USE OF DISEASE REACTIONS TO IDENTIFY RESISTANCE IN WHEAT TO BACTERIALSTREAK

An inoculation technique capable of uniformly infiltrating portions of leaves and rating scales for categorizing disease reactions were used to differentiate levels of resistance in wheat to bacterial streak, c aused by Xanthomonas campestris pv. translucens. An inoculum concentra tion of 1 X 10(6) colony-forming units (cfu) per milliliter gave the b est differentiation of resistance on flag leaves of adult plants, and concentrations of 1 X 10(4) to 1 X 10(5) cfu/ml gave the best differen tiation of resistance on primary leaves of seedlings. Seven categories (rating scale 0-6) of disease reaction could be discerned on primary leaves, but only five categories (rating scale 0-4) could be discerned on flag leaves. Disease reactions were positively correlated with pat hogen population size in inoculation sites on primary leaves (r = 0.67 , P less-than-or-equal-to 0.0001) and flag leaves (r = 0.82, P less-th an-or-equal-to 0.0001). For the cultivars tested, disease reactions on primary leaves were similar to disease reactions on flag leaves. Cult ivars FFR 525W and Coker 983 had the highest disease reactions; Terral 101 and Twain had the lowest disease reactions; and Florida 302 and K eiser were intermediate. Disease reactions were associated with bacter ial streak severities observed in the field. The inoculation technique and disease reaction scales should be useful for evaluating wheat cul tivars and breeding lines for resistance, and for assessing pathogen s trains for host range, virulence, and aggressiveness.