Results point to need for further research

After witnessing dramatic crop losses to late blight of tomato in 2009, the Northeast Organic Farming Association of Massachusetts (NOFA-Mass) decided to survey members about their growing practices in order to collect data that might help organic growers in the future. The results of the survey were presented at NOFA’s winter conference in January.

Defining late blight

Phytophthora infestans is a destructive parasitic fungus that causes late blight in tomatoes, potatoes and other solanaceous plants.

Several varieties of late blight exist; among them are mating types and strains. The late blight in the Northeast has one mating type, which needs live tissue to survive. Other Phytophthora have two mating types and can survive in the soil. In 2008, strain US-8 devastated potatoes, but didn’t touch tomatoes. In 2009, strains US-14 and US-19 (both single-mating types) attacked both potatoes and tomatoes with equal virulence.

Leaf and stem symptoms in tomatoes.
Photos courtesy of the University of Massachusetts.

Late blight lesions produce white fuzz on the underside of leaves. Each lesion is a sporangium that contains zoospores. Phytophthora’s reproduction cycle is rapid. When the spore lands, it germinates, penetrates the leaf and allows the mycelium to move through it. The fungus infests stem, foliage and fruit. On the stem, it looks like a canker. On the fruit, it’s a hard lesion with rings around it. In potatoes, it initially affects the stem and the leaves. Spores can also be washed down into the soil and infect the tubers that way.

Symptoms may appear within five to seven days of when a spore lands. Once it sporulates, it produces another large amplification of spores that move within the plant canopy. Late blight spores also travel long distances via winds. “When we hear about late blight in Pennsylvania and New Jersey, we pay attention, because that presents a risk to us,” says Ruth Hazzard of the UMass Extension Program.

The effects of this rapid-moving fungus can be devastating. Hazzard and her colleagues observed that once a field was infected, it tended to go down quite fast. When Phytophthora infected a CSA in Hadley, Mass., they were dead within a week. “Around July 15, I went to a meeting in eastern Massachusetts, and the grower’s field was completely loaded with it. He had to destroy his whole crop,” she says.

Although initial reports blamed 2009’s outbreak on one plant supplier, there is no definitive evidence pointing to a single point of origin of the airborne fungus. However, it is clear that garden centers and big box stores that housed infected plants outside, where they were subject to the weather, helped to disseminate the spores.

University of Massachusetts Extension vegetable specialists started getting reports in Pennsylvania and New Jersey around June 18, and by June 25, it had spread to a big box store in New York. “Then we started hearing about it in New England,” says Hazzard. “We found it in every big box store, so we started issuing alerts immediately.”

Conditions ripe for late blight

Weather conditions in 2009 presented an unusual situation for growers nationwide. In the Northeast, it rained from the second week of June until August. After a short respite from the cool, wet weather, conditions again became cooler and foggy. As a result, tomato and potato leaves were wet for long periods and received little sunlight, ideal conditions for late blight to develop and sporulate.

Desiccation and UV light kill late blight spores, so if the weather cooperates in 2010, farmers may face less of a threat. In addition, late blight spores need living tissue to last the winter. Since tomato plants die with hard frosts, the only concern is infected potato tubers. Hazzard and her colleagues worry that some of the inoculum so widely distributed in 2009 may be lying in wait on unharvested potato crops or infected tubers that haven’t decomposed in compost piles. If infected tubers do not freeze, there’s the possibility that seedlings will emerge from them and present a new problem.

“We need to minimize the re-inoculums in 2010 and that’s everybody’s job,” says Hazzard. “If you’re farming in a community and working with people who are gardening, you have an opportunity to educate your friends.”

The life cycle of potato late blight.

Defend crops from late blight

Several methods in concert work to protect crops from late blight. Hazzard suggests the following:

  • Destroy infected tissue before the new growing season by burning it, freezing it or feeding it to livestock.
  • Buy certified disease-free potato seed (but there is a 1 percent tolerance for late blight, so look at the seed).
  • At the start of the season, scout and destroy volunteer potatoes.
  • Allow 2009’s growing fields to lie fallow in 2010.
  • Grow from seed instead of plugs, since seeds do not carry Phytophthora.
  • Let greenhouse temperatures drop to freezing. Greenhouse growers who battled late blight in tomatoes in 2009 and keep their greenhouses warm over the winter to grow greens can allow spores to survive the winter and re-infect solanaceous crops in 2010.
  • Plant resistant varieties of tomato, such as Legend (Territorial Seed Co.); Mountain Magic and Plum Regal (Johnny’s); Juliet, Stupice and NC03320 (Organic Seed Alliance); Matt’s Wild; Ferline; and Fantasio.
  • Plant resistant varieties of potato, such as Kennebec, Sebago, Allegany and Chieftain.
  • In greenhouses, focus on ventilation and keeping humidity down.
  • Listen to or read reports about late blight, available from cooperative extension programs throughout the country.

