In the spring dull, olive-green spots with a velvety texture develop on either side of the leaves. Secondary infections produce spots that are irregular and are found along the veins. Symptoms can vary slightly with different cultivars of crabapple, but scab is usually easily identified in the landscape once the symptoms are recognized. Under favorable conditions for disease development, infected leaves will turn yellow and drop, causing premature defoliation. Crabapple fruit can become infected when spores produced on the leaves land on the fruit.
Infected fruit have small, dark, raised scabby areas on the fruit surface.
If spots are numerous, the fruit becomes deformed, cracked and falls early. The scab fungus survives the winter in fallen, infected leaves and fruit. During wet spring weather, about the time of vegetative bud-break, spores are produced in reproductive structures in the fallen leaves and fruit. These spores are forcibly discharged and young, wet leaves become infected. The leaf spots develop from 8 to18 days later, depending on the temperature.