insect poop) around tomato and pepper plants. The frass may be your first sign there is a problem. The frass of young hornworms is small, but the hornworms quickly grow large and so does the frass. I don’t bother looking for the caterpillar because they blend in so well – I just look for the frass.

Powdery Mildew of Cucurbits

Powdery mildew is a common disease of pumpkins and winter squash. All cucurbits are susceptible, but many common cucumber and melon varieties are resistant. The disease can cause infected leaves to die prematurely, reducing yields and lowering fruit quality, especially taste. Winter squash from diseased plants won’t store as long as fruits from healthy plants. The fungus that causes the disease does not overwinter in Maine — spores blow up every year from southern overwintering sites. If they arrive late in the season, you may not need any control; but if they arrive in early to midsummer, exercise some control or you may have no leaves by mid-August, or bland, starchy squash.

I have not seen any yet, but it’s good to scout regularly. Check upper and lower surfaces of leaves of older plants every few days starting now. The first symptoms usually are white, powdery fungal patches on the undersides of older leaves. Yellow spots may form opposite these, on the upper leaf surfaces.

No products with systemic activity (products that move through the plant) are approved for organic production, and applying fungicide to the lower leaf surface is difficult. In experiments, foliar applications of sulfur (such as Microthiol disperss) have been more effective than most other organic products for powdery mildew — apparently because sulfur deposited on the upper leaf surface can volatilize and be redistributed to the lower surface. Sulfur can be phytotoxic on melons, however, especially if applied when temperatures are hot.

There are some products made from potassium bicarbonate (Kaligreen and Milstop are two of these) that are showing good efficacy. For a good discussion of these, including efficacy, see the new edition of the Resource Guide to Organic Insect and Disease Management. Potassium bicarbonate combined with a horticultural oil (such as JMS Stylet Oil) has been shown to be more effective than either product alone. Several bio-fungicide products are available as well.

This material is based upon work supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, through the Northeast Sustainable Agriculture Research and Education program under subaward number ONE19-334.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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Three-Lined Potato Beetles

Three-lined potato beetles are typically a minor pest, but can cause real damage to small plantings of tomatillos or husk cherries. Read more about three-lined potato beetles here.

Leek Moth

Overwintered leek moths had an early first flight this year — about a month ago. That means the first generation of the year is expected to emerge from their cocoons soon. The eggs they lay will hatch out to the second generation caterpillars which typically cause more damage. The pest is still confined to only a few areas in the state (as far as we know … ) but it may be a good idea to make sure they haven’t spread to you, as now would be the critical time to prevent damage from the second generation. Read more about leek moth here.

Striped Cucumber Beetle

Striped cucumber beetle is our most serious early-season pest in cucurbit crops. These beetles spend the winter in plant debris in field edges and with the onset of warm days and emergence of cucurbit crops they move rapidly into the field. Densities can be very high, especially in non-rotated fields or those close to last year’s cucurbit crops. Adult feeding on cotyledons and young leaves can cause stand reduction, delayed plant growth and reduced yield. Critically, the striped cucumber beetle also vectors Erwinia tracheiphila, the causal agent of bacterial wilt, and this can be much more damaging than direct feeding injury. Crop rotation, transplants and floating row cover are cultural controls that help reduce the impact of cucumber beetles. Many growers use row covers or exclusion netting on cucurbits for both growth benefit and insect protection, removing them when flowering begins to allow for pollination. Parthenocarpic varieties do not require pollination and can allow for delayed removal of row covers or netting.

Avoid early season infection with wilt. Cucurbit plants at the cotyledon and first one-to- two leaf stage are more susceptible to infection with bacterial wilt than older plants, and disease transmission is lower after about the four-leaf stage. Using row covers to keep the beetles out at this most vulnerable stage can be very effective.

Beetle numbers should be kept low, especially before the five-leaf stage. Scout frequently (at least twice per week before emergence, and for two weeks after crop emergence) and treat after beetles colonize the field. The threshold depends on the crop. To prevent bacterial wilt in highly susceptible crops such as cucumber, muskmelons, summer squash and zucchini, we recommend that beetles should not be allowed to exceed one beetle for every two plants. Less wilt-susceptible crops (butternut squash, most pumpkins) will tolerate one or two beetles per plant without yield losses. Spray within 24 hours after the threshold is reached.

Insecticides that are allowed in organic production (listed by OMRI, the Organic Materials Review Institute) include kaolin clay (Surround WP), pyrethrin (PyGanic Crop Spray 5.0 EC), and a mixture of pyrethrin and azadirachtin (Azera). Surround should be applied before beetles arrive because it acts as a repellent and protectant, not a contact poison. With direct-seeded crops, apply as soon as seedlings emerge if beetles are active. Transplants can be sprayed before setting out in the field.

Perimeter trap cropping with a preferred cucurbit crop (usually a Curbita maxima such as Blue Hubbard or Buttercup) can give excellent control with a dramatic reduction in pesticide use. For details on using perimeter trap cropping, see University of Connecticut Integrated Pest Management Program’s “Directions For Using a Perimeter Trap Crop Strategy to Protect Cucurbit Crops.”

