An assessment of the durability and susceptibility of scab resistance in apple cultivars

Durable resistance to scab (Venturia inaequalis) of newly introduced cultivars is needed to achieve a considerable reduction of pesticide use. However, the assessment in the field on the level and durability of scab resistance is time and space consuming. A proper and fast indoor test should be a convenient tool to discard cultivars with a limited level of resistance at an early stage. As a result, field tests can include only the better cultivars. This approach has been tried out by comparing field data with data from a glasshouse test, comprising twelve new and seven well-known susceptible and resistant (with and without Vf) cultivars which have been tested with six isolates of scab from different origins, virulence and aggressiveness, including race 6 and 7. Young trees on M.9 rootstock were infected and after a 17 day period at high humidity, sporulation and chlorosis have been observed. If the set of isolates is broad and well-chosen it seems possible to assess susceptibility and possibly also durability. To optimize the choice of isolates, it is desirable to know the pedigree of the cultivars that are tested. In addition, this procedure allows the use of highly virulent and aggressive races, that should be avoided in field tests for the risk of spreadin