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item Cowger, Christina

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/5/2005
Publication Date: 9/1/2005
Citation: Cowger, C., Wallace, L., Mundt, C. 2005. Velocity of spread of wheat stripe rust epidemics. Phytopathology. 95:972-982.

Interpretive Summary: A three-year experiment was conducted at two locations in Oregon to investigate whether plant disease epidemics that spread out from centers, or foci, do so with constant or increasing velocity. Wheat stripe rust epidemics were initiated by artificial inoculation in plots of susceptible plants or susceptible plants mixed with immune plants. The velocity of epidemic spread was consistently found to increase over both space and time.

Technical Abstract: Controversy has long existed over whether plant disease epidemics spread with constant or with increasing velocity. We conducted a large-scale field experiment with wheat stripe rust at Madras and Hermiston, Oregon, to test these competing models. In each of three years, a susceptible winter wheat cultivar was planted in pure stand and also in a 1:4 or 1:1 mixture with a cultivar immune to the stripe rust race utilized in the experiments. Plots were 6.1 m wide and varied from 73 to 171 m in length. A 1.5 x 1.5 m focus was inoculated either in the center (2001) or upwind of the center (2002 and 2003) of each plot. Disease severity was evaluated weekly throughout the epidemics in each plot at the same points along a transect running upwind and downwind from the focus. Velocity was calculated from the severity data and regressed separately on time and on distance from the focus. In all location-years and treatments, and at all levels of disease severity, velocity consistently increased linearly with distance, at a rate of approximately 0.59 m/wk per m, and exponentially with time. This finding has important implications for plant diseases with a focal or partially focal character. In particular, it may be relevant to the effectiveness of disease management strategies at different spatial scales.