Otherwise, insects may become resistant to the insecticide. It is a good idea to switch insecticides periodically. Insects can be harmful, but disease can be disastrous. Diseases must be prevented, not cured. There are two main diseases which cause this disaster every spring.
Early blight is characterized by irregular, brown spots that first appear on older foliage.
With age, the spots show concentric rings forming a target pattern. A yellow, diffuse zone is formed around each spot. Although this fungus disease can be observed throughout the year, it is most common during the fruiting period. The more tomatoes a plant produces, the more susceptible to and disastrous are the effects of an early blight infection. The fungus is favored by high humidity and high temperatures.