42 Scab is the most common disease of crabapple in Delaware. In years with warm, wet springs, scab can completely defoliate susceptible crabapples.
Wet seasons produce epidemics while dry ones produce very little scab.
The disease is caused by the fungus Venturia inaequalis , which occurs everywhere apples and crabapples are grown. 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.