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Genetic Variation for Virulence and Resistance in the Wheat-Mycosphaerella graminicola Pathosystem I. Interactions Between Pathogen Isolates and Host Cultivars. Gert H. J. Kema, DLO-Research Institute for Plant Protection (IPO-DLO), P.O. Box 9060, 6700 GW Wageningen, Netherlands; Juan G. Annone(2), Rachid Sayoud(3), Cor H. Van Silfhout(4), Maarten Van Ginkel(5), and Joop de Bree(6). (2)EEA Pergamino-National Institute for Agricultural Technology (INTA), CC 31, 2700 Pergamino, BA, Argentina; (3)Institut Technique des Grandes Cultures (ITGC), Station Experimentale Agricole, B.P. 126, Guelma, Algeria; (4)DLO-Research Institute for Plant Protection (IPO-DLO), P.O. Box 9060, 6700 GW Wageningen, Netherlands; (5)International Maize and Wheat Improvement Center (CIMMYT), Lisboa 27, Apdo Postal 6-641, 06600 Mexico, D.F., Mexico; and (6)DLO-Agricultural Mathematics Group (GLW-DLO), P.O. Box 100, 6700 AC Wageningen, Netherlands. Phytopathology 86:200-212. Accepted for publication 19 October 1995. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/Phyto-86-200.

Genetic variation for virulence in 63 Mycosphaerella graminicola isolates, originating from 13 countries, was studied in two seedling experiments. Each experiment was performed according to a partially balanced incomplete block design with four replications over time. The first experiment put emphasis on M. graminicola isolates that originated from bread wheat, and comprised 50 isolates that were inoculated on a set of testers containing 19 bread wheat cultivars, four durum wheat cultivars, and one triticale cultivar. In the second experiment more attention was paid to M. graminicola isolates that originated from durum wheat, and comprised 15 isolates that were inoculated on a set of testers containing 17 durum wheat cultivars, four bread wheat cultivars, one triticale cultivar, and a Triticum turgidum subsp. dicoccoides accession. Two disease parameters, the presence of necrosis (N) and pycnidia (P) estimated as percentages of primary leaves, were employed to measure disease severity. Genetic variation for virulence in the pathogen isolates and genetic variation for resistance in the host cultivars were estimated by analyses of covariance. The significance of cultivar isolate interactions in both experiments and for each disease parameter suggested a gene-for-gene interaction between resistance and virulence loci in host and pathogen, respectively. An agglomerative hierarchical clustering procedure, that used one df component of interaction between isolates and cultivars as a proximity measure, was employed to study the similarity between isolates and cultivars. Discrepancies between N and P resulted in nonidentical clusters of isolates and cultivars when considering these parameters separately, which suggested that N and P were under different genetical control. Evidently, isolates of M. graminicola were specialized to either bread wheat or durum wheat. This was particularly evident when considering P. It is proposed, therefore, to designate two varieties in M. graminicola that refer to the host species specialization in this pathogen.

Additional keywords: pathogenic variation, pathotypes, races, Septoria tritici, Triticum aestivum, Triticum turgidum subsp. durum.