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ARS Home » Pacific West Area » Corvallis, Oregon » Forage Seed and Cereal Research Unit » Research » Publications at this Location » Publication #221834
Title: Field assessment of a model for fungicide effects on within-plant spread of stem rust in perennial ryegrass seed crops

Author
item Pfender, William
item EYNARD, J - OREGON STATE UNIVERSITY

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/23/2009
Publication Date: 6/30/2009
Citation: Pfender, W.F., Eynard, J. 2009. Field assessment of a model for fungicide effects on within-plant spread of stem rust in perennial ryegrass seed crops. Phytopathology. 99:696-703.

Interpretive Summary: Stem rust is the most important disease of perennial ryegrass and tall fescue seed crops in the NW USA, and is closely related to the stem rust disease of wheat. Infection of the seed head is the major component of epidemic development and crop damage. A mathematical model was developed to estimate the effectiveness of fungicides, as a function of application timing, on seed head infection in perennial ryegrass. This model was tested in field experiments using the two most-commonly applied fungicides, which we previously found to differ in important aspects of their efficacy. The model accurately estimated disease severities on the flower head for most treatments and observation times. The model can now be incorporated into decision support tools for fungicide type and timing in management of this disease.

Technical Abstract: Within-plant spread of stem rust (Puccinia graminis subsp. graminicola) in perennial ryegrass during tiller extension is a major determinant of epidemic severity and is dominated by stem extension dynamics. Simple equations for extension of inflorescence and internodes are presented and parameterized. These equations are combined with previously-published equations for pathogen latent period and for post-infection efficacy of fungicides, to produce a model for effects of fungicide material and timing on within-plant spread. The model is driven by thermal units, calculated from air temperature measurements. Model performance was assessed in three field experiments conducted over two years. Either propiconazole or azoxystrobin, the two most commonly used fungicides for stem rust control, was applied to tillers that had stem rust pustules on the flag sheath and in which the inflorescence was partially extended. Within-plant spread of disease to the extending inflorescence (stem and flowerhead) was observed at several dates following treatment, and compared with modeled severities. The model estimated accurately (within 1 standard deviation of observed values) the severities of inflorescence infection for most treatments and observation times. Disease severity in azoxystrobin treatments were overestimated in some cases, i.e. modeled as very low severity whereas observed severities were zero. The model correctly estimated the rank order of final severities among the treatments (fungicide type and timing). The model can be extended to within-plant spread of stem rust at all internodes, and incorporated into decision support tools for fungicide type and timing in management of this disease.