Effects of the Mycoparasite Sphaerellopsis filum on Overwintering Survival of Stem Rust in Perennial Ryegrass.

Gordon, T. C., and Pfender, W. F. 2012. Effects of the mycoparasite Sphaerellopsis filum on overwintering survival of stem rust in perennial ryegrass. Plant Dis. 96:1471-1481. Sphaerellopsis filum is a mycoparasite of Puccinia graminis subsp. graminicola, a rust fungus that causes widespread crop damage on perennial ryegrass grown for seed. In observations taken over the winter months, S. filum was found naturally colonizing 2% of P. graminis subsp. graminicola uredinia on first-year plantings and 25% of the uredinia on second-year plantings of perennial ryegrass. In controlled experiments conducted in glasshouses and growth chambers, S. filum applied to rust-inoculated plants reduced the lifetime spore production of P. graminis subsp. graminicola pustules by half, from 39,000 to 18,000 spores/pustule . Mist duration, temperature, and P. graminis subsp. graminicola pustule age at the time of S. filum inoculation had significant effects on the proportion of P. graminis subsp. graminicola pustules infected by S. filum. Fifty percent of all P. graminis subsp. graminicola pustules were infected when S. filum was inoculated onto erumpent pustules and incubated above 5°C for 48 h while exposed to mist. Plants inoculated with both fungi under controlled conditions and then planted into the field had a significantly greater proportion of P. graminis subsp. graminicola pustules infected with S. filum, and a significantly reduced P. graminis subsp. graminicola overwintering population, compared with plants inoculated with P. graminis subsp. graminicola only. First-year stands of perennial ryegrass treated in the field with monthly applications of S. filum had more than 10 times the proportion of pustules infected with S. filum and 50% less P. graminis subsp. graminicola disease compared with the nontreated controls. In comparison, plants with one application of fungicide during the winter had 98% lower P. graminis subsp. graminicola severity than the P. graminis subsp. graminicola-only control There were no effects of S. filum or fungicide application on rust severity in 2- or 3-year-old perennial ryegrass stands. Perennial ryegrass (Lolium perenne L.) is a cool-season seed crop intensively cultivated in the Pacific Northwest of the United States, New Zealand, and parts of Europe (3,37,38). The most damaging disease of perennial ryegrass in the Pacific Northwest is stem rust, caused by the fungus Puccinia graminis subsp. graminicola Z. Urb. If untreated with fungicide, stem rust can cause reductions in seed yields exceeding 90% (24). Weather conditions and host growth during the cropping season (spring and summer) play a major role in determining the severity of the stem rust epidemic. These factors have been combined into a mathematical model that predicts disease severity based on daily weather inputs, host stage, and initial spring rust severity (25,26,28). The model predicts that disease severity during the course of the epidemic is proportional to the initial amount of rust in the spring, if all other variables are held constant. Other studies have found that autumn planting date, cultivar, and stand age all affect spring and summer epidemic severity of other rust diseases in the Pacific Northwest (6,19,33). Late-autumn plantings of perennial ryegrass have significantly less stem rust severity during the epidemic than those planted earlier in the fall, probably due to