The Plant Disease Triangle


The Plant Disease Triangle

By Paul Schnare

Monday, June 23, 2003

Three factors are required for plant diseases to run rampant. There must be the presence of a susceptible host, a disease causing organism, and the right environmental or weather conditions. If any of these three factors are missing, no plant diseases will be found.

Certain plants, such as tomatoes, are susceptible to different diseases, such as blight or wilt. When the right weather conditions and the disease causing fungi are present, these diseases will destroy tomato plants. Plant breeders are constantly trying to find new hybrids that are resistant to these diseases. Tomato varieties that are resistant to blight or wilt will have V or F behind their name.

Most native soils in the Heartland contain verticillium and fusarium, different fungi that cause disease in tomatoes. It is difficult to sterilize the soil in order to eliminate these disease causing organisms. These fungi enter tomato plants through leaves. When it rains, soil containing these organisms splatter onto leaves, and then inoculation can occur. You can reduce disease incidence by mulching around tomato plants so that soil does not splatter on leaves during a rain.

For the disease triangle to close and for disease to run rampant in tomatoes weather conditions must include lots of rain, high relative humidity, and warm temperatures. If you plant tomatoes in soil containing verticillium or fusarium, but the weather stays cool and dry, you will find very little disease incidence on your tomatoes.

If you keep in mind the three legs of the disease triangle, you may be able to reduce incidence of disease in the plants growing in your garden. This would eliminate the need for applications of fungicides.