In vitro influence of phosphite on chlamydospore production and viability of Phytophthora cinnamomi

Chlamydospores of four isolates of Phytophthora cinnamomi were produced on mycelium grown on medium with or without 100 μg phosphite/ml, then tested for germination on medium containing no phosphite or 100 or 160 μg phosphite/ml. For each isolate when spores were produced on medium without phosphite, there was no effect of phosphite on germination. However, chlamydospores produced in the presence of 100 μg phosphite/ml showed significantly lower germination on media with no phosphite, 100 or 160 μg phosphite/ml than those produced in the absence of phosphite. For isolates known to be phosphite sensitive, the level of viable non‐germinated chlamydospores was higher amongst those produced on medium containing phosphite than amongst the controls, suggesting phosphite induces chlamydospore dormancy in some isolates of P. cinnamomi. Chlamydospores were produced uniformly across the radius of colonies on solid or liquid control media, but on media containing 100 μg phosphite/ml, chlamydospore production was lower in the centre of the colony and peaked at a point where mycelial morphology changed from tightly packed to sparse. This change in mycelial morphology and the peak in chlamydospore numbers did not occur when the pathogen was grown on liquid medium renewed every few days. In renewed medium, chlamydospores were evenly distributed across the radius of the colony.