Preventing Late Blight of Tomatoes and Potatoes

Late Blight on a Tomato Leaf
Gerald Holmes, California Polytechnic State University at San Luis Obispo,

Late blight is a disease that affects mainly tomatoes and potatoes. Cool, wet weather encourages the development of the disease. Once it takes hold, the spores spread rapidly and cover great distances. If one of your plants becomes infected, chances are excellent all of your tomatoes and/or potatoes will be.

While there is no cure for late blight, there are a few steps you can take to prevent late blight in your garden.

Plant Resistant Varieties to Prevent Late Blight

Keep in mind that a handful of varieties are resistant to late blight, but not totally immune. They may be slower to get and spread the disease, but they're not fail safe.

  • Potatoes: Resistant potato varieties include 'Defender' and "Elba'. 'Kennebec', 'Sebago', 'Allegany', and' Rosa' also show some resistance.
  • Tomatoes: There is a lot of breeding work being done. 'Mountain Magic', a cherry tomato, may show resistance. Some heirloom tomatoes that show resistance include: 'Pruden's Purple', 'Mr. Stripey', and 'Matt's Wild Cherry'.

Keep the Disease out of Your Garden

This may sound like double-talk, but the spores can over winter in potatoes that were infected. So if you saved some tubers to use as seed potatoes, you could be reintroducing the problem in your garden.

Late blight of potatoes is the same late blight that affects tomatoes—it will spread from one to the other. When late blight has been a problem in your area during the growing season, you should not save potatoes to replant. However they're fine to eat. Late blight does not affect humans or the flavor of the potatoes. But the spores will over winter on potatoes, so do not save them to replant and do not dispose of them in the compost pile. Plant only certified seed and seed potatoes.

Keep a close eye on your plants, to catch problems early. If you're having a rainy summer and late blight has been reported in your area, you can gain some protection by spraying plants with a fungicide. These fungicides will help prevent late blight infection, but they will not cure plants that are already infected.

For organic control, researchers at Cornell University recommend using a fungicide that has "... fixed copper as an active ingredient and has tomato and potato late blight on the label. Of these fungicides, only some of the fixed copper products are approved for organic production. There are some organic labeled products that also seem to help, if applied prior to infection. Serenade, is probably the easiest to find in garden centers, but there are more options listed on North Carolina Extension's fact sheet."

The folks at your local Cooperative Extension can probably help you find a good product. Always follow the label directions. More is not better.