Question: Our tomatoes have had blight the last two years. Is there anything we can do to keep them from getting it again?
Answer: There are three major blights that can attack your tomatoes: Septoria leaf spot, early blight and late blight. All are fungal diseases spread by spores, which require dew or rain to infect the plant. These are most severe in wet weather. Septoria leaf spot, sometimes called Septoria blight, is caused by the fungus Septoria lycopersici and usually appears on the lower leaves after the first fruits set. Fruits are rarely infected. All the leaf loss reduces fruit yield and quality, and exposed fruits are more susceptible to sunscald. The fungus is spread by splashing water and by working among the plants when they are wet. It overwinters on tomato and weed refuse.
Early blight, caused by the fungus Alternaria solani, appears on the lower leaves, usually after a heavy fruit set. The spots are dark brown to black. Concentric rings develop in the spot forming a bull’s eye. The leaf area around each target spot turns yellow, and soon the entire leaf turns yellow and drops. Early blight fungus also infects stems and may produce stem cankers. It occasionally attacks the fruit, producing large sunken black target spots on the stem end of the fruit. Infected fruits often drop before they mature. This disease is most common late in the growing season. The fungus overwinters on old tomato vines and on weeds in the nightshade family.
Late blight, caused by the fungal-like organism Phytophthora infestans, occurs in moist weather with cool nights and moderately warm days. Dark-green to nearly black wet-looking spots begin spreading in from the leaf edge. In wet weather, the spots may have a downy, white growth on the lower leaf surface near the outer portion of the spot. Spots also develop on the fruits. At first, the spots are gray-green and water-soaked, but they soon enlarge and turn dark brown and firm, with a rough surface. When conditions are favorable, the disease may progress very rapidly.
Avoid these diseases by rotating crops. Plant tomatoes in the same place only once in three or four years. Remove and destroy tomato vines in the fall. Plow or rototill to bury the remaining crop refuse. Use healthy transplants. Remove badly diseased lower leaves, as these are a source of leaf spot fungus spores that help spread the disease.
Water at the base of the plants to avoid splashing water, which spreads the spores. Avoid watering with overhead sprinklers in late afternoon or evening. If the plants stay wet all night, leaf spot infections are likely to occur.
Use fungicides when needed. These diseases spread rapidly and are difficult to control once established. Fungicides must be applied before the disease first appears and reapplied throughout the growing season. Chlorothalonil fungicide, sold as Ortho Multi-Purpose Fungicide, can be applied up to the day of harvest.
-Beverly Shaw, Advanced Master Gardener Purdue University