Q. Tomato blight damaged my crop this summer. Is there anything I can apply to the soil to reduce the likelihood of this happening again next year?
A. The best course of action this fall is to thoroughly clean out your vegetable garden. Remove spent plants, stakes, ties and/or cages. Scrub stakes and cages and allow them to air dry, and launder ties if they are reusable. There is nothing you can apply to the soil now to reduce the likelihood of tomato diseases next year. Rotate crops so that tomatoes and related crops -- such as peppers, eggplants and potatoes -- are not grown in the same part of the garden every year. Three- to four-year rotations are best if you have enough room. It is also important to stake or cage tomato plants because that allows increased air circulation around them, and it makes them easier to spray if you choose to use fungicides.
There are a number of "blights" that have impacted tomatoes in our area this year, including early blight, septoria leaf spot and late blight. Early blight is caused by the fungus Alternaria solani and is characterized by leaf spots with a bull's-eye appearance. Spotted leaves yellow and die prematurely, which leads to early defoliation. This reduces yields and opens up fruits to sunscald. The fungus overwinters on plant residue and can persist for at least a year. It is also borne on seeds, and can be introduced to the garden on infected transplants. Early blight occurs under a variety of weather conditions, but it is favored by periods of leaf wetness from dew and rainfall.
Early blight usually starts on the oldest, lowest leaves when spring rain hits the ground and splashes overwintered spores onto the leaves closest to the ground. Good garden sanitation can help remove much of the fungus, and mulching transplants can reduce splashing. Clean oat straw, composted grass clippings (not from herbicide-treated lawns), shredded leaves. Red or black plastic are suitable mulches in the vegetable garden. Fungicide applications work best when they are used to protect plants from infection rather than after the fact. Start making applications about two weeks after setting transplants into the garden. Fungicides labeled to control early blight include chlorothalonil (Ortho Vegetable Disease Control), maneb (Dithane), copper-based fungicides (Bonide Copper) and Bacillus subtilis (Seranade).
Septoia leaf spot is caused by the fungus Septoria lycopersici and is characterized by numerous small, circular leaf spots. They are much smaller than early blight leaf spots and lack the bull's-eye appearance. Infected leaves yellow and fall prematurely, resulting in reduced yields and fruits damaged by sunscald. Like early blight, septoria overwinters on infected plant debris and is splashed onto the lower leaves of new transplants by rain or overhead irrigation. Septoria is favored by wet weather and dew, but it is not usually a problem until plants begin to set fruit. Garden sanitation, crop rotation, mulching and fungicide applications all help reduce the severity of this disease. The same fungicides labeled for early blight are also labeled to control septoria leaf spot.
Late blight is caused by the oomycete Phytophthora infestans and is one of the most destructive tomato (and potato) diseases. Leaf spots caused by late blight are irregularly shaped and appear greasy or water-soaked at first. The spots enlarge rapidly during wet weather, merging together to kill entire leaves. You may also see masses of white spores on the underside of affected leaves under humid conditions. In the northern U.S., the causal organism can only overwinter on potato tubers overlooked during harvest or in cull piles on commercial potato farms. Otherwise, it overwinters in the South and the spores are blown north on storm currents. Cool, wet weather favors development. If you grow potatoes, avoid planting tomatoes or potatoes in those areas more than once every three or four years.
While garden sanitation and staking to improve air circulation are always good practices, fungicide applications are the most important control for late blight. The same fungicides labeled to control early blight and septoria leaf spot also control late blight.
Email questions to Sandy Feather at firstname.lastname@example.org; by regular mail c/o Penn State Extension, 400 N. Lexington Ave., Pittsburgh 15208.