Author
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Pfender, William |
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Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 1/9/2009 Publication Date: 5/31/2009 Citation: Pfender, W.F. 2009. A Damage Function for Stem Rust of Perennial Ryegrass Seed Crops. Phytopathology. 99:498-505. Interpretive Summary: Stem rust is the most important disease of perennial ryegrass and tall fescue seed crops in the NW USA, and millions of dollars are spent by growers to manage it each year. To make optimum economic decisions regarding stem rust management, it is necessary to know the relationship between severity of the disease and amount of yield lost. In field experiments conducted over 9 years, we measured disease development and seed yield in hundreds of field plots. Our analysis of the data shows that the amount of rust disease from about one week after flowering to one week before harvest is the best predictor of yields that will be lost to the disease. The equation relating this disease quantity to yield loss shows that seed yield will be reduced by 22% if the disease covers 5% of the plant area during this critical period. Reduction is 42% for 10% disease. These results will be used in combination with the stem rust decision aid to help growers manage crop disease to optimize economic return. Technical Abstract: Observations of naturally occurring stem rust epidemics and seed yields in perennial ryegrass were taken in 19 field experiments conducted over the course of 9 yr. Yield loss (difference between attainable and actual yield) in the non-protected treatments ranged from 0% to 98% due to yearly differences in epidemic conditions. Disease severity in the upper canopy was estimated at approximately weekly intervals, and converted to proportion of the plant area diseased. The complementary value, proportion of area healthy, and its integral over time, healthy area duration (HAD), were calculated. Regression analyses were conducted using various phenological time intervals of HAD as the independent variable. The best intervals of HAD for predicting relative yield were centered on the midpoint time between anthesis and harvest. The regression equation (r2 = 0.89) for relative yield as a function of HAD during the 3 wk interval was selected and rearranged to produce the damage function: Unrealized yield = - 4.621 x2 + 4.668x, where x = 1 - HAD. This damage function estimates yield loss at 5%, 22% and 42% for critical-interval diseased proportions of 1%, 5% and 10%, respectively. Yield data collected from field experiments not used in model development correlated well (r2 = 0.9) with yields predicted by the damage function from their observed disease severity. |
