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Focus Expansion in Plant Disease. IV: Expansion Rates in Mixtures of Resistant and Susceptible Hosts. F. van den Bosch, Institute of Theoretical Biology, State University of Leiden, Kaiserstraat 63, 2311 GP Leiden, The Netherlands; M. A. Verhaar, A. A. M. Buiel, W. Hoogkamer, and J. C. Zadoks. Department of Phytopathology, Agricultural University, P.O. Box 8025, 6700 EE Wageningen, The Netherlands. Phytopathology 80:598-602. Accepted for publication 21 December 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/Phyto-80-598.

The radial expansion of plant disease foci in ideal (i.e., infinitesimally fine grained) mixtures of a susceptible and a resistant cultivar is described. If the only difference between the cultivars is in their susceptibility to disease, theory predicts that the radial velocity of focus expansion increases linearly with the logarithm of the fraction of susceptible plants. This hypothesis was tested in the field by using the wheat-stripe rust pathosystem. The results of the experiment were generally consistent with the hypothesis, although minor deviations were present. There was an indication that in plots with a small fraction of susceptible plants, the velocity of focus expansion did not attain its final value.

Additional keywords: cultivar mixtures, epidemiology, model, Puccinia striiformis, Triticum aestivum.