Answer: It sounds as though your crabapple has developed a fungal disease called apple scab. This disease attacks both apple and crabapple trees. Symptoms first appear in the spring as spots (lesions) on the lower leaf surface, the side first exposed to fungal spores as buds open. At first, the lesions are usually small, velvety, olive green in color, and have unclear margins. On some crabapples, infections may be reddish in color. As they age, the infections become darker and more distinct in outline. Lesions may appear more numerous closer to the mid-vein of the leaf. If heavily infected, the leaf becomes distorted and drops early in the summer. Trees of highly susceptible varieties may be severely defoliated by mid to late summer.
Fruit symptoms are similar to those found on leaves. The margins of the spots, however, are more distinct on the fruit. The lesions darken with age and become black and "scabby." Scabs are unsightly, but are only skin deep. Badly scabbed fruit becomes deformed and may fall before reaching good size.
Once the disease has infected a tree, it's very difficult to control. The first step is to avoid reinfection by raking up fallen leaves and fruit at the end of the season. This will stop fungal spores from splashing back up onto your tree during rainy weather.
Apple scab is more common when spring weather is wet, and if this past year was wetter than usual, your tree may not develop the disease again next year, especially since you've already done some pruning to open the tree canopy to better air circulation.
Some organic gardeners protect their apple and crabapple trees with sulfur sprays early in the growing season. If you're not opposed to using a preventative fungicide, sulfur sprays might be the answer if the problem continues in the years to come.
Hope this information is helpful.
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