by Sally Colby
Dr. Lori Hoagland, Purdue University, said that tomatoes are one of the most important vegetable crops in the world. “Organic and local production is increasing throughout the nation,” she said. “Tomatoes have a lot of production issues, so the driving force behind the project was to learn more about the production challenges in order to answer them.”
Hoagland is describing the scope of the NIFA (National Institute of Food and Agriculture) OREI (Organic Agriculture Research and Extension Initiative) funded Tomato Organic Management and Improvement (TOMI) project. TOMI is a collection of researchers and extension representing a variety of states across the country.
The project started with a national survey of organic and conventional growers to identify challenges and traits they were interested in. “Flavor is the number one priority, particularly among growers serving local markets,” said Hoagland. “We learned that growers are turning to heirlooms because they’re perceived to have better flavor. They’re also more susceptible to production challenges.”
The second most important factor to growers was disease outbreaks. The top three disease issues rated by growers included were late blight, primarily caused by Phythopthera infestans, early blight caused by Alterneria solani, and septoria leaf spot. Several soil borne pathogens, including fusarium and verticillium, and other foliar pathogens, including bacterial canker, were also mentioned by growers.
Hoagland outlined some of the steps in keeping pathogens at bay. First, avoid planting susceptible hosts by selecting disease-resistant varieties. Big Beef (F1) is commonly grown and is genetically bred to resist Alternaria stem canker, Fusarium wilt, Verticillium wilt, tobacco mosaic virus, nematodes and gray leaf spot.
Because seed can arrive already loaded with pathogens, use disease-free seeds and transplants. Hoagland referenced the major outbreak of late blight in 2010 in tomatoes that was traced to infected transplants. Growers who save seed should use treatment protocols to reduce pathogens on seed. Avoid planting transplants that show signs of disease.
It’s important to keep tools clean because pathogens can survive on materials that are used in tomato production. “Bacterial canker, which was problematic to control, has been shown to survive on fabric mulches and tomato stakes,” said Hoagland. “Make sure you clean materials, especially if you have had an outbreak in the past. It’s a good idea to check with your certifier to make sure products you’re using for cleaning are approved.”
Ensure that plants have plenty of air flow. Pathogens thrive in warm, moist conditions, so orienting row with prevailing winds to help dry the canopy and make the environment less favorable for the pathogen. “Go as wide as you can go with spacing,” said Hoagland. “Both between rows and in-row can help dry out the canopy and make a less favorable environment for pathogens.”
Avoid splashing water from soil onto plants. Mulch can help reduce splash, drip irrigation helps keep the canopy from becoming wet, and trellising and pruning help to create air flow. “Disinfect tools and avoid moving through fields in morning when things are wet,” said Hoagland.
Some growers use high tunnels to eliminate the risk of rain splash onto plants. “High tunnels can reduce late blight,” said Hoagland, “but they can also create an environment that becomes more favorable for other pathogens we don’t see in the field, like leaf mold. Make sure the high tunnel is vented.”
Hoagland said that several breeding lines are being trialed in several states and show good resistance to late blight, early blight and septoria. The varieties were tested last year during on-farm trials that also included taste trials and flavor rating. The new varieties have good yield and resistance, and will be trialed in different states this summer. Hoagland is hoping some varieties will be released for additional large farm trials.
Dr. Dan Egel, Purdue University, discussed foliar disease management. Egel said it’s important to be able to identify diseases in order to identify the problem. “The most common question I get from growers is ‘my tomatoes are turning brown from the ground up,’” said Egel. “That’s probably either early blight or septoria leaf blight.”
Egel said that if every year tomato plants start to brown from the ground up, the grower should make a timely treatment application. It’s important to know the proper intervals for applications for best results. “Start applications before disease appears or during the very early stages,” said Egel. “By the time you start seeing disease in the field, it’s already worse than it appears.” Egel added that if you see the first five lesions from septoria leaf spot and think you’re ahead of the game and start applying fungicides, what you don’t know is that there are microscopic lesions that are too small to see.
Be ready to repeat applications as necessary. “The product weathers,” said Egel. “In the field you’ll see that fungicides wear off the leaf. New growth will not have the fungicide on it. For a biological product, you may need to reapply the living organism to make sure it’s living on the leaf. Most foliar diseases need warm, rainy weather, so the rainier and wetter it is, the more you’ll have to apply fungicide.” Egel added that copper and sulfur products are more likely to wear off and may need to be applied more often.
It’s important to understand how products work on or in the plant. “Products with hydrogen dioxide (OxiDate®) have no residual after they’ve dried,” said Egel. “If a pathogen or fungal spore blows into the field after that, the plants aren’t protected. Frequent applications are necessary.”
Egel said that biological control products may require longer intervals or less frequent applications. In the experiments he conducted with Prestop®, a living organism, application every two or three weeks was sufficient. “Look to the label,” he said. “Some products work better shortly before disease starts. If you apply Serenade® Max and there’s no disease in the field, it doesn’t protect the plant and you’ll need to reapply it. Seranade® Opti works better applied right before disease starts, which might indicate more frequent application.”
Some products should not be tank mixed. “If a biological control product, which counts on the organism living on the leaf, think twice about mixing it with a copper product,” said Egel. “Again, look at the label to know for sure.” Egel reminds growers to use caution when mixing in alternation. Copper and OxiDate® products may kill the biological control product. If copper is necessary, the biological control product might have to be reapplied. AgriPhage, a microbe that attacks bacterial diseases of tomatoes, cannot be applied with copper. Egel recommended applying copper to flowering plants, then AgriPhage three or four times, then finish the season with copper.
Because pathogens can survive on materials used in tomato production, what’s the best way to clean? Egel says that wooden stakes are more likely to have residue, but pathogens can survive on any stake that has crop residue. “Most important is to clean with soap and water, and make sure there’s no crop residue,” he said, adding that it’s important to check sanitization products for organic approval. “When leaves fall from early blight and get on fabric, you can sweep them up and it doesn’t go into ground. Take the cloth, clean it with a hose and place in large tubs with bleach or OxiDate® to sanitize.”