Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/23/2012
Publication Date: 10/31/2012
Citation: Gordon, T.C., Pfender, W.F. 2012. Effects of the mycoparasite Sphaerellopsis filum on overwintering survival of stem rust in perennial ryegrass. Plant Disease. 96:1471-1481. Interpretive Summary: Stem rust, caused by Puccinia graminis subsp. graminicola (Pgg), is the most damaging disease of perennial ryegrass grown for seed in the Northwest U.S. The pathogen overwinters in the crop as active infections, and severity of the epidemic during the subsequent growing season depends directly on the amount of rust that survives the winter. This research examined the biology of a biological control fungus (S. filum) and tested its ability to reduce overwinter survival of Pgg. We determined that S. filum can infect Pgg pustules at temperatures between 5 and 25 C when leaves are wet for at least 6 hrs. Infected rust pustules produced only half as many rust spores as non-infected pustules. In field experiments, application of S. filum to ryegrass plants resulted populations of the biocontrol fungus that were 10 to 27 times greater than natural levels, and reduced overwintered stem rust disease by about one-half.
Technical Abstract: Sphaerellopsis filum is a mycoparasite of Puccinia graminis subsp. graminicola (Pgg), a rust fungus that causes widespread crop damage on perennial ryegrass grown for seed. In observations taken over the winters of 2000-2003 S. filum was found in 10% of Pgg uredinia on 1st year plantings of perennial ryegrass and 40% of the uredinia on 2nd year plantings of perennial ryegrass. S. filum reduced the lifetime spore production of a Pgg pustule by half, from 39,000 to 18,000 spores (P < 0.001). Mist duration, temperature and Pgg pustule age at the time of S. filum inoculation had significant effects (P < 0.001, P < 0.05, and P < 0.001, respectively) on the proportion of Pgg pustules infected by S. filum. Fifty percent of all Pgg 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 planted into the field had 2.5 times greater proportion of Pgg pustules infected with S. filum (P = 0.045), and a Pgg overwintering population one third the size, compared to plants inoculated with Pgg only (P = 0.013) at two field sites in one year. First-year stands of perennial ryegrass treated with monthly applications of S. filum had 10 to 27 times the proportion of pustules infected with S. filum (P < 0.05), and 0.5 times (P = 0.02) as much Pgg disease, as the non-treated controls. In comparison, plants with one application of fungicide during the winter had Pgg severity 0.02 times that of the Pgg-only control (P < 0.001). There were no effects of S. filum application on rust in 2- or 3-year old perennial ryegrass.