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Sexual Recombinants Make a Significant Contribution to Epidemics Caused by the Wheat Pathogen Phaeosphaeria nodorum

First and second authors: Plant Pathology, Institute of Integrative Biology, ETH Zurich, LFW, Universitaetstrasse 2, CH-8092 Zürich, Switzerland; third author: Swiss Federal Research Station for Agronomy, Changins, CH-1260 Nyon, Switzerland; and fourth author: Industrial Crop Research Institute, Yunnan Academy of Agricultural Sciences, Longtou Street, Kunming, 650205, People's Republic of China.

We conducted a 2-year mark-release-recapture field experiment to quantify the relative contributions of immigration and sexual and asexual reproduction to epidemics of Stagonospora nodorum blotch caused by Phaeosphaeria nodorum. The epidemic was initiated using nine genetically distinct P. nodorum isolates. Infected plants were sampled four times across two growing seasons. In total, 1,286 isolates were recovered and assayed with 10 microsatellite markers and 1 minisatellite marker. The proportion of isolates having multilocus haplotypes (MLHTs) identical to the inoculated isolates decreased steadily from 86% in the first collection to 25% in the fourth collection. The novel isolates that had different MLHTs compared with the marked inoculants originated through immigration and sexual recombination. By the end of the experiment, nearly three-quarters of the novel isolates originated from sexual recombination. Our results indicate that recombinant offspring and airborne immigrant ascospores can make significant contributions to epidemics of Stagonospora nodorum blotch during a growing season.

Additional keywords: Bayesian theory, maximum likelihood estimation, population genetics, primary inoculum.