Hazzard emphasizes the importance of taking action to protect the crop before infection. Choose the right inputs and apply them properly.

Before applying any crop treatment, farmers should make sure the product has an agricultural use label and follow the directions on the label. To apply a foliar feed on a large field, little backpacks with little nozzles are neither effective nor safe. To apply foliar feed properly, growers should use a well-designed sprayer. David Fisher, of Natural Roots farm in Conway, Mass., utilized a sprayer with drop nozzles to foliar feed his tomatoes in 2009.

Farmer shares his successful strategy

Despite challenges with late blight, 2009 was the best tomato production year in Fisher’s 12-year history. At the NOFA-Mass conference, Fisher shared the methods he used in 2009 to create high yields on his organic farm.

Because Fisher utilizes a one-year rotation period for his fields, his tomato fields lay fallow in 2008. During that period, Fisher overwintered with a grass legume sod, which he plowed down in summer and followed with a six-week bare fallow period. During that time the ground was open to germinate as many weed seeds as possible. Fisher tilled the weeds under for weed control. In August, he composted with sheep manure and inoculated with minerals and various biological inputs to address nutrient shortfalls. Finally, he planted a fall cover crop of oats and peas.

In the spring of 2009, Fisher applied another round of biologicals and minerals. Lancaster Ag products composed Fisher’s custom dry blend based on soil testing and consultations on his farm. The blend includes high-calcium limestone, gypsum, dolomite, potassium, Epsom salts, micromate (a humate product, super concentrated organic matter to bond and hold these products), kelp, fish emulsion, molasses, sugar, microseed treat, fungal inoculant and trace minerals magnesium and zinc.

Fisher applied about 75 percent of the dry blend on his fields. By May 4, he planted early tomatoes in an 8-inch-deep trench to create a micro tunnel in the earth. (The soil is mounded on the sides and the tomato plants are set below the tops.) Before planting, Fisher soaked the plants in a solution of fish emulsion and a broad-spectrum inoculant called MPM, which contains fish emulsion, kelp and a host of biological inoculants. By late May, he started regular primary cultivation for weed control.

One of the few things Fisher did differently in 2009 compared to previous years was to apply regular foliar feeds, also custom blended by Lancaster Ag Labs, which he applied every two weeks using a drip sprayer. The blend included bean juice (provided by nearby South River Miso Company), micronized pearl calcium and MPM.

He started the main season on May 26. On July 2, Fisher saw some leaf spot on the early tomatoes. “We were pretty sure it wasn’t late blight, but we got out there anyway and started hand-pruning,” he says. Because Natural Roots farm is only 3.5 acres, this was a manageable task. Fisher included an oxidate product in the foliar tank to see if it would do anything to the septoria. “In retrospect,” he says, “it probably wasn’t a good idea to combine it in the foliar tank, because a lot of the biological inoculants are killed by the oxidate, but in the hustle we did that … wouldn’t do it again.”

At that point, Fisher suspected late blight in his tomatoes and potatoes. On July 6, his main season tomatoes were starting to flower. He side-dressed the tomatoes along the surface, applying the remaining 25 percent of the custom dry blend. On July 9, late blight was confirmed in Fisher’s potatoes, so he began foliar copper applications in potatoes and tomatoes at the rate of 4 pounds per acre. He also laid down new film. On July 15, he mulched tomatoes with oat straw to suppress soilborne diseases. The late blight was spreading rapidly in potatoes, and Fisher was concerned about spreading the spores from infected plants to uninfected plants via his equipment. To mitigate the risk, he sprayed the least infected plants first. By the end of the month, the potato vines were mostly dead due to late blight. Fisher mowed the vines to try to preserve any existing crop underground. In August and September, he applied additional copper with the drip sprayer.

