Crabapples Resistant to Apple Scab and Japanese Beetle in Indiana

The range of form, flower, and leaf color that crabapples can offer has made them one of the most popular small trees in Indiana landscapes. Two defoliating pests, apple scab and Japanese beetle, have also given this plant a reputation of being prone to insect and disease problems. Both these pests are widely distributed in Indiana. Apple scab is a fungal disease that infects leaves throughout the growing season when temperature and leaf wetness favor spore germination. Infected leaves eventually turn yellow and drop prematurely. The Japanese beetle is a flying insect that consumes leaves when it is in the adult stage, from late June through August. Planting crabapple cultivars with "good" resistance to these pests can greatly reduce the need for pesticides. In this bulletin we place crabapple cultivars into one of 3 classes of resistance/susceptibility to each of these pests: Class I plants are highly resistant; Class II plants are moderately resistant; and Class III plants are highly susceptible. Cultivars with a Class I or II rating for both apple scab and Japanese beetle are likely to require little or no pesticide use to maintain their appearance in the landscape. In contrast, cultivars with a Class III rating for either Japanese beetle or apple scab are likely to require pesticide use to maintain their aesthetic appeal throughout the growing season. These cultivars are listed in table 3. Evaluation Process Our ratings are based on 3 years of field evaluations of 43 cultivars of trees in LaPorte County, Indiana. Most crabapples were planted in 1990 as one-year-old whips. Evaluations used in the study are based on average pest incidence on 10 trees from each variety taken between 1994 and 1996. Years with the greatest pest-associated defoliation were used to produce our lists. For apple scab, these years were 1995 and 1996, and for Japanese beetle they were 1994 and 1995. Japanese beetle ratings were tempered by comparing our findings with those of other researchers.