Effect of Citrus Rootstocks on Soil Populations of Phytophthora parasitica. J. P. Agostini, Graduate Assistant, Citrus Research and Education Center, University of Florida, Institute of Food and Agricultural Sciences, 700 Experiment Station Road, Lake Alfred 33850. L. W. Timmer, W. S. Castle, and D. J. Mitchell. Professor, Professor, Citrus Research and Education Center, University of Florida, Institute of Food and Agricultural Sciences, 700 Experiment Station Road, Lake Alfred 33850; and Professor, Department of Plant Pathology, University of Florida, IFAS, Gainesville 32611. Plant Dis. 75:296-300. Accepted for publication 17 September 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0296.

Soil and root samples were collected from December 1987 to February 1989 from 12-yr-old Valencia sweet orange rootstock trials near St. Cloud and Avon Park, FL. Propagule densities of Phytophthora parasitica were determined by plating soil on a selective medium. The lowest populations occurred on the least susceptible trifoliate orange and Swingle citrumelo and the highest on the most susceptible sweet orange, Volkamer lemon, and Palestine sweet lime in the field experiments. Average numbers of propagules per cubic centimeter of soil over this period in the field experiments ranged from 8.3 to 32.2 at St. Cloud and from 5.4 to 14.6 at Avon Park for rootstocks with the lowest and highest populations, respectively, with the highest populations occurring in summer. In a screenhouse pot test, populations were highest on sweet orange and sour orange and lowest on Swingle citrumelo and trifoliate orange but were not significantly different from the unplanted control. In experiments at St. Cloud, where roots were removed from cores of soil and new roots were allowed to grow into infested and disturbed soil, propagule densities were high and were correlated with the density of new roots formed (r = 0.929). Abundant immature fibrous roots, a highly susceptible rootstock, and favorable environmental conditions promote development of root rot and, consequently, high propagule densities.