EVALUATION OF APPLE SCAB RESISTANCE IN SELECTIONS OF MALUS

Abstract

APPLE VARIETIES grown commercially in the United States are notably susceptible to the apple scab disease caused by Venturia inaequalis (Cke.) Wint. Certain varieties, for example, McIntosh and Rome Beauty, are recognized as extremely susceptible while Jonathan and Duchess are generally considered less susceptible. A source of resistance of a higher degree than that found in any of our commercial varieties must be demonstrated and exploited in an intensive improvement program before resistant varieties can serve as a satisfactory method of controlling apple scab. Sources of high level resistance are available. Schmidt (1938, 1939) reported Malus micromalus Mak., M. coronaria (L.) Mill. and certain forms of the Russian variety Antonovka as resistant to scab in Germany. Hough (1944) reported that certain species of Maius and hybrids of one of these with Rome Beauty remained free from attack in the field in Illinois during the 1943 season when the disease was epiphytotic. To explore the possibilities these reports suggested, a cooperative project between the Agricultural Experiment Stations of Illinois and Purdue University was established in 1945. The aims of the project were two-fold: (1) to search for and to evaluate resistance to scab in Malus species and hybrids, and (2) to breed for resistant commercially desirable apple varieties. This paper reports the finding of several scab resistant Malus species and describes the results obtained so far in the evaluation of their resistance. Preliminary accounts have been published by Shay and Hough (1948, 1951). The breeding work with the resistant Malus is under way (Hough and Shay, 1949) and will be the subject of a later report. MATERIALS AND METHODS.-Surveys of the Illinois Agricultural Experiment Station fruit breeding plots and the Malus planting at Morton Arboretum, Lisle, Illinois, were conducted in early and late summer of 1945, 1946 and 1947. Fruit breeding plots of the Minnesota Agricultural Experiment Station and of the Morden Experiment Station in Manitoba, Canada, were examined in August, 1947. Individuals were selected during these surveys that were free from natural scab infections on fruit and foliage. These apparently resistant selections were propagated by budding in August or September of