GENETIC UNIFORMITY IN BARLEY AND ITS POWDERY MILDEW PATHOGEN

A recent epidemic of powdery mildew, caused by Erysiphe graminis f.sp. hordei, on barley cultivars carrying the resistance allele Mla13, was largely initiated by two clones. One of these was found throughout the British Isles, and the second in three parts of England. Clones were identified by race-specific virulences, genetic fingerprints and responses to two fungicides, a sterol demethylation inhibitor triazole compound, triadimenol, and the adenosine deaminase inhibitor, ethirimol. Other isolates, with diverse virulences, fungicide responses and fingerprints, were found at low frequencies. Two sets of near-isogenic lines of isolates, differing in virulence or fungicide response phenotypes but not fingerprints, were detected. The epidemic may have originated in continental Europe. The genetic uniformity of agricultural crops imposes very strong selection for virulent pathogen clones. Populations of airborne pathogens therefore build up and spread rapidly, allowing severe epidemics to be caused by one or a few physiologic races. This contrasts with the diversity of many natural plant populations, which may reduce the impact of diseases.