Apple scab is a fungus that causes spotting and scabbing on the fruit and leaves. This fungus can also be a problem on ornamental crabapple trees. A similar, yet different subspecies of fungus can affect pears. One fungus will not affect the other type of tree.
Scab overwinters in the infected leaves on the ground . Rainfall or sprinklers will release the spores, where wind or water will carry them to infect the; leaves, flowers or fruit. Pear scab can also overwinter in lesions on pear twigs.
The first symptoms appear as yellow spots on the leaves (above), these spots produce spores. Then 8 to 17 days later, dark, olive-colored spots form on the leaves and fruit (below).
Spots on the underside of the leaf often look velvety due to fungus growth . On pear these can also occur on twig lesions . Severely affected leaves turn yellow and drop, if flower stems are affected the flower will drop, fruit may become distorted and fall from
the tree. Occasionally the fruit will crack allowing other problems.
Cultural control should always be the first step, this involves removing all of the dropped leaves in the fall. Either compost them or destroy them by burning or removing off site. If you have scab often, or live in a region prone to cool moist
conditions, apply FL Fruit, Citrus and Pecan Tree Food. This fertilizer has extra Zinc which is used to produce fruit and helps resist scab. This application should be made in the fall when leaves begin to turn yellow, but before they fall.
Fungicide sprays are only necessary if the weather is rainy and the leaves remain wet for 9 hours or more. Applications during full dormancy have no effect. Make your first application as soon as you see the tips of the leaves emerge, then a second application 10 to 14 days later. If it remains rainy spray a third time when most of the flower petals have dropped.
The first choice of chemical is FL F-Stop the second best would be NG Copper Soap. Avoid Copper after full bloom. Some older products would be HY Captan 50W Fungicide or HY Wettable Dusting Sulfur.
With Sulfur applications, do not apply within 3 weeks of an oil application or when temperatures are over 90°.