Tomato Blight FAQ

Question:

 What can I do about blight on my tomatoes?

Minnesota Master Gardeners say:

 There are three important "blights" of tomatoes in Minnesota.

The most common is Septoria leaf spot, which appears toward the end of July. It first appears as small round black or brown lesions on the lower leaves. It works its way up the plant starting at the bottom.

The second most common is Early blight. It appears about the same time as Septoria, and is characterized by concentric "target" shaped lesions.

The least common but most destructive is late blight. It usually appears in August and the first symptoms are watery lesions on the lower leaves. Late blight will destroy an entire crop within a week.

The prevalence of these blights is affected by seasonal conditions and varies greatly from year to year. Heirloom tomatoes seem more susceptible.

Septoria does not usually affect the fruit. We have gotten very large crops of excellent tomatoes even on plants with a substantial infection.

The most effective way to treat tomato blights is to prevent them. Here is a list of do's and don'ts:

* Mulch to prevent splash-up from rain.
* Don't water overhead.
* Don't water in the evening.
* Give your plants plenty of space.
* Don't work around your plants when they are wet.
* Don't plant tomatoes in the same place where tomatoes, potatoes, peppers or eggplants were grown last year.
* Clean up all debris in the fall and don't compost it.
* Prune out diseased branches promptly and destroy.
* Keep weeds at a minimum.
* Plant resistant varieties when available.

These recommendations are especially important if your crop has late blight. In that case, skip planting tomatoes, peppers, potatoes or eggplant in the same location for at least one season.

We have greatly reduced the Septoria in our tomatoes by using landscape fabric as a mulch. It can be purchased at garden centers.

We do not generally recommend using fungicides for tomato blights. By the time gross symptoms appear on your plants, it is too late to apply a fungicide. If you do use a fungicide, you must use it at the very first sign of the disease. Fungicide recommendations can be found in the publications below.

For further reading:

Parasitic diseases of tomatoes (including early blight, late blight and Septoria):
http://www.extension.umn.edu/distribution/horticulture/DG1155.html

Septoria leaf spot of tomato:
http://www.extension.umn.edu/projects/yardandgarden/ygbriefs/p232septoria-tomato.html

Early blight of tomatoes and potatoes:
http://www.extension.umn.edu/projects/yardandgarden/ygbriefs/p256earlyblight-potato-tomato.html

Late blight of potato and tomato:
http://www.extension.umn.edu/projects/yardandgarden/ygbriefs/p230lateblight-pot-tom.html

Controlling diseases in the home garden:
http://www.extension.umn.edu/distribution/horticulture/DG1427.html