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ARS Home » Pacific West Area » Corvallis, Oregon » Forage Seed and Cereal Research Unit » Research » Publications at this Location » Publication #311084
Title: A simulation model for epidemics of stem rust in ryegrass seed crops

Author
item Pfender, William
item UPPER, D - Oregon State University

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/24/2014
Publication Date: 1/1/2015
Citation: Pfender, W.F., Upper, D. 2015. A simulation model for epidemics of stem rust in ryegrass seed crops. Phytopathology. 105:45-56.

Interpretive Summary: A mathematical model was created to calculate epidemic development of stem rust, the most important disease of perennial ryegrass grown to maturity as a seed crop. The model uses weather data to estimate disease development for a particular year and location. Key aspects of plant growth are modeled. The disease increase is calculated to be affected by daily weather variables, plant growth and fungicide application. Fungicide effects on disease cycle components are modeled for two commonly-used active ingredients, applied pre-infection and/or post-infection. The complete model was calibrated with field data from 10 stem rust epidemics. On average, there was no difference between modeled outcomes and the field data, but individual predictions could vary appreciably. An action threshold, by which the model suggests optimum timing for fungicide application, was determined. This action threshold is a level of disease below which the disease cannot cause economic damage to the crop.

Technical Abstract: A simulation model (STEMRUST_G, named for stem rust of grasses) was created for stem rust (caused by Puccinia graminis subsp. graminicola) in perennial ryegrass grown to maturity as a seed crop. The model has a daily time step and is driven by weather data and an initial input of disease severity from field observation. Key aspects of plant growth are modeled. Disease severity is modeled as rust population growth, where individuals are pathogen colonies (pustules) grouped in cohorts defined by date of initiation and plant part infected. Pathogen cohorts progress through life stages that are modeled as disease cycle components (colony establishment, latent period, infectious period, sporulation) affected by daily weather variables, plant growth and fungicide application. Fungicide effects on disease cycle components are modeled for two commonly-used active ingredients, applied pre-infection and/or post-infection. Previously validated submodels for certain disease cycle components formed the framework for integrating additional processes, and the complete model was calibrated with field data from 10 stem rust epidemics. Discrepancies between modeled outcomes and the calibration data (log10 [modeled] - log10 [observed]) had a mean near zero but considerable variance, with 1 standard deviation = 0.5 log10 units (3.2-fold). An action threshold for fungicide application was derived empirically, using a constructed weather input file favorable for disease development. The action threshold is a negative threshold, representing a level of disease (latent plus visible) below which damaging levels of disease are unable to develop before the yield-critical crop stage.