Variables such as weather conditions, cultural control, fungicide toxicity and spray application techniques will have some effect on your attempts to control a disease. But, the one factor that will have the most influence is the time at which you begin your spray program.

Using the right amount for the right fungicide is essential for effective plant disease control, according to Roy Lee Carter, Gulf County (Florida) extension director. But, that's only half the battle. You must also know when to apply the selected materials.

Fungicides works by preventing plant disease. They serve as shields against infection not cures. These chemicals can't save plants that are already infected. They can only limit the spread of a disease to a healthy plant. So, it's important to begin a fungicidal spray program at the first sign of attack or, better yet, even before you notice any symptoms at all.

When you notice the first signs of fungus disease, usually leaf spots or blight, you can be fairly sure that the affected plants have been infected for at least three days, and perhaps as long as two weeks. Symptoms of bacterial leaf spots and downy mildew usually appear three to 10 days after infection. Other leaf diseases have incubation periods of from seven to 10 days, in some cases as long as from 10 to 14 days.

Generally, the longer the incubation period, the slower a disease spreads, and the more easily it can be controlled. Obviously, then diseases that have short incubation periods, such as bacterial blights, and downy mildews are the most difficult to control.

So, you should always inspect your plants carefully and frequently, and begin spraying at the first sign of infection. It may seem that extensive disease symptoms develop overnight. However, a few spots always appear on the leaves before a sudden explosion of symptoms. Remain alert for these early warnings. Also, if you know from past experience that a disease problem is likely to develop, you should begin a spray program before you see any symptoms at all. Then, continue spraying at the intervals suggested on the product label.

If disease symptoms that have appeared seem to get worse after you've sprayed, don't get discouraged. Remember that fungicides can only prevent – not cure – an infection. So, a disease may continue to produce symptoms for some time after spraying. However, you should notice a slowdown in symptom development within about 10 days following application of a fungicide.