is it late blight or septoria on my tomatoes?

Asked August 12, 2014, 6:38 PM EDT

I called the MSUE hotline and discussed the condition that is affecting my tomatoes but after looking at the pictures and thinking further about the questions perhaps I wasn't keying on the correct signs to distinguish Late Blight from Septoria. the three pictures attached show the spots that have and are appearing on my tomato plants. They have bloomed and produced fruit as you can see. The condition seems to affect my Romas and Early Girls more than the Beefsteaks but those too are now showing signs as in the photos. But perhaps at this point the distinction is more academic as the treatment seems to be pretty similar?

Shiawassee County Michigan

1 Response

This is Septoria: bottom leaves get small dark spots that grow larger and develop a tan or gray center. The leaf turns yellow around the spot, then all yellow, then brown and then crispy and falls off. It works from the bottom of the plant upwards. It usually takes weeks for a plant to end up with clusters of leaves at the end of the branches. Fruit is unmarked unless it gets sunburned.

This is late blight: Spots develop on leaves in irregular brown-purple patterns. On damp mornings, the dark spots may appear almost fuzzy. Stems are darkened in places and fruit develops bubbled, tan-brown skin. The entire plant develops a stinky, moldy odor when put into a plastic bag and smelled. The plants are dead in three or four days. The disease moves so fast you can almost stand and watch your plants die.

There is no doubt that the photos are Septoria. Like all fungal diseases, it can be prevented but not cured. Many more leaves are infected before they show symptoms.

Even if you start spraying now, you may not be able to stop it at this point. The best control is really prevention. Begin spraying plants in late June or when there are tiny fruit, whichever comes first. Follow label direction which usually involves spraying every 7-10 days. You must repeat if the plant is hit by overhead irrigation or it rains. The product is water soluble. Chlorothalonil which can be found in Daconil or Fungonil is usually used. Serenade is an organic product that is labeled for this use but I have not talked to anyone that has used it so I do not know how effective it is.

You can treat for Septoria but it is almost impossible to treat for late blight. By the time you know the plants have it, it is too late.

Septoria is the most common tomato disease that I have seen for a number of decades.