If you have a crabapple variety susceptible to scab and you live in Illinois, you most likely see this disease every year. You can count on a flowery show and a nice flush of foliage, followed by spotting, blighting, and yellowing of leaves and 80% defoliation by mid-June. I still have not determined why these trees don’t die from scab! Instead, they may repeat this early defoliation for decades and exhibit a slow decline. Details about scab can be found in the plant disease fact sheet, “Apple and Crabapple Scab,” RPD no. 803, available in local Extension offices or on the Web at http://www.ag.uiuc.edu/~vista/horticul.htm.
Most of you know the management options. Numerous crabapple varieties have resistance to scab. You can even pick ones with tolerance to scab, cedar apple rust, and fire blight and still have some choices in flower color. An excellent source of such information is a paper, Recommended Crabapples for Illinois Landscapes, by horticulture specialists David Wil-liams and Gary Kling. It can be downloaded from http://www.extension.uiuc.edu/IPLANT/. Also, an international ornamental crabapple society has a Web site that might be helpful: www.malus.net.
Many crabapple growers do not want to replace their trees but do want to control the disease. Pruning out dead wood and opening the center of the tree to allow better air flow in the tree helps some. Watering in drought, fertilizing in the fall, and raking fallen leaves are also helpful in reducing disease severity, but the disease still occurs.
Fungicide applications are available and control the disease for one season. If you go this route, apply when leaves begin to emerge from the buds and continue with sprays at labeled intervals (varies with product) until frequent and prolonged wetting periods are uncommon. Leaves are emerging from the buds (April 6) in Champaign. If you have not applied the first fungicide application, do so as soon as possible.
Most questions concerning scab involve what chemicals to use and why sprays have not been effective. Sprays protect the foliage from infection. If you are late for the first application, spores may have already germinated and penetrated the young leaf tissue. Using a systemic product for the first application may help with some “kickback” activity. You need to consider chemical mode of action—whether the chemical is classified as contact, protective contact, or systemic. Spray intervals vary with the mode of action, so read the labels of chemical options. Rains shorten those intervals, especially if you are using a protective–contact type of product.
Chemical options for homeowners are listed in the Illinois Homeowners’ Guide to Pest Management (1997). The new version, renamed the Home, Yard, and Garden Pest Guide, should be hot off the press by the end of the month. Chemical options for commercial applicators can be found in the 2001 Commercial Landscape and Turfgrass Pest Management Handbook. Both publications are available in Extension offices for a fee, or call (800)345-6087 to order.
Another concern of applicators is the development of resistance to chemicals. When the same chemical is used repeatedly, year after year, the target fungus may develop resistance to the chemical, making sprays useless. For this reason, commercial growers usually rotate fungicides from different chemical families. There are nine systemic products listed in the commercial handbook to control scab. To avoid development of fungal resistance, growers can alternate between products containing propiconazole (demethy-lation inhibitor), thiophanate-methyl (miotic poisons), and oxystrobin (mitochondria disrupters). The only systemic fungicide marketed for homeowner scab control is thiophanate-methyl.