Usually the question with this fungal disease is not when it will show, but how severe it will be on sus-ceptible crabapple trees. The disease starts as olive green spots on the leaves. These turn brown to black, elongate along veins, and have a velvety appearance. Eventually the leaves turn yellow and drop, leaving a nearly denuded crabapple tree in June. We have all seen this on Illinois landscapes.
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. One would hope that the drought we have experienced this spring would lessen the severity of scab. Keep in mind that the minimum wetting period on the leaves is only about 6 hours if temperatures stay near the optimum of 68 degrees F. If temperatures are cooler, the wetting period has to be longer. In a normal spring, scab symptoms might start to show on the leaves anywhere from 8 to 18 days after infection. Under cool, dry conditions, this incubation period might be longer.
The best disease management for scab, of course, is to use a resistant crabapple variety, of which there are now many widely available. If you have a variety that is susceptible to scab, and you are not able or willing to replace it now, then spraying with fungicides might be your course of action against this disease. Fungicides are used as protectants, before infection occurs. From what we have been discussing here, it should be obvious that you cannot wait until symptoms show before you make the first fungicide application. The first spray should have been applied at budbreak to protect new leaves. If you have 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. Refer to the 2000 Illinois Commercial Landscape and Turf-grass Pest Management Handbook or the Illinois Homeowners’ Guide to Pest Management for chemical options. For details on the scab disease, refer to Report on Plant Disease No. 803, available in your local Extension office or on the Web at http://www.ag.uiuc.edu/~vista/horticul.htm.