According to Johnny's Selected Seeds Website "Tomatoes are among the world's most cultivated crops, and given proper conditions and regular maintenance, they are relatively easy to grow. They are far from trouble-free, however, being prone to a host of production problems and pathogens, especially when conditions and maintenance are less than ideal.
As prevention is the best medicine, it helps to have some familiarity with the threats that you and your plants may face. To help acquaint you with the most common and damaging problems that may arise we developed this Guide to Common Tomato Pests, Diseases and Physiological Disorders. Many tomato problems closely resemble each other and are continually evolving; therefore, we always recommend seeking professional assistance from your cooperative extension for the definitive identification of disease and pests. This guide will provide a starting point toward understanding some of the most common issues that may arise, and their causes, while also helping you to select varieties resistant to specific diseases."