Barley powdery mildew populations on volunteers and changes in pathotype frequencies during summer on artificially inoculated field plots

Pathotype frequencies in barley powdery mildew populations were assessed in artificially inoculated barley plots. The field experiment was organised in eighteen 3 m x 3 m plots with different inoculum compositions, obtained by sequential inoculation with three isolates, gl-1, gl-2 and gl-3, shortly after seedling emergence. In the conidia populations before summer, pathotypes corresponding to the inoculated isolates were detected at frequencies in the range 11-42% for GL1, 0-14% for GL2 and 2-34% for GL3. On the volunteers appearing after harvest these three pathotypes were observed at lower frequencies: 0-37% for GL1, 0-12% for GL2 and 0-23% for GL3. The overall ranking of GL1, GL2 and GL3 frequencies was thus the same before and after summer. The populations on volunteers were influenced by both sexual and asexual populations present in the same field at the end of the previous growing season. However, at the small-scale level no simple correlation was found between the frequencies in the conidia populations on volunteers and those in the airborne population, or the conidia populations on the crop before summer, or the calculated expected frequencies in populations of ascospores. During the summer survival, chance events may also have a large influence on pathotype frequencies leading to a high variation between repeated events of transition from the crop to the volunteers.