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Apple scab (Venturia inaequalis) overwinters on infected leaves which have fallen to the ground. "You won't believe how much can come from your orchard floor. Sanitation will go a long way," Dr. Peter explains. Spraying urea on the orchard floor increases the numbers and activity of microbes that break down leaves. Chopping starts the breakdown process and makes the leaves more accessible for the microbes. Chopping and spraying urea is an important practice. It is effective and this is an area which is within your (the grower's) control.
"The key is to be vigilant," Dr. Peter explains.
If apple scab spores remain wet for a few hours after landing on wet apple buds, leaves, or fruit, they germinate and grow into the apple tissue. The time required for germination and penetration depends on the temperature and the presence of a wet surface. When average temperatures are between 61 and 75oF you want to be worried about infection, especially if you have at least six hours of leaf wetness.
Apple scab infections periods
Apple scab factsheet
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