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Use of Disease Reactions to Identify Resistance in Wheat to Bacterial Streak. E. A. MILUS, Assistant Professor; Department of Plant Pathology, University of Arkansas, Fayetteville 72701. A. F. MIRLOHI, Former Research Associate, Department of Plant Pathology, University of Arkansas, Fayetteville 72701. Plant Dis. 78:157-161. Accepted for publication 12 September 1993. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0157.

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, caused by Xanthomonas campestris pv. translucens. An inoculum concentration of 1 106 colony-forming units (cfu) per milliliter gave the best differentiation of resistance on flag leaves of adult plants, and concentrations of 1 104 to 1 105 cfu/ml gave the best differentiation 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 pathogen population size in inoculation sites on primary leaves (r = 0.67, P ≤ 0.0001) and flag leaves (r = 0.82, P ≤ 0.0001). For the cultivars tested, disease reactions on primary leaves were similar to disease reactions on flag leaves. Cultivars FFR 525W and Coker 983 had the highest disease reactions; Terral 101 and Twain had the lowest disease reactions; and Florida 302 and Keiser were intermediate. Disease reactions were associated with bacterial streak severities observed in the field. The inoculation technique and disease reaction scales should be useful for evaluating wheat cultivars and breeding lines for resistance, and for assessing pathogen strains for host range, virulence, and aggressiveness.