University of Illinois Extension

What's Wrong with My Tomatoes?

If you grow tomatoes and do not treat for diseases, then expect one of two fungal foliar diseases to cause the loss of leaves, said a University of Illinois Extension PSEP plant pathologist.

"The two diseases that cause tomato leaves to usually die from the bottom up are Septoria leaf spot (Septoria lycopersici) and Early Blight (Alternaria solani)," said James Schuster. "Both pathogens are fungal diseases and both like free standing water on the leaves. The water can be due to rains, dews or over-head watering. In addition, both diseases over-winter in the plant debris.

"In the spring, splashing rains and blowing winds carry the pathogen onto the new plants. Fall sanitation can help delay serious infection the following year since the amount of inoculum has been removed or reduced and the pathogen causing these leaf spots may have to blow in from another site."

If you want to determine which of these diseases is on your plants, it will require close inspection of the disease symptoms.

Septoria leaf spot first starts as tiny, water-soaked spots that soon enlarge to circular or angular lesions 1/8 to 1/4 inch across. As the spots enlarge, the lesions develop a dark margin with a grayish white center. The grayish white center usually contains tiny black reproductive bodies of the pathogen that contain spores. Leaves that are heavily infected, turn yellow, wither, die and fall off in large numbers starting at the base of the plant. Septoria lycopersici can infect other plant parts.

Early Blight (caused by Alternaria solani) spots are usually brown to black and develop target-like concentric rings as they enlarge to 1/4 inch or more.

"Major leaf veins frequently limit the size of the lesions," he said. "This disease can attack the tomato causing blackish, sunken and rotten spots on the fruit. Fruit lesions are often covered with a dark brown, velvety layer of spores. The early blight pathogen can survive the winter on tomato debris, on other solanaceous plants and in the soil. Spores can blow in the air as well as on blowing dust."

Some Alternaria fungi are saprophytic (live on dead tissue) while others are pathogens (able to infect and kill live tissue). In addition, these fungi can cause internal decay while showing little external damage.

"Symptoms can vary between individual fungal species as well as among different hosts," Schuster noted. "Spots may have either sharp margins or diffused margins. Spots may be sunken and the color can be brown to black. If a mat of Alternaria mycelium forms on plant surfaces, the mycelium usually starts white before changing to a darker color."