COMPARISON OF RELATIVE VIRULENCE AND DIFFERENTIAL CAPACITY OF 15 STAGONOSPORA NODORUM MONO-PYCNIDIOSPORE ISOLATES USED SINGLY AND IN A MIXTURE TO SCREEN TRITICALE AND WHEAT GENOTYPES FOR RESISTANCE UNDER FIELD CONDITIONS E. ARSENIUK AND P.C. CZEMBOR Department of Plant Pathology, Plant Breeding and Acclimatization Institute, Radzik6w, 05-870 Blonie, Poland Background and objectives Since resistance in cereals to S. nodorum is relative and not absolute (no immunity) screening of cereal breeding materials against the pathogen with one up to a few highly pathogenic isolates might suffice for an efficient selection of genotypes with moderate resistance levels (Nelson & Marshall, 1990). This way, the plant response may not be blurred up and the inoculum producion is also easier. The study was aimed to compare the screening efficiency of triticale and wheat cultivars for stagonospora nodorum blotch (SNB) resistance with single isolates and their mixture. Concurrently, the relative virulence of S. nodorum isolates and the relative resistance of the cereal genotypes used in the study was determined. In addition, isolates were characterised with the use of microsatellite primed polymerase chain reaction (MP-PCR) and their molecular genotypes with virulence phenotypes were correlated. Materials and methods Winter triticale (n=11) and wheat (n=4) and spring triticale (n=5) and wheat (n=3) cultivars varying in resistance to S. nodorum were inoculated individually with 15 isolates of the pathogen and with their mixture. Isolates varied by geographical and host origins. The composite isolate (a volumetric mixture of isolates) was prepared by mixing equal volumes of spore suspensions of all isolates. Plants were inoculated at the late boot stage (5x106 spores/ml) and after heading (2x106 spores/mi). SNB was rated on a 10 digit scale [0 (resistant) - 9 (susceptible)]. Experiments were conducted over three years under field conditions in a split-plot design. Results and conlusions All isolates of S. nodorum were virulent on leaves and on heads of all triticale and bread wheat cultivars. The main effects of cultivars and isolates were highly significant. The effect of isolates was smaller than one of cultivars. The cultivar by isolate interaction effect, though significant, only slightly contributed to the total variance. Spearman correlations of cultivar rankings on their reaction to S. nodorum isolates, and of isolate rankings on their virulence on cultivars were often significant. Thus, it was found that the host specificity among isolates was low. Scoring dates differed significantly. Since interactions of disease scoring dates with cultivars and isolates were significant it was concluded that SNB progressed with a different rate on individual genotypes. The interactions of years with isolates and cultivars, although significant, were low and solely slightly contributed to the total variation. The composite isolate was never the most virulent. Positive relationships between the mean virulence of S. nodorum isolates on leaves and heads and the variances in SNB reaction among cultivars to the respective isolates proved that virulent isolates were more efficient in differentiating of the cereal genotypes on their SNB resistance. The differentiating capacity of the composite isolate was lower as single virulent isolates. Screening of triticale and wheat genotypes with one or just a mixture of a few highly virulent isolates of S. nodorum is reommended for practical breeding purposes. Similarly, a highly susceptible cereal genotype was not necessarily an efficient differential of virulence among isolates. The MP-PCR assay revealed considerable genetic variability among isolates. However, from the same location isolates with two distinct molecular genotypes were obtained what indicates that isolates belonging to more than one haplotype at that location were present.. DNA polymorphisms determined virulence in the most virulent isolates up to 64%. However, a DNA band common for any of the isolate virulence patterns (groups) was not identified. The association for low and intermediately virulent isolates was weak and most often not significant. The interdependence between DNA polymorphisms and virulence of S. nodorum isolates requires further studies. References L.R. Nelson and D. Marshall 1990. Advances in Agronomy 44: 257-277.