Scab—Fusicladium, Spilocaea, and Venturia spp.
Various fungi cause leaf and fruit spotting and drop. Hosts of scab disease fungi include apple, loquat, olive, pear, photinia, pyracantha, Rhamnus, toyon, and willow. Scab generally is important only on fruit trees grown for harvest, especially apple and pear.
There are many causes of leaf spots.
Scab first appears as pale or yellow spots on leaves. As disease progresses, dark, olive-colored spots form on leaves, fruit, and sometimes stems. On the undersurface of leaves, spots may have velvety fungal growth. Affected leaves may twist or pucker, turn yellow, and drop prematurely. Often only a few scattered leaves are affected, but severe disease can affect most of a plant's leaves and fruit.
When scab affects flower stems, flowers may drop prematurely. On fruit, spots begin as velvety or sooty gray-black, or greasy lesions that sometimes have a red halo. Lesions become sunken and tan and may have olive-colored spores around their margins. Severely infected fruit distort or crack and drop.
Scab fungi overwinter on fallen leaves and on evergreen hosts on leaves on the plant. Spores on infected tissue are discharged into the air or splashed by water, usually in the spring. Spores landing on fruit, leaves, and stems germinate and infect the host if they remain wet for more than about 9 hours.
The disease is worst when rainfall is abundant in the spring. Mild temperatures when plants are continuously wet promotes scab disease. Hot, dry weather slows or stops disease development.
Solutions
When planting apple, crabapple, or pear, choose one of the many scab-resistant species or varieties. For example, Asian pears, Pyrus pyrifolia, are less susceptible to scab than European pears, P. communis.
Remove and dispose of or thoroughly compost fallen leaves. Avoid sprinkling foliage, but if water does hit trees, irrigate early in the day so that foliage dries more quickly. Prune branches to thin canopies, which improves air circulation and reduces humidity.
Various copper, oil, sulfur, and synthetic fungicides may prevent the disease if properly applied from about bud break through a month after petal fall. Spraying is generally not warranted except where the disease is severe on apple or pear fruit. See Pest Notes: Apple and Pear Scab for more information.
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Scab
lesions on toyon leaves
Scabby spots on apples
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