MOSES Conference Friday Workshop: Tomato Diseases

We recently took part in the annual MOSES (Midwest Organic & Sustainabile Education Service) Conference in La Crosse, WI, with our potato study poster. We were also able to attend workshops, meet inspiring organic growers, and eat great food. MOSES has the enviable problem of too many good programs happening at the same time, as the schedule shows. We attended as many as we could and took notes to share here.

Ecological Management of Tomato Diseases

This workshop focused on behavioral, varietal, and the careful use of organic treatments to prevent typical maladies. The presenters — Erin Silva, Tina Wu, Julie Dawson, and Amanda Gevens, all of UW–Madison — provided a list of resources, including:

Ecological management requires an understanding of the pathogenic triangle: the host, pathogen, and environment, all of which are required for a malady to take root. By identifying which problem you’re dealing with, you may be able to use strategies to disrupt one or more leg of the triangle.

Late Blight

Late Blight is one of the most widely observed problems for tomato plants in the second half of the growing season. A wet and cool (60–70ºF/day and 50–60ºF/night) environment coupled with potato or tomato plants support the propagation of the Late Blight fungus (actually the same fungus that caused the Irish potato famine: Phytophthora infestans). Symptoms include small brown lesions with fuzzy white down on the underside of the leaf. The pathogen is brought in from off site; it does not overwinter on seeds or in the soil, but it can survive on seed potatoes.

Late Blight can be combated by changing our behavior: choosing a site with good air flow, using drip irrigation, and destroying infected materials. Additionally, we can monitor for outbreaks, make sure our compost is hot enough to kill pathogens, use resistant and disease-free seeds, and if needed, approved fungicides (better if specific to this malady rather than a systemic fungicide, which also kills beneficial fungi) or biopesticides. At the end of the season, all plants should be buried or burned.

Early Blight

As the name implies, this one strikes early. It likes warm and wet days and can come from infected seeds, soil, or compost. This fungus causes dark lesions with a distinctive yellow halo on the leaves.

To keep down Early Blight, rotate the crop on a two- or three-year cycle (this includes tomatoes and potatoes). Avoid broadcast irrigation and bare soil below the plants, which allows splashing of infected medium up onto the plants. Seeds can be treated with steam, hot water, and maybe even bleach to kill the fungus (do some of your own research on this).


Another fungus! This one has small brown spots with white or tan centers that start on the bottom of the plant. It also prefers warm and moist locations, so pruning the bottom branches of the plant can help reduce humidity and contact with splashed ground. Light mulch and drip irrigation also helps reduce infection. Like Early Blight, Septoria can be reduced by rotating tomatoes on an at-least-two-year rotation. Unlike Early Blight, Septoria overwinters on equipment, which can be treated with heat (130ºF+ for 30+ min.). I just use twine for my trellises each year and burn them with the plants.

Variety Selection

Julie Dawson has been running tomato trials on resistant varieties for the UW. Her data is here, but you can see a similar study here. She noted different maladies for tomatoes grown in the field versus in a high tunnel greenhouse. In the former, blights, Septoria, bacterial speck, cracking, and a short season were limiting factors. In the latter, her team saw blossom-end rot, green shouldering, horn worms, and blossom abortions.

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