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Frequency of Sexual Reproduction by Mycosphaerella graminicola on Partially Resistant Wheat Cultivars

November 2002 , Volume 92 , Number  11
Pages  1,175 - 1,181

Christina Cowger , Bruce A. McDonald , and Christopher C. Mundt

First and third authors: Department of Botany and Plant Pathology, 2082 Cordley Hall, Oregon State University, Corvallis 97331-2902; and second author: Institute of Plant Sciences, Phytopathology Group, ETH Zentrum/LFW, Universitätstrasse2, CH-8092 Zürich, Switzerland

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Accepted for publication 19 June 2002.

The frequency of sexual reproduction has a profound effect on the population structure and the adaptive potential of a facultatively sexual parasite. Little is known about the relationship of quantitative host resistance to the frequency of sex in pathogens. We sampled over 5,000 fungal fruiting bodies from eight different wheat cultivars over a 3-year period. The cultivars possessed varying degrees of susceptibility to Mycosphaerella graminicola, a facultatively sexual pathogen that is hetero-thallic and bipolar. The fruiting bodies were classified as M. graminicola pycnidia or ascocarps (asexual and sexual fruiting bodies, respectively), other identifiable fungi, or unidentified. In all 3 years, area under the disease progress curve (AUDPC) explained a significant proportion of the variation in ascocarps as a percentage of M. graminicola fruiting bodies (P < 0.0005). The mean percentage of M. graminicola ascocarps from all cultivars was 63% in 1998, when the epidemic was intense, and 14% in 1999, a year of low disease levels. In 2000, samples were taken at 7-day intervals from 6 June to 27 June from two cultivars with substantially different AUDPCs (788 and 2,185 percentage-days). The less diseased cultivar yielded its first M. graminicola ascocarps 1 week later than the more diseased cultivar, and respective means of ascocarps as a percentage of M. graminicola fruiting bodies across sampling dates were 20.2 and 59.3%. The ratio of sexual to asexual reproduction by M. graminicola is likely to be strongly conditioned by infection density.

Additional keywords: microconidia, micropycnidiospores, Phaeosphaeria nodorum, Red Queen hypothesis, Septoria tritici blotch, Triticum aestivum.

© 2002 The American Phytopathological Society