Scab is a fungal disease caused by Venturia inaequalis. Undoubtedly, most of you have seen or heard of this disease. It is very common on crabapples and apples, but it also infects pear, mountainash, and pyracantha. Look for olive green leaf spots in the spring, followed by leaf yellowing and extensive defoliation of susceptible varieties by late June.
A wetting period is necessary for infection to occur. The length of that period depends on the temperature, with the minimum period on leaves being only about 6 hours if temperatures stay near the optimum of 68oF. Prolonged wetness allows for a more severe infection. If rains accompany the warm trend of late, infection is inevitable on susceptible varieties. Look for symptoms to show 8 to 18 days after infection.
Fungicides may be used to protect susceptible varieties (before infection occurs). Chemical applications should be repeated according to label directions but continued until 2 weeks after petal fall. Such applications protect the plant for only one season. The first application should have been applied by now.
If you choose to use a fungicide, consider selecting a systemic product to avoid loss by rains and to reduce the number of applications necessary. The Commercial Landscape and Turfgrass Pest Management Handbook and the Home, Yard, and Garden Pest Guide both list chemical options. In addition, at the end of each disease chapter, you can find a table listing all fungicides recommended in the chapter, their active ingredient, and mobility (protective contact or systemic).
To avoid fungicide resistance, do not use the same systemic product repeatedly. Determine the mode of action of the systemic products and rotate between different types. As an example, homeowners could use three systemic active ingredients to protect against scab: thiophanate-methyl, propiconazole, and myclo-butanil. Because the latter two are both demethylation inhibitors, rotating them would not help avoid fungicide resistance. Thiophanate-methyl is a miotic poison and could be effectively rotated with the other two.
If you are planting new crabapples this year, look for varieties with resistance to scab, rust, fire blight, and powdery mildew. One publication that may help is by U of I professors Dave Williams and Gary Kling: Recommended Crabapples for Illinois Landscapes. Look for it on the Web at http://www.extension.uiuc.edu/IPLANT/plant_select/trees/Selecting_Crabapples.pdf. For details on the scab disease, refer to Report on Plant Disease, no. 803, available in your Extension office or on the Web at http://www.ag.uiuc.edu/~vista/horticul.htm.