Evaluation of the Ryegrass Stem Rust Model STEMRUST_G and Its Implementation as a Decision Aid.

STEMRUST_G, a simulation model for epidemics of stem rust in perennial ryegrass grown to maturity as a seed crop, was validated for use as a heuristic tool and as a decision aid for disease management with fungicides. Multistage validation had been used in model creation by incorporating previously validated submodels for infection, latent period duration, sporulation, fungicide effects, and plant growth. Validation of the complete model was by comparison of model output with observed disease severities in 35 epidemics at nine location-years in the Pacific Northwest of the United States. We judge the model acceptable for its purposes, based on several tests. Graphs of modeled disease progress were generally congruent with plotted disease severity observations. There was negligible average bias in the 570 modeled-versus-observed comparisons across all data, although there was large variance in size of the deviances. Modeled severities were accurate in >80% of the comparisons, where accuracy is defined as the modeled value being within twice the 95% confidence interval of the observed value, within ±1 day of the observation date. An interactive website was created to produce disease estimates by running STEMRUST_G with user-supplied disease scouting information and automated daily weather data inputs from field sites. The model and decision aid supplement disease managers' information by estimating the level of latent (invisible) and expressed disease since the last scouting observation, given season-long weather conditions up to the present, and it estimates effects of fungicides on epidemic development. In additional large-plot experiments conducted in grower fields, the decision aid produced disease management outcomes (management cost and seed yield) as good as or better than the growers' standard practice. In future, STEMRUST_G could be modified to create similar models and decision aids for stem rust of wheat and barley, after additional experiments to determine appropriate parameters for the disease in these small-grain hosts.

First Report of Choke, Caused by Epichloe typhina, on Orchardgrass in Oregon.

During July 1997, Epichloe typhina (Pers.:Fr.) Tul. in Tul. & C. Tul., the cause of choke disease, was found in four fields of an unnamed, experimental cultivar of orchardgrass (Dactylis glomerata L.) grown for seed near Halsey, OR. Disease occurrence in each of three fields was estimated by counting choked tillers in about 50 quadrats, 1 × 0.3 m, taken at 30-m intervals along three or four diagonal transects. In two fields, the disease was present in most quadrats (3% tillers infected). In the third field, choke was clustered in two areas, each with 1 to 8% infected tillers. A collection of E. typhina was deposited at the Oregon State University Mycological Herbarium (accession number 56,395). The disease had not been previously observed in commercial cultivars grown for seed in Oregon, with the exception of an infected tiller collected from an orchard-grass seed field during 1996. This is the first report of choke in Oregon on orchardgrass. Choke is an important disease in France, where it reduces seed yields of orchardgrass. Ten Oregon cultivars of orchardgrass were evaluated under field conditions in France in 1993 and 1994 for susceptibility to E. typhina. All cultivars were found susceptible to the disease; incidence of infected tillers ranged from 4 to 11%, with a mean of 7% (G. Sicard and R. E. Welty, unpublished). During 1996, several fragments of stroma of E. typhina were found among seed from a seed lot submitted to the Oregon State University Seed Lab for purity testing. This indicates that stroma may occur as a contaminant with seed, although it is not known if E. typhina would survive with the seed. E. typhina has not been reported to be seed-borne in orchardgrass.