Fungicides can be a great help in preventing diseases when properly applied to the plant foliage. Since fungicides are preventive, they should be applied before the disease occurs, or as soon as the first symptoms of disease appear. Some vegetable diseases require specific fungicides for their control. An outline of diseases of specific vegetables, disease description, suggested cultural and chemical control and comments follows this section.
Fungicides are available primarily as wet-table powders, dry flowables and dusts, but a few are sold as emulsifiable concentrates, flowables and liquids. Wettable powders (WP) and dry flowable (DF) are formulated in such a manner as to be readily suspendable in water. Dusts (D) should not be mixed in water, but applied directly to the plant. Emulsifiable concentrate (EC) fungicides contain an emulsifying agent that makes them readily suspendable in water. Flowable (F) fungicides are finely ground wettable powders that are suspended in a liquid.
Some soil fungicides are available as granules and are applied in the furrow at planting. Dust and spray fungicides may also be used as in furrow treatments for seedling disease control.
Never attempt to use dust formulations of fungicides in spray solutions. Dusts will not suspend in the spray solution. Wettable powder formulations may appear similar to dusts, but they are formulated to be suspended in spray solutions. Foliar sprays will aid in controlling leaf spots, rusts, mildews, anthracnose and fruit rots. Foliar sprays are not effective against vascular wilts or root rots. Foliar sprays are protectants, because they form a protective layer of fungicide over the surface of the fruit and foliage. Disease agents (bacteria and fungi) that land on these fungicide-coated surfaces are killed or prevented from infecting the plant.
Most fungicides are not effective in inhibiting disease organisms once they have infected a plant. It is imperative that foliar fungicides be applied prior to infection of the plant. A spray schedule should be followed that maintains a protective fungicide layer on the foliage and fruit during favorable infection periods. By carefully monitoring their vegetables, some gardeners can usually delay the first fungicide application until the first sign of disease. Then a 7-14 day spray schedule should be followed. During rainy or humid weather, spray application intervals should be shortened.
Proper pesticide mixing and spraying plays a very important part in achieving disease control. Most home gardeners will find a 1-2 gallon compressed-air sprayer adequate for applying foliar sprays. A nozzle with a cone pattern will provide the most effective coverage of plant foliage. Keep the pressure up to insure small spray particle size and good coverage.
Sprayers should be cleaned and rinsed after each use. Hose-end sprayers are not very effective in applying fungicides to vegetables. Never use the same sprayer for fungicides and insecticides that has been used for herbicides. Residues of certain types of herbicides are very difficult to remove from sprayers. These residues may cause crop injury if a herbicide-contaminated sprayer is used in applying fungicides or insecticides.
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