Effects of relative humidity on infection, colonization and conidiation of Magnaporthe orzyae on perennial ryegrass

Grey leaf spot, caused by Magnaporthe oryzae, causes severe damage on perennial ryegrass (Lolium perenne) turf. In this study, the effects of relative humidity (RH, 88 to 100% at 28°C) on infection, colonization and conidiation of M. oryzae on perennial ryegrass were investigated in controlled humidity chambers. Results showed that the RH threshold for successful M. oryzae infection was ≥92% at 28°C. The advancement of infection on the leaf tissue was further examined with a green fluorescent protein (GFP)-tagged M. oryzae strain. No appressorium formation was found when the inoculum was incubated at RH ≤ 88%. Additionally, the GFP-tagged staining provided a rapid method to quantitatively compare the fungal colonization from leaf tissue at different levels of RH. The fluorescence intensity data indicated that the fungal biomass was highest at 100% RH and there was no fluorescence intensity observed at 88% RH or below. Conidiation was only observed when RH was ≥96%, with the most abundant conidiation occurring 8 days after inoculation. Reduced conidiation was associated with decreasing RH, and no conidiation occurred at RH ≤ 92%. This study indicates that infection and conidiation of M. oryzae on perennial ryegrass required different thresholds: 92% and 96% RH for infection and conidiation, respectively. The quantitative data from this research will assist in prediction of grey leaf spot disease outbreaks and of secondary infection of perennial ryegrass.