Late blight on tomatoes

Asked February 4, 2015, 11:08 AM EST

I grow tomatoes in Lansing, MI in high (32 inch) raised beds. These beds are constructed of sheet roofing steel and are partially filled with partially rotten logs - hugelkultur. This system works great and reduces need for water. Last summer, my tomatoes were affected in August with late blight, which I understand is Phytophthora. I understand that this pathogen overwinters and will likely affect my crop this year. I'm trying to find the best information for sanitizing the soil in these boxes and managing any recurrence of blight this summer. Also, I run a large neighborhood website and know that a lot of our neighbors have also been affected. I would like to pass on the best information I can on soil fumigation, selection of resistant tomato varieties, and crop management to minimize blight effects this summer. I have attached a photo of unaffected plants in the raised beds.

Ingham County Michigan

1 Response

Late blight requires several things to make it happen: any kind of tomato or potato plant, the right weather conditions which involves cool and damp, and a source of the disease. There were several times last summer when we had the conducive weather conditions with cool, almost cold, nights and fogs or mists.

Late blight does not overwinter if the plant debris has been frozen. It could overwinter if infected potatoes were left in the soil and they did not freeze and were dug up the next year.

There are no tomatoes that have any resistance to late blight. Your choice is to protect them with a fungicide before the plants get late blight. That means beginning to spray plants just as they are starting to create small fruit or at the beginning of July, whichever comes first.Most directions indcate spraying on a 7-10 repeat. Repeat after a rain and do not overhead water the plants. The fungicides are water soluble.

If you are a traditional gardener, you can use a fungicide that contains chlorothalonil. If you are an organic gardener, you can use a fungicide containing copper hydroxide. There is another organic product called Serenade but I have not talked with any gardeners that were pleased with its performance so keep this in mind if you use it.

The soil does not need to be treated and there is nothing to treat it with. This is a disease that floats in the air. If dead plants were destroyed by burning or burying somewhere they will not be unearthed, it should be enough. Wild plants in the nightshade family can get the disease but it is rarely fatal to them.

The big thing is the weather. If we do not have cool, damp weather, chances are small there will be a problem.