Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/19/2006
Publication Date: 9/30/2006
Citation: Pfender, W.F. 2006. Interaction of fungicide physical modes of action and plant phenology in control of stem rust of perennial ryegrass grown for seed. Plant Disease.90:1225-1232.
Interpretive Summary: Stem rust is the most economically damaging disease or pest problem for farmers producing seed of perennial ryegrass or tall fescue. The disease can be managed with the use of fungicides, but the number of fungicide sprays needed to maintain yield varies from year to year. Better information is needed about the effectiveness of fungicides against stem rust, particularly the persistence of the fungicide in the plant after it is applied, and the fungicide's capability to inhibit infections that had started before application. The two fungicides most commonly used in the grass seed crop were found to have protective and curative activity, but to differ in their duration of activity. One of the fungicides was found also to reduce fungus spore production and viability. A major finding of this research is that the fungicides differ in their ability to interfere with a special type of disease spread that occurs within each grass plant when it is infected with stem rust. The fungicide that reduces spore viability is capable of inhibiting this within-plant disease spread regardless of the plant growth stage during which it is applied. In contrast, the other fungicide has a much narrower time window during plant growth when it is effective at inhibiting the within-plant disease spread. Results of these experiments will allow effects of fungicide application to be incorporated into predictive disease management models, reducing cost and environmental impact from unneeded sprays.
Technical Abstract: Azoxystrobin had protective and curative effects against stem rust (caused by Puccinia graminis subsp. graminicola) of inoculated perennial ryegrass under field conditions. Either fungicide was very effective when applied near the time of infection, and was less effective as time between infection and spray (whether curative or protective) increased. The effectiveness of each fungicide was well described by a second- or third-order polynomial using time (days or stem rust latent periods) as the independent variable, with propiconazole having a more rapid decline in effectiveness than azoxystrobin with time. When symptomatic plants were sprayed with a fungicide, urediniospore production per pustule was reduced by 50% in propiconazole-treated plants and by 85% in azoxystrobin-treated plants. Azoxystrobin significantly reduced viability of urediniospores from sprayed pustules, unlike propiconazole. These differences between the two fungicides in physical modes of action resulted in a marked difference in their effects on secondary, within-plant spread of the disease. Azoxystrobin inhibited sporulation from the inner face of pustules on the flag-leaf sheath, thereby interrupting inoculum production that otherwise would lead to numerous contiguous secondary infections along the length of the emerging inflorescence. Propiconazole had little effect on such infections, so that tillers treated with this fungicide early in the process of within-plant spread had significantly greater final stem rust severity than those treated with azoxystrobin at this time.