Detect Tomato Diseases by Using This Step by Step Method
From Roger Harris, former About.com Guide
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You May Have to Dig for the Truth to Identify Tomato Diseases
Keep in mind that plant disease symptoms change depending on what stage the disease is in. For example, primary symptoms occur at the point of infection, such as spotted leaves on tomatoes. As the disease progresses, the leaves may drop off and expose the fruit to sunlight, scalding the fruit, so the obvious is really the secondary symptom. Or wilted leaves may be evident, but digging up the plant will show the primary symptoms really are the nematode-damaged roots.