Properly identify a tomato disease by visually inspecting all parts of the plant, making a record of symptoms and comparing these symptoms to a tomato disease key. Keep factors such as time of year, moisture level, tomato variety and potential pests and vectors in mind when attempting to identify a tomato disease. Some tomato diseases are extremely similar, and these require microscopic assays to unequivocally determine the pathogenic organism.
Different tomato pathogens tend to affect different parts of the tomato plant. Wilting diseases, such as the fungal fusarium and verticillium wilts, primarily affect stems and vascular tissue on tomato plants, turning healthy green tissue brown and sickly. Fusarium can also cause both crown and root rot, where the highest and lowest parts of the tomato plant turn brown. Powdery mildew is another fungal pathogen, but one that almost exclusively targets the leaves, causing extensive defoliation but leaving stems and vascular tissue untouched.
Time of year can be a key factor in determining what type of disease affects a tomato plant. Viruses and bacterial infections are fairly common at all times of year. Fungal pathogens, on the other hand, prefer damper, cooler conditions and are therefore more common in fall, at the end of the growing season. Late blight, a fungal disease, even takes its name from the time of year in which it usually strikes tomato plants.