As predicted, 2002 is a banner year for apple scab on crabapples. Untreated susceptible varieties are showing symptoms in central Illinois. For details on how and why this occurs, see issue no. 2 of this newsletter.
Recently, we have seen several severe cases of scab on crabapple. Besides the usual scattered lesions and infection along the vein, we saw entire leaves affected and some large, blotchy areas--dark brown rather than the usual greenish brown. We ruled out fire blight, anthracnose, and frogeye leaf spot. All samples were infected with scab fungus, with no other pathogens. The term "sheet scab" is used for these large areas of infection. Affected leaves may drop. Cold-temperature injury may have occurred along with scab, resulting in extensive leaf damage.
Remember, symptoms of scab do not show for 8 to 18 days. If a tree is showing scab now, there is probably not much benefit to spraying. Fungicides mainly protect new growth, and many of the healthy-looking leaves have already been infected. Mark your calendar to spray next spring when new growth starts. Meanwhile, rake and remove fallen leaves and concentrate on watering in periods of extended drought this summer.