Some fungicides are broad spectrum and will control numerous foliage diseases on numerous plant species, but they should be used only on those plant species listed on the label. Some fungicides will control certain diseases but not others. Thus, your selection of a fungicide is based primarily on what is legal and what your county agent recommends. A recommendation from your county agent means that the fungicide has been tested and is effective in Florida for the purpose recommended.

Where numerous fungicides are recommended for a specific disease, it is reasonable to assume that some are better than others. This is true especially when disease is severe, either because you waited too long to begin a spray program or the disease is one that spreads rapidly (downy mildews, late blight of potato, and others). However, where a spray program is initiated before disease becomes severe, a so-called "less effective" fungicide may perform as well as "the most effective" fungicide or "the most effective" fungicide may not be available at your store.

Remember, control measures other than fungicide sprays, such as crop rotation, resistant varieties, sanitation, healthy transplants and others reduce disease inoculum which, in turn, improves the effectiveness of fungicides simply because "disease pressure" is less.

Use only the rate of fungicide listed on the label. Label rates are often listed as a range such as 1 to 1½ tablespoon/gallon. Generally, the lower range is used early in the season or when plants are small and the higher range when plants are full grown, or when disease has become severe. Unless the label specifies otherwise, use of the higher rates earlier will minimize disease early, thereby making disease control more effective later in the season.

A disease already present will remain until the plant or plant part dies or is removed. With the initiation of a spray program and proper plant maintenance, a plant may be revived by putting on new growth. This is more apt to be successful with perennials (trees, shrubs, turf) than with annuals (vegetables, field crops).

Some fungicides have what is often referred to as "kickback action." That is, they may intercept the life cycle of the organism (fungus or bacterium) by reducing inoculum (spore) production on a lesion. In some cases, a fungicide may delay or inhibit growth of the organism in or on the plant during infection (penetration) of plant tissue or prior to when symptoms occur. Suck kickback activity by fungicides should not be relied upon for effective plant disease control except for greasy spot of citrus.

Because fungicides control plant diseases by protecting the plant against infection, like a coat of paint, spraying prior to infection will allow you to get the most for your money. Symptoms are the result of infection which occurred at an earlier time. The earlier you begin your spray program in relation to the amount of disease, the more successful you will be in controlling plant disease. You will be more satisfied with your efforts if you begin spraying at the first sign of disease or before (assuming you know from past experience that some disease will occur). Then continue spraying at intervals suggested on the label of the fungicide.

Repeat applications are necessary because fungicides are not chemically stable. They break down when exposed to light, organic matter; water and other factors. Rain washes fungicides off leaves and stems.

Disease-causing organisms such as fungi and bacteria can live in the soil on organic debris, thus providing inoculum for next year's crop. Crop rotation therefore, is recommended for disease control. You may need to spray earlier and with the highest rate on the label when you do not use crop rotation, as crop rotation delays the onset of disease by reducing available inoculum.

From Plant Protection Pointers,
Gainesville, FL. Oct. 1978.
by Tom Kuchared,
Extension Plant Pathologist.
R.F.C. International Inc. Newsletter,
May 1983

DATE: July 1983

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