Apple Scab needs organic material to overwinter

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.

Monitor to determine infection time

"The key is to be vigilant," Dr. Peter explains.

1. Look at the veins and the leaf stalk as well as the leaf tissue. Scab can start on any leaf tissue.
2. A good rule of thumb is to scout 20 leaves on 5 limbs, on each of 10 trees.
3. Follow a W pattern in order to randomly select trees to scout in the orchard.
4. Make sure to look at branches closer to the ground, not just at branches that are at eye level. Lower branches are closer to where overwintering spores would land as they shoot out of fruiting bodies in the leaves on the orchard floor.
5. Focus on blocks that have a history of apple scab.
6. Once fruit has set, monitor the fruit as well. You will often tend to see scab on or near the calyx first if it is there.
7. Be vigilant through the end of June.

The beginning to mid-May is an important time for possible infection

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 75^oF you want to be worried about infection, especially if you have at least six hours of leaf wetness.

For more information on apple scab see:

Apple scab infections periods

Apple scab factsheet