We have recently received several samples in the Plant Disease Clinic of apple and pear with severe infections of scab.
Scab is a common fungal disease found on many varieties of apple, pear, and crabapple. On leaves, scab appears as olive-green spots with feathery, indistinct margins. Infected leaves often curl and turn yellow before falling off, and heavily infected trees can be completely defoliated by midsummer. Lesions on fruits initially look similar to those on leaves, but their color may progress to brown or black and they may have a corky or scabby texture. Deep cracks also may develop on fruits.
Two distinct but closely related fungi cause scab, Venturia inaequalis, on apple and crabapple and Venturia pirina on pear. These fungi survive the winter on infected plant debris, such as leaves left on the ground. During rainy periods in the spring, spores are released from this debris and can infect the new leaves and fruits. Scab is favored by extended moist conditions.
Scab is best managed using a multi-faceted approach. Several varieties of apple, pear, and crabapple are resistant to scab, so these varieties should be used when planting new trees. When scab occurs on established, susceptible trees, raking up all diseased leaves in the fall to minimize spring infection can lessen its effects. Trees should be pruned to maximize airflow so leaves dry off as quickly as possible. Fungicide sprays, including those containing maneb, mancozeb, or captan, may also be needed for adequate disease control. Fungicides must be applied when new leaves appear until about mid-June.
This article originally appeared in the 8/27/2004 issue.