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Ecology and Epidemiology

Effects of Habitat and Population Structure on Powdery Mildew Epidemics in Experimental Phlox Populations. A. M. Jarosz, Graduate student, Department of Biological Sciences, Purdue University, West Lafayette, IN 47907, Present address: Division of Plant Industry, CSIRO, G.P.O. Box 1600, Canberra, A.C.T., 2601, Australia; M. Levy, Associate professor, Department of Biological Sciences, Purdue University, West Lafayette, IN 47907. Phytopathology 78:358-362. Accepted for publication 15 September 1987. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-358.

The development of powdery mildew epidemics caused by Erysiphe cichoracearum on its natural Phlox hosts was monitored from 1979 to 1982 in a series of experimental populations provided with a central inoculum source. Populations were established in both shaded woodland and adjacent, exposed old-field habitats and contained 33, 67, or 100% of individuals susceptible to the local pathogen isolate used. Generally, shaded populations accrued 3–5 times more infected plants and 5–40 times higher disease severity (i.e., area under the disease progress curve) than comparably structured, exposed populations. The persistence of infections was also greater on shaded plants. Between-year variation in disease severity was largely explained by temperature and rainfall effects on pathogen growth. Population structure did not appear to affect disease severity. Expected reductions in epidemic severity with increasing proportions of resistant plants may have been negated by the low host density (typical of woodland Phlox taxa) and large inoculum sources used. A previous survey established that resistance levels among wild Phlox populations were significantly correlated with the degree of habitat shading; in fully shaded sites, populations also tended to be homogeneous for high levels of resistance. Consequently, the experimental epidemiology supported the hypothesis that natural Phlox populations have evolved and/or maintained resistance levels primarily in response to habitat-mediated levels of E. cichoracearum pathogen pressure.