Throughout the summer, Fisher took Brix readings to measure plant health, as well as nutrient density. In early June, tomatoes measured 9 on the Brix scale. A minimum Brix reading of 12 would have indicated tomatoes with a healthy disease and insect resistance. Successive Brix readings were higher, until mid-August, when the Brix reading on Fisher’s tomatoes dived down to a 5. At that time, the sap was brownish.

Dan Kittredge of the Real Food Campaign was another panelist at the conference. He responded to Fisher’s information, saying, “If our plants don’t have sufficient nutrition, they’ll start to pull minerals out of the soil. David illustrated this with his Brix levels. They were going along pretty well, 10 is not bad. Then, as Ruth showed, we had the cloudy rainy weather.” Kittredge explained that in those conditions, when there’s not as much sun for photosynthesis and the soil becomes waterlogged, the biological symbiotes in the soil asphyxiate and die and the entire biological process begins to shut down. Under those environmental conditions, Brix levels drop as the whole plant’s ability to function drops significantly. Fisher’s timely and efficient response saved his crop. By September, after additional copper applications, the Brix was back up to a 10.

Ultimately, Fisher’s tomato yields were 238 percent of his previous five-year average. Seventy percent of all Natural Roots farm’s crops, including greens and chard, showed significantly higher yields than in previous years.

Although Fisher grew several varieties in 2009, his top-producing variety was Sapho, a salad tomato slightly bigger than a golf ball. He harvested most of his tomatoes until the first or second week of October, but the Sapho resisted frost and produced into November. “It was pretty much indestructible,” he boasts.

The NOFA Survey

Fisher was one of 327 respondents to the survey put out by NOFA-Mass in the fall of 2009. While it is hardly a scientific study, the survey begins to answer questions about methods growers can use to prevent or treat late blight in tomatoes.

On the survey, NOFA asked questions about soil fertility, growing methods, pruning, mulching and treatments.

According to respondents, late blight losses on tomatoes grown outside were significantly higher than losses of crops grown in greenhouses. According to survey results, mulching resulted in somewhat lower incidence of loss than bare ground, and black plastic mulch resulted in lower losses than other mulches. More intense pruning seemed associated with lower loss due to late blight both inside and outside.

Seventy-one percent of respondents sprayed nothing on their tomatoes. Thirty-seven growers applied copper in the greenhouse; they reported an average of 15 percent crop loss, compared to a higher percentage of crop loss among growers who did nothing. Eleven growers applied biological fungicide alone, and reported results similar to those growers who applied no treatments.

Growers were asked about the perceived effectiveness of treatments on a scale of 1 to 3, 1 being generally ineffective, 2 being somewhat effective and 3 being highly effective. In each instance, each product appeared more effective in a greenhouse than outside. Other questions on the survey were equally perception-dependent. Consequently, the results of the survey are highly subjective. Although the survey provided NOFA with a general base of information, Ben Grosscup, who designed and administered the survey, says more scientific research is needed.

Kittredge Presents Additional Research on Late Blight

To gather data supporting the success levels of growers utilizing Nutrient Dense Crop production, 20 farms, on a combined 50 acres throughout the Northeast, followed the same protocols in 2009.

Participants hoped to create an environment on the leaf surface inhospitable to leaf blight and molds by applying a solution of 2 gallons raw milk, 2 quarts Sea Crop, 8 ounces Phytostim (kelp) and 5 ounces micro5000 (a biological innoculant).

Of the 20 participants, seven had been using sophisticated methods to boost soil prior to joining the research project; these farmers realized 100 percent crop harvest. Although they got late blight in their field, they minimized it using the protocols (above). Their crops went to full term with almost no problem. The other 13 farmers kept late blight at bay for two to four weeks, but then succumbed.

Dan Kittredge of hte Real Food Campaign said the case study demonstrates that biological supports are beneficial, but not sufficient, for farms lacking a basic nutritional foundation in their soil and plants. Plants grown on soil without that nutritional foundation need more sophisticated fungicides and chemical inputs to survive.

The study also shows that growers utilizing Nutrient Dense Cropping protocols realize significant increases in yields. NDC protocols work under the assumption that with every harvest, minerals are removed from the field. Therefore, the logical response is to put back into the field what one has taken out. Presuming one’s field did not have what it needed to start with, Kittredge suggests testing the soil to discover what deficiencies are present. A proactive fertility protocol is a valuable piece to have in place as a growing operation evolves.

The author is a freelance writer based in Massachusetts and a monthly contributor to Growing.