Winter survival and rapid PCR-based detection of Magnaporthe oryzae, the gray leaf spot pathogen on perennial ryegrass

Philip Frederick Harmon, Purdue University

Abstract

Gray leaf spot, caused by Magnaporthe oryzae, recently emerged as a serious but sporadic disease on perennial ryegrass in Indiana. Outbreaks occur later in the summer and are less severe in northern Indiana than in the southern part of the state. This research was designed to investigate uncertainties regarding the survival of the pathogen in north central Indiana and the source of primary inoculum for epidemics during the summer months. Overwinter survival of the pathogen was addressed by subjecting infested host residue to various environmental conditions then periodically assessing the potential of the pathogen to sporulate. Pathogen populations were monitored in the field by quantifying the density of airborne conidia of the pathogen with air sampling techniques. Pathogen populations were drastically reduced by winter conditions, providing some insight into the sporadic nature of the disease. Airborne conidia were detected in very low numbers before disease symptoms were observed. Numbers of conidia gradually increased and peaked when disease symptoms were most severe. Symptoms were not observed every year of the experiment but conidia were collected. Disease diagnosis is difficult because symptoms of several other diseases resemble those typically observed during the early stages of a gray leaf spot epidemic. In order to facilitate the differentiation of gray leaf spot from nondescript leaf spot lesions, a PCR-based detection method was developed to quickly and accurately identify this disease. The method utilized a commercially available kit designed to extract plant DNA and was designed to amplify a repetitive element in the M. oryzae genome. The pathogen could be detected from suspect leaf lesions in approximately 4 to 8 hours with this method.

Degree

Ph.D.

Advisors

Latin, Purdue University.

Subject Area

Plant pathology

COinS