A detached leaf technique for studying race-specific resistance to Cladosporium caryigenum in pecan

A detached leaf screening technique was developed for studying specific interactions between pecan [Carya illinoinensis (Wangenh.) C. Koch] cultivars and isolates of the pecan scab fungus, Cladosporium caryigenum. Monoconidial Isolates were obtained from leaf scab lesions on 'Wichita', 'Desirable', 'Cape Fear', and 'Elliot'. Each isolate was then inoculated onto detached leaves of each of the four cultivars and fungal growth was observed under the microscope after eight days. 'Wichita', 'Desirable', and 'Cape Fear' isolates produced subcuticular hyphae at a much higher frequency when inoculated back onto the cultivar from which they were isolated in comparison to the other cultivars. The 'Elliot' isolate was able to produce a high frequency of subcuticular hyphae when inoculated onto 'Elliot' and 'Cape Fear', but not when inoculated onto 'Desirable' and 'Wichita'. Field inoculations conducted with the 'Wichita' and 'Desirable' isolates validated the detached leaf protocol. The results obtained indicate that pecan scab is composed of multiple races with a high degree of specificity for host cultivars. A rapid whole-leaf staining system is presented which appears to have wide applicability to assessing fungal growth in leaves.