(Modified from the UMass Vegetable Notes Newsletter, written by Ruth Hazzard and Andrew Cavanagh)

This material is based upon work supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, through the Northeast Sustainable Agriculture Research and Education program under subaward number ONE19-334.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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Cucurbit downy mildew has been verified on cucumbers and cantaloupe in New Jersey and Pennsylvania. Because of prevailing west-to-east weather patterns, and the projected path of tropical storm Elsa, there is a chance that this disease will continue its spread toward us. The Cucurbit Downy Mildew Forecasting website is predicting much of New England and Maine to be at risk of downy mildew spread.

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Excess mineral salts on greenhouse soil surface.

SaltSome composts are high in soluble salts. Even if the salts are nutrient salts such as nitrates, high salts will cause water absorption problems and may prevent seeds from germinating. Salty composts are not a problem in field use because they become diluted with the soil, but in a potting mix it is a real problem. Compost used for potting mix should be the best you can get and should not have salts – measured as conductivity on a compost analysis – higher than 1 mmhos.

EDEMA (oedema)Edema is an abiotic symptom that can look concerningly like a disease, but typically isn’t a major concern. For more information check out the Edema Factsheet at MOFGA.ORG

FLEA BEETLESWhy do my brassicas look like they were shot by a shotgun? Learn more about flea beetles in the Flea beetle factsheet at MOFGA.org

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In late July 2017 I discovered that my potato plants were not doing well. Leaves were yellowing and the plants were not growing as expected. Suspecting some sort of blight, I sent photos to Eric Sideman at MOFGA. He thought the problem was potato leafhoppers and referred me to a MOFGA pest report. Potato leafhoppers (PLH) are tiny buggers that suck juices from the host plant and inject a toxin that clogs the vascular system of the plant and kills it. The only insecticide approved for organic production is pyrethrum, but you must treat before you see the symptoms; once symptoms appear, it is too late. For me last year, it was too late. Looking at my potato plants very carefully, I could see that Eric was right. The little beasties were there!

Last year was my first encounter with PLH in more than 30 years of gardening, but suspecting it was not my last, I decided this year to protect my potatoes from PLH with floating row covers. I guessed that row covers would also be effective against Colorado potato beetles, an annual problem for me. With potato beetles in mind, I also planted in late June, a few weeks later than in past years, to avoid the early summer rush of those pests.

I plant potatoes in 50-foot-long shallow furrows that I make with a triangular hoe. I spread compost in the furrows, then place seed potatoes a foot apart on top of the compost, cover with more compost, and pull enough soil over each furrow to cover all. When each furrow is complete, I lay out a soaker hose along its length, securing it with ground staples.  

When the first leaves appeared above ground, I spread lightweight row cover over each furrow, securing the fabric with ground staples. To hill furrows, I removed staples from one side and pulled the fabric aside, then replaced the row cover when I was done. I hilled each furrow twice, first when plants were 6 inches high and again when they had grown another 6 inches. I left the row covers on until the plants bloomed, by which time they needed more room than the row covers allowed. During dry spells in the growing season, I watered each row for two to four hours weekly, using a water timer.

When I removed the row covers, I saw no evidence of PLH or potato beetles. Plants were healthy and happy and remained so. Eventually potato beetles, but not PLH, did appear, but they never caused significant damage.

I planted about 45 pounds of seed potatoes; my harvest totaled about 350 pounds, and the potatoes were scab free – my best crop ever by far! Row covers are not cheap, but with care they can be reused for two or more years, so I think they are a worthwhile investment. 

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By Will Bonsall

Note: Certified organic producers should check with their certifier before using any pesticides (including pest repellents) not mentioned on their organic farm plan. When using pesticides on crops grown commercially and intended for human consumption, an applicator’s license may be required. See https://www.maine.gov/dacf/php/pesticides/applicators/licensing.html.

One of the main problems with growing the foods we love is that other critters love them just as much as we do. In my article “Controlling Pest Insects in the Organic Garden” in the fall 2020 issue of The Maine Organic Farmer & Gardener, I discussed remedies for insect attacks. But what about bigger pests – the furry and the feathered ones? While mammals and birds tend to be far fewer in numbers, a single individual can do far more damage. In one night, a single deer can wipe out more kale than a whole army of cabbage worms. Fortunately, the larger critters are often easier to deter.

Shooting, trapping and poisoning are undeniably effective. A complication for me is that I’m a vegan. Oh, I certainly will kill things before going hungry myself, but I always prefer to use less lethal strategies – not only out of kindliness but also because these strategies often bring collateral damage. If I set out poison for mice, how will this affect the foxes, weasels and skunks that eat the dead mice? I care very much for those (non-vegan) predators who keep my competitors in check. After all, it is my fellow vegans in the world (i.e., potato bugs, woodchucks and deer) whom I cannot abide, since we’re after the same stuff. As I don’t keep chickens, predators are not a concern; I rather welcome them. Whenever I can, I greatly prefer to exclude or deter the pests rather than destroy them.

