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Spots on tomato leaves

Bob Dluzen
Special to The Detroit News

By now if you have tomatoes in your garden, I'm sure you have been seeing leaves that have started to turn yellow and develop spots.  These are symptoms of any one of the fungal diseases that infect tomato plants in our area. Early blight and septoria leaf spot are the most common.

A third disease, late blight, is a less common but much more serious infection that occurs during cool, moist years. We won’t have to worry about that this year because of the hot, dry summer we’ve been having.

These leaf spots diseases are often collectively referred to by many gardeners as just “tomato blight” or “tomato leaf spots”.

Early blight spots usually form concentric rings as the spots get bigger, while septoria spots appear smaller and separated.

Both early blight and septoria happen every year, so many gardeners accept this as a normal part of tomato growing.  There are a number of tomato varieties that are resistant to early blight, however all varieties are susceptible to septoria leaf spot.

These two tomato plants are growing on a shared trellis.  The green leaves toward the right belong to Matt’s Wild Cherry, a resistant variety available from JohnnySeeds.com. The yellow, spotted leaves  belong to Sweet 100’s, a susceptible variety.

Early blight will infect the fruit as well as the leaves, while septoria only infects leaves. Both will cause reduced yields. Damage from sun scald because of the lack of leaves to protect the fruit from the direct rays of the sun reduces fruit quality. Also, fewer leaves also means less photosynthesis.

The treatment is about the same for both diseases, spraying the plants with a fungicide.  The treatment will not "cure" the disease but will help keep the symptoms from progressing further. You need to be vigilant in applying your fungicides if you decide to spray. 

Rotation of your planting from year to year helps somewhat by reducing the amount of spores in the soil.  These blights also infect potatoes which are closely related to tomatoes so rotate your  planting so they don’t follow one another.

Dead and dying plants should be removed, bagged up and sent to the landfill along with your regular trash to reduce the number of spores over wintering in your garden.