Where Are They Now? High humid condition at turf canopy

from Pennsylvania Turfgrass - Winter 2019

Where Are They Now?

A major mediating factor for infection, colonization, and conidiation process in gray leaf spot pathosystem

Gray leaf spot, caused by Magnaporthe oryzae Couch, is a serious disease of perennial ryegrass turf, causing extensive damage to ryegrass fairways in various regions of the United States. Environmental conditions are important determinants in gray leaf spot epidemic development in perennial ryegrass fairways. The disease usually first develops in higher cut turf, particularly the rough where prolonged leaf wetness often remains for an extended period of time. Three environmental components, temperature, leaf wetness duration, and relative humidity (RH) govern gray leaf spot development in a uniquely interactive fashion. The effects of temperature and free leaf moisture have been previously documented; however, the effects of RH, particularly the infection process have not been well understood.

In our study on the effects of relative humidity (88, 92, 96, and 100% RH), the RH threshold for successful infection by the pathogen was at 92% or higher under moderately warm condition (82°F). The advancement of infection on the leaf tissue was further examined with a green fluorescent protein (GFP)-tagged pathogen strain. No appressorium formation was found when the inoculum was incubated at 88% RH or below. The GFP-tagged straining provided a rapid procedure to quantitatively compare the fungal colonization from colonized leaf tissue at different relative humidity. The fluorescence intensity data showed that the level of fungal pathogen biomass was greatest at the highest level of humidity (RH 100%) and there was no fluorescence intensity observed at 88% RH or below. The conidiation (production of fungal spores) occurred at 96% RH with the most abundant spore production occurring 8 days after inoculation. Reduced spore production was associated with decreased RH, and no spore production occurred at RH 92% or lower. Our study indicates that infection and production of spores by the gray leaf spot pathogen on perennial ryegrass host required different thresholds of RH which were 92 and 96%, respectively. This study reveals the importance of the presence of highly humid conditions for disease epidemic development and secondary infection process in gray leaf spot pathosystem.

Research Summary submitted by Dr. Wakar Uddin, Professor, and Dr. Yinfei Li, former Graduate Research Assistant, Department of Plant Pathology and Environ. Microbiology, Penn State University

Where Are They Now? High humid condition at turf canopy

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