Extension Educator, Horticulture
As I was grazing in the garden last night on my first tomato of the season, (a variety called fittingly 4th of July) I noticed the lower leaves of my tomato plants were browning. With further inspection, I noticed the leaves had spots. Some leaves were then yellowing while others had progressed to the brown and "hanging limp on the stem" stage.
Tomatoes can get several diseases so proper identification is crucial before any action is taken. My tomatoes are suffering from leaf spot diseases which can include septoria leaf spot and early blight. Both cause spots and can cause the leaves to quickly yellow and drop, starting at the bottom of the plant. Septoria causes small water-soaked spots. These spots become circular to angular with dark margins and grayish white centers. Early blight causes small brown leaf spots with a target-like series of concentric rings in each lesion.
To control leaf spots, improve air circulation in the garden, mulch, stake or cage vines, and remove and destroy infected leaves as soon as the symptoms start. Keep tomatoes watered with one inch of water weekly to improve vigor. Weekly fungicide sprays of maneb or chlorothalonil can be helpful. Be sure to read and follow all label directions especially the days between application and harvest. Leaf spot diseases can be easily controlled in the garden in the beginning stages of the disease. At the end of the garden season, remove and destroy tomato vines and unharvested fruit. Varieties such as Jetstar, Roma VF, and Supersonic have some tolerance to early blight.
In my experiences people often mistake leaf spot diseases for the fungal wilt diseases - fusarium and verticillium. The major difference is the wilt diseases will not show the spots on the leaves. However, as in the leaf spot diseases, leaves of the plants infected with the fungal wilt diseases will turn yellow and die starting at the base of the plant and progressing upward. In the wilt diseases infected plants may also appear stunted. Leaves on only one side of the plant may show symptoms. Wilting might occur during the hottest part of the day even when plants appear to have adequate water. Leaves may dry before wilting is detected so initial symptoms may appear as stems with dead leaves. Both fusarium and verticillium cause discoloration of the inner stem tissue.
Both fusarium and verticillium are soil-borne diseases and infect plants through the root system. Both are able to survive in the soil in the absence of susceptible plants for many years. Fusarium can also be seed-borne. Selecting resistant varieties is the most common and practical means of controlling fusarium and verticillium. Fungicide spray will not control these wilt diseases.When selecting tomatoes, look for varieties labeled "VFN." These show resistance to verticillium, one or more races of fusarium and nematodes.
Rotation out of an affected area for 5 to 7 years can help to reduce the incidence of verticillium wilt. Shorter rotations will have little effect. Removal of infected plant parts including roots may help.
Other factors can cause wilting of tomatoes including root rots, mechanical injury to roots or stems, toxins from walnut trees and a bacteria. The bottom line is to get an accurate identification of the problem before you reach for the fungicides or prune at ground level.
Suspected plants can be taken to your local Master Gardener office or the University of Illinois Plant Clinic at 1401 St Mary's Road in Urbana. There is a $12.50 fee for plant clinic diagnosis.
It's amazing even with all the problems that tomatoes can have, you still have to lock your car doors to make sure the neighbors don't leave you another batch.