Undestanding Late Blight Disease of Tomatoes and Potatoes

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

Late blight is a fungus-like disease that mainly affects tomatoes and potatoes. While there are many diseases that can affect your tomato and potato plants that wont severely impact your harvest, late blight should be taken seriously. Once late blight takes hold, the pathogen,Phytophthora infestans, spreads rapidly and great distances. Late blight was responsible for the Irish potato famine.

As its name implies, late blight generally occurs later in summer, it is possible to see it earlier in the season.

Cool, wet weather encourages the development of the fungus.

What are the Symptoms of Late Blight?

Symptoms of late blight include: greasy looking, irregularly shaped gray spots on the leaves. A ring of white mold can develop around the spots, especially in wet weather. The spots eventually turn dry and papery. Blackened areas may also appear on the stems. The tomato fruits can develop large, irregularly shaped, greasy gray spots. Potatoes can take on a reddish brown discoloration and will become susceptible to soft rot.

The Late Blight pathogen needs a living host to sustain itself. Even after the growing season, it can overwinter in plant debris. In frost free areas it will easily overwinter in potato debris left in the garden, as well as in potatoes saved as seed. Infected seed potatoes can be hosts to the pathogen, even in colder areas. If your garden is infected with late blight, it is extremely important that you remove and dispose of all debris at the end of the season.

Do not compost plants or fruits with late blight  and don't save seed potatoes.

What Can You Do About late Blight?

Unfortunately there are three major hurdles with controlling late blight.


  1. Once it’s detected, it’s usually too late to save the plant. Late blight develops quickly, so by the time you see symptoms, it has probably been around for awhile.
  1. The spores are air born and can travel hundreds of miles. An infected plant in a neighboring town can mean the ruin of crops for that whole region.
  2. Late blight can over winter in plant debris and seed. You should dispose off all plant debris by the end of the season. As I mentioned above, do not compost it.

    Since late blight can even over winter in potatoes, if you saved some to plant for next year or even if you simply missed one in the ground and it sprouts next year, you could be in for another season of trouble.

If you suspect you have Late Blight, contact your Cooperative Extension Service for definite ID. They will appreciate knowing that late blight is in the area and will be able to contact farmers and get the word out to other gardeners to be on the alert. And if late blight is detected in your area, here are some ways to prevent late blight from finding its way into your garden.

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