Live Animal Traps

A popular strategy for many compassionate gardeners is live-trapping. While I have and use several Havahart traps, I’m also skeptical of them. With live-trapping, the underlying assumption is that you remove the offending animal and release them somewhere where they’re not a nuisance. But where, pray tell, is that? My town has enough gardens scattered around that it is increasingly hard to find a release point where I’m not merely foisting my problem onto someone else. Even if I’m willing to drive far enough afield to find a wilder area, am I really doing the trapped animal a favor? Wildlife scientists claim that at any given time any habitat is probably filled to carrying capacity, and an interloper is likely to be attacked and dispatched by the habitat’s current residents (who already know the territory and its resources much better than an intruder). I will point out that wildlife populations are very un-static and that surplus animals are always roaming around seeking new territory, even without my dumping of captives there. At least then the animals have a chance, which is surely better than my summarily executing them. So while trap-and-release is no panacea, it has its place (assuming you can get the critter to go into the trap).

Physical Barriers

What I find more effective than live-trapping is excluding large pests through physical means. Row cover such as Reemay can be very effective against large browsers – provided you secure it well at the base so woodchucks, raccoons and birds don’t work their way underneath. I generally dislike using petroleum-based materials when they’re only good for one season. I haven’t bought any new row cover for years, but I have accumulated lots of old pieces which are torn and generally considered useless, though not to me. A ripped section may not stop cabbage moths, but it will discourage a deer, and even woodchucks don’t like it. An imperfect cover will likewise keep birds out of your strawberries, though some may find their way in and become trapped.

I’ve also found string to be very effective in different situations. When crows and wild turkeys were wiping out my newly planted corn seed, I stretched a length of twine right over the row, a foot or two above the ground. While the birds could very easily reach under it, they seemingly felt uncomfortable with the twine over their heads. Moreover, string will interfere with a bird’s ability to quickly takeoff if panicked. For extra effectiveness, especially with wild turkeys, I place strings at different heights: one row 2 feet high and the next row about 3 feet high. While cheap, string is a slight nuisance and still doesn’t deter voles or cutworms, so I’ve taken to starting most of my corn (and sunflowers and amaranth) in 72-cell seedling trays, three seeds per cell.

For very small areas like a single garden bed, I’ve been surprised at how a single string seems to exclude deer – especially at night. I place stakes surrounding the small plot and stretch string taut between them, no more than 40 inches off the ground. Obviously deer could step right over it; someone has suggested that the feel of the barely visible filament against the deer’s sides feels spooky to them. I don’t know about that, but I do know that when you enlarge the enclosure string ceases to work.

Hex wire fencing can be used to keep deer out of small areas as well. It also works to deter raccoons from corn if you take this precaution: Staple the wire to sturdy posts around your plot but leave the top foot or so of wire loose, snipping the corners so it remains floppy. Raccoons will fall when trying to climb it. Electrifying that fence is really effective; use insulators and keep the bottom edge of the wire 6 inches above the ground. If you’re going to use electric wire anyway, you can skip the fencing and just use two bare wires: one 6 inches high and the other 12-14 inches high. I usually use fiberglass rods with spring clips, which are easy to adjust to the perfect height. A simple 6-volt battery will not work unless you also use a fence charger to dam up the current and send it out in little 5,000-volt pulses – not lethal but very deterring. To deter both deer and raccoons with the same system, I add deer tape at a height of 40 inches to the same fiberglass rods as the double coon wire.

Repellents

One hears about bars of soap, sweaty shirts or balls of human hair, as well as various noise makers, radios and lights, for keeping animal pests away from the garden. My own experience is that any and all of those things work for a little while, but in time most critters get accustomed to them. It’s good to have a plan B or to simply rotate these strategies, so they never get used to one.

There are commercially available products which repel animals due to an unpleasant smell or taste, although some need to be reapplied after heavy rains. If they cover a large enough area, they might be worth the price. I’ve heard that with some products you can merely spray the ground around your garden to prevent critters from venturing further. Though generally harmless, produce should be washed thoroughly at harvest to remove any detectable residue.

A remedy which I’ve read about and intend to try soon is solar-powered ultrasonic transmitters. They are supposed to repel all kinds of animals, and I can supposedly tweak the frequency so I can keep out deer without offending my skunk, fox and weasel friends. The system could be quite pricey as most plots would require several transmitters for adequate coverage, but maybe that’s acceptable if they last a long time.

About the author: Will Bonsall lives in Industry, Maine, where he directs Scatterseed Project, a seed-saving enterprise. He is the author of “Will Bonsall’s Essential Guide to Radical Self-Reliant Gardening” (Chelsea Green, 2015). And indeed, he is also a distant cousin of another exemplary Maine horticulturist: Tom Vigue. You can contact Will at wabonsall@gmail.com.

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