Integrated Practices for Managing Late Blight without Chemicals

By Paul Hepperly

Farmers, chefs and tomato lovers throughout the Northeast are on edge due to a fungal disease threatening  the crop across many states. Late blight, caused by the fungus Phytophthora infestans, causes tomato and potato foliage to collapse prematurely and the fruit and tubers to rot. Under cool and wet conditions epidemics can wipe out fields of these crops.

In Pennsylvania, we’re anticipating a significant epidemic of this fungus, perhaps worse than what we had in 2005, both very wet years.

The late Alan MacNab from Pennsylvania State University developed an epidemic computer model for predicting the need for applying late-blight controls based on the duration of cool and wet conditions. His system was called BLITECAST. Such modeling tools allows farmers to be ready for action to prevent losses from the disease when conditions are favoring severe epidemics.

Organic farmers can’t use synthetic fungicides, but that doesn’t mean we’re helpless in managing the disease. In certified organic production we have approved materials including copper, sulfur and biologicals. Always check with your certifier when using a product for the first time, or for a different application than you did previously.

Although we know of no high levels of resistance in either tomato or potato to late blight, there are some varieties of both potatoes (Sebago) and tomatoes (Red Cherry) which are tolerant to the disease. “Tolerant” means losses are not complete even in conditions favoring disease. These varieties should be employed in areas where blight is periodically a problem.

When no one practice will work alone, combining techniques may allow organic farmers to live with serious diseases and pests. We suggest that as soon as Blight Cast would predict epidemic conditions, copper fungicide be applied. Copper would be alternated with Serenade, a biological control based on Bacillus subtilus bacteria.

We also suggest that you consider the use of Trichoderma harzianum. In a commercial product called Root Shield, this material has been shown to activate the defense metabolism of the treated plants.

Using the trichoderma h. to stimulate plant defenses combined with tolerant varieties mounts a strong natural defense. Strengthening the response with applications of copper and Serenade after epidemic environments are predicted by BLITECAST is the best integrated approach to mitigate losses from this challenging. devastating and opportunistic disease. Take great care not to transfer the disease from infected plants via tools, baskets, machinery or your own hands and clothes.

In environments which are consistently susceptible to fungal diseases, moving tomatoes under roofed structures prevents the rain splashing that exacerbates the disease epidemics. Production greenhouses or plastic hoop houses, using drip irrigation prevents the leaf moisture that creates susceptibility to fungal infection.

Earlier yield coupled with insect protection—and possible preventing a fungal epidemic—helps to offset the added expense relative to open-field cultivation.

Paul Hepperly, a Fulbright Scholar , serves as senior scientist at the Rodale Institute, Kutztown, Pennsylvania.

MacNab’s recommendations for prevention

eOrganic-produced materials on eXtension:
Organic Management of Late Blight of Potato and Tomato 
Organic Management of Late Blight of Potato and Tomato with Copper Products