Avant Gardener: Fungal disease can ruin tomatoes: Prevention is key

  • By Kyle Valentini
  • July 29, 2013
  • 368
                        
Summary: Tomato blight can turn a great harvest into a lousy harvest overnight. Take measures to prevent fungal disease because once it strikes the garden, there is no quick fix. It has been a good season for tomatoes in my garden. My neighbor has not been so lucky. Her plants are dying from the bottom up. Tomatoes are notorious for their lack of disease and insect resistance. They really are a lot of trouble but because we love them so much, especially the homegrown variety, they remain one of the most common garden plants in the region. Three common fungal diseases plague tomatoes and one of them is probably responsible for my neighbor’s tomato issues. Each of these diseases has a botanical Latin name but you need not worry about proper pronunciation or even remembering them at all. No one in Rome ever spoke botanical Latin anyway. Septoria leaf spot, early blight and late blight are fungal diseases spread by spores that infect the plant when foliage is moist. Septoria leaf spot usually appears on the older lower leaves after the first fruits set. Fruits are rarely infected directly, but the loss of good foliage reduces fruit yield and quality, and exposed fruits can get sun burned. The fungus is spread by splashing water, equipment and our own gloves and boots when we are working among the plants when they are wet. It overwinters on tomato and weed debris, and can linger for three years. It does not live in the soil but rather on debris that is in the soil. Early blight is caused by the fungus Alternaria solani and appears on the lower leaves, usually after a heavy fruit set. The spots are dark brown to black and form concentric rings that form what is often referred to a bull’s eye. The tissue around each target spot turns yellow, and soon the entire leaf turns yellow and drops. Early blight fungus infects stems and may produce stem cankers. Sometimes it infects 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, a result of tophthora infestans occurs in moist weather with cool nights and moderately warm days. Wind-dispersed spores cause dark, wet-looking spots to spread in from the leaf edge and may develop a downy white growth on the lower leaf surface during wet weather. Fruits may also develop spots that are gray-green and water-soaked at first, but they soon enlarge and turn dark brown and firm, with a rough surface. When conditions are favorable, the disease may progress very rapidly. Crop rotation is essential in managing fungal disease. Tomatoes should be rotated every season and only on the fourth year should they be planted in their original location. This remains a challenge in a small garden. Raised beds can help as they allow better drainage and help disease from spreading. Container gardening is an option. It is important to keep the garden tidy. Once detected remove any diseased plants immediately and never compost them. After the first killing frost, remove all plants from the garden. Keep the foliage of the plants as dry as possible. Allow plenty of space between plants for good air circulation. Avoid overhead watering especially in late afternoon or evening. Plants that stay wet all night provide the perfect conditions for fungal spores to infect. Water the soil rather than the foliage. In conventional gardening, fungicides can help protect healthy foliage from becoming infected, but they cannot cure an infection once it is present. These diseases can spread rapidly and, once established, are difficult to control. Organic gardeners have limited options. Copper sprays, while approved in organic farming can build up in the soil and can harm beneficial insects. Disease resistant tomatoes are being developed remarkably enough by growers creating hybrids from wild tomatoes found in their native South America.


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