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ADDITIONAL INDEX WORDS. Carya illinoinensis, scab, vertical resistance, fungus, microscopic, histology, stain, trypan blue, chlorazole black E ABSTRACT. 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. germplasm because all relevant races are not likely to be present at any one testing location. Typical techniques of screening for resistant germplasm involve field ratings for the presence of scab lesions on the leaves or nuts (Goff et al., 1998; Thompson and Grauke, 1994). A disadvantage of this method is dependence on favorable environmental conditions for pathogen development before ratings can be taken. Replication of these tests is hindered by annual rainfall variations and difficulty in grafting plant materials. Variation in race makeup of the local scab pathogen populations also will likely influence these tests, producing differences in observed cultivar susceptibility between locations. To bypass these inherit difficulties in conducting field tests for resistance, Yates et al. (1996) developed a detached leaf method for assessing cultivar resistance to pecan scab. This method has the advantage of a standardized test environment in which the inocula- tion is being conducted, facilitating reproducibility between tests. Additionally, rare isolates of the pathogen can be tested without their release into the environment. However, this procedure made use of a cumbersome staining procedure and was not tested with multiple isolates of the pathogen. This protocol was modified by adopting a faster staining system and tested with multiple isolates of the pathogen to evaluate its potential for screening germplasm and breeding progenies for race-specific resistance to pecan scab.

A Detached Leaf Technique for Studying Race-specific Resistance to Cladosporium caryigenum in Pecan