Effects of relative humidity on infection, colonization and conidiation ofMagnaporthe orzyaeon perennial ryegrass

Abstract

Grey leaf spot, caused byMagnaporthe 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 ofM. oryzaeon perennial ryegrass were investigated in controlled humidity chambers. Results showed that theRHthreshold for successfulM. oryzaeinfection was ≥92% at 28°C. The advancement of infection on the leaf tissue was further examined with a green fluorescent protein (GFP)‐taggedM. oryzaestrain. No appressorium formation was found when the inoculum was incubated atRH ≤ 88%. Additionally, theGFP‐tagged staining provided a rapid method to quantitatively compare the fungal colonization from leaf tissue at different levels ofRH. The fluorescence intensity data indicated that the fungal biomass was highest at 100%RHand there was no fluorescence intensity observed at 88%RHor below. Conidiation was only observed whenRHwas ≥96%, with the most abundant conidiation occurring 8 days after inoculation. Reduced conidiation was associated with decreasingRH, and no conidiation occurred atRH ≤ 92%. This study indicates that infection and conidiation ofM. oryzaeon perennial ryegrass required different thresholds: 92% and 96%RHfor 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.