Most landscapers know how to identify apple scab. If you need some help, consult Report on Plant Disease, no. 803, “Apple and Crabapple Scab,” available in your local Extension office or on the Extension VISTA Web site. Many crabapple cultivars have resistance to scab, and resistance is definitely the long-term answer to this disease. Still, there are many susceptible crabapples in established landscapes. At this time of year, we always get questions about timing for fungicide applications to prevent infection.
The apple scab fungus infects under a wide range of temperatures but requires a wetting period to become established on a tree. Usually, midwestern weather in the spring provides just what the scab fungus needs. The minimum wetting period on the leaves is only about 6 hours if temperatures stay near the optimal 60°F. If temperatures are cooler, the wetting period must be longer for infection to occur. In a normal spring, scab symptoms might start to show on the leaves from 8 to 18 days after infection. Under cool, dry conditions, this incubation period might be longer.
If you have a susceptible variety and you are not able or willing to replace it, then spraying with fungicides might be your course of action against this disease. Fungicides are used as protectants, before infection occurs. You cannot wait until symptoms show to make the first application. Our recommendations say that the first spray should be applied at budbreak to protect new leaves. The ascospores that cause initial infection are released when edible apples are flowering, which is usually after crabapple budbreak. We use budbreak as a guide because it is easier to observe in the landscape and this gives us a grace period if sprays are late. If you had cool, dry weather in your area and you wish to spray your crabapple to protect it from the scab fungus, you may have been granted a reprieve this year due to the weather. You may still see some protection from sprays. Each area of Illinois is a bit different, depending on local weather. A systemic fungicide provides a bit of kick-back action as well and will not be washed off by rains. Refer to the 2003 Illinois Commercial Landscape & Turfgrass Pest Management Handbook or the Home, Yard, & Garden Pest Guide for chemical options. Don’t forget that chemical mobility is listed at the end of each chapter.