Tomato Blight

Several gardeners have called the MCA Environment Department recently to inquire about a disease that has been affecting their tomato plants. The disease is called tomato blight. There are actually two types of blight, an early blight and a late blight. Both diseases are caused by fungi. Alternaria and Phytophthora are the fungi that cause these blights. The following excerpt from the Purdue University Plant and Pest Diagnostic Laboratory website describes each blight in further detail and provides recommendations for gardeners that have blight in their gardens.

“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 fungus 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 “

Tomato blight will also affect any plant that is related to the tomato, such as potatoes and peppers. These plants all belong to the Solanaceae or nightshade family. There are also weedy plants that may grow near the garden (bittersweet nightshade) that can be infected and will be able to infect other plants in your garden.

There are organic methods of treating fungi in the garden rather than using all types of chemicals in gardens. Most of the organic treatments involve the use of sulfur or copper –based anti-fungal treatments.

It is very important for gardeners to rotate their crops once their gardens have been stricken by blight or any other plant disease. The fungal spores persist in the soil for a number of years. Once your garden has been infected with blight, you should plant tomatoes, potatoes and peppers in different areas of your garden.

DO NOT COMPOST AFFECTED PLANTS. Composting the infected plants will not kill the fungus that causes the blight. It is better to either burn the plants or put them in plastic bags and dispose of them in the garbage.

The attached photos show what the two blights look like. The photographs were taken by T.A. Zitter of Cornell University in Ithaca, NY.

References: http://www.ppdl.purdue.edu/PPDL/expert/tomato-blight.html. Purdue University, Purdue Plant and Pest Diagnostic Laboratory, West Lafayette, Indiana. Accessed online 8/17/2009.

http://vegetablemdonline.ppath.cornell.edu/PhotoPages/Impt_Diseases/Tomato/Tomato_Photos2.htm. Cornell University, Vegetable MD Online, Department of Plant Pathology, Ithaca, NY, 14853. Accessed online 8/17/2009.