Factors Influencing The Development Of Gray Leaf Spot Of Perennial Ryegrass Turf And Seasonal Availability Of The Inoculum

Open Access
Li, Yinfei
Graduate Program:
Plant Pathology
Doctor of Philosophy
Document Type:
Date of Defense:
November 26, 2012
Committee Members:
  • Wakar Uddin, Dissertation Advisor
  • John Edward Kaminski Iii, Dissertation Advisor
  • Beth Krueger Gugino, Committee Member
  • Scott Alan Isard, Committee Member
  • James Landis Rosenberger, Committee Member
  • Turfgrass disease management
  • Magnaporthe oryzae
  • Gray leaf spot
  • Perennial ryegrass
  • Epidemiology
  • Blast disease
Gray leaf spot, caused by Magnaporthe oryzae Couch, is a devastating disease of perennial ryegrass turf. Severe disease epidemics have been reported in various regions of the United States, and it has been most prevalent in the northeastern region. Airborne conidia of M. oryzae serve as the major inoculum for the development of disease epidemic that can cause extensive damage on perennial ryegrass turf under favorable environmental conditions. However, the effects of environment on infection, fungal colonization, disease development and conidiation are not fully understood. Therefore, understanding the role of environmental factors on inoculum and disease development of gray leaf spot of perennial ryegrass turf is warranted. The objective of the first study was to determine the effects of intermittent and interrupted leaf wetness periods on incidence and severity of gray leaf spot of perennial ryegrass turf. Results indicated significant effects of wet and dry cycles on disease incidence and severity. Under optimum temperature (28°C), the highest disease incidence and severity were observed on plants exposed to 18h continuous leaf wetness. The frequencies and durations of dry periods significantly reduced gray leaf spot incidence and severity. Further, strong correlations were found between frequency and duration of dry periods and disease incidence or severity reduction rates. Interruption of leaf wetness durations with longer dry periods significantly reduced gray leaf spot incidence and severity. Additionally, there were negative correlations between the length of interrupted dry periods and gray leaf spot incidence or severity. The relationship was best described by a quadratic model for disease incidence and a linear model for disease severity. Results of this study indicate that turfgrass canopy moisture management strategies may be employed as an important cultural practice as part of a gray leaf spot integrated disease management program in perennial ryegrass turf. In the second study, the effects of relative humidity (RH, 85% to 100%) on infection, colonization, and conidiation by M. oryzae were investigated at 28ºC in controlled environment chambers. Results of the study showed that the RH threshold for successful M. oryzae infection was 91% at 28°C. The fungal biomass from colonized leaf tissue increased with increasing levels of RH. Green fluorescent protein-tagged M. oryzae strains provided a rapid and accurate method for visual and quantitative determination of the fungal colonization. Further, daily conidiation rates were quantified at 88% to 100% RH. The most abundant conidiation of M. oryzae was found at eight days after inoculation at 100% RH. Reduced conidiation was associated with decreased RH, and no conidiation occurred at RH of 91% or lower. This study showed that relative humidity at >90% is required for infection of perennial ryegrass plants by M. oryzae and the subsequent disease development under the favorable temperature regime in the gray leaf spot pathosystem. The results from this research can be integrated to improve the current disease forecasting model. The objectives of the third study were to examine the availability and the dispersal pattern of conidia in the field, and to determine the relationship between certain environmental parameters and the concentration of airborne inoculum. The field monitoring of airborne conidia was conducted in Dillsburg and Leesport, Pennsylvania in 2010 and 2011. Analyses of the air samples indicated that airborne M. oryzae conidia were present at all sampling sites. Average hourly conidia counts indicated that the peak concentration was generally observed during the early part of the day (0500 to 0900 hour). Concurrent occurrence of both high concentration of airborne conidia (>1000 conidia/m3) and favorable environmental condition (percent infection index>90) was always confirmed before the reported disease epidemics which is the important determinant for predicting disease epidemics. Correlation analyses between environmental parameters and conidia concentration indicated that the temperature may be used to forecast initial inoculum availability and humidity levels may be a critical indicator for the hourly conidia concentration. Results of this study will be employed as a component of the current gray leaf spot forecast system based on inoculum level.