Correct spray timing during the primary scab cycle lessens the need for extensive fungicide applications during the later stages of disease development. The critical period for scab control is from the beginning of bud growth until the apples are 1/2 inch (1.27 cm) in diameter.
In order to properly control secondary scab, apple trees should be closely monitored for primary scab. Fungicides should be applied if one or two scab lesions per tree are present. Irrigation sets 12 hours or longer should be avoided.
There are two approaches to control of apple scab using fungicides. Protective, post-infective (kickback or eradicant), or a combination of both types of programs can be followed. The protective schedule is the least complicated, but usually requires more applications.
Sprays are applied as soon as susceptible tissue is exposed in the spring and every 7-10 days throughout the season if scab is present on the leaves, or until all of the spores are gone. The interval between sprays is dependent on the rate of growth of the host, weather conditions, and stability of the fungicide. Fungicides such as captan and dodine (dodine can also be used in a post-infective program) are examples of Protective materials.
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