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Increased Germination of Propagules of Phytophthora parasitica by Heating Citrus Soils Sampled During Winter. A. L. Lutz, Department of Plant Pathology, University of California, Riverside 92521; J. A. Menge, and D. M. Ferrin. Department of Plant Pathology, University of California, Riverside 92521. Phytopathology 81:865-872. Accepted for publication 5 April 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-865.

The influence of heat treatment on the germination of propagules of Phytophthora parasitica was examined in three citrus groves during 1986. Temperatures between 12 and 34 C and lengthened incubation time increased recovery of propagules from soil. This effect could be expressed in terms of accumulated heat units calculated as degree-days. Similar numbers of propagules germinated from soils exposed to different temperatures when the durations of heat treatments were adjusted to ensure that soils received equivalent numbers of heat units. Germination of propagules was correlated directly with accumulation of heat units from 0 to 150 degree-days but was correlated negatively with accumulation of heat units from 150 to 1,650 degree-days. Maximal stimulation of propagule germination occurred when soils had acquired 100150 degree-days. Population densities at this time were up to 9.5 times greater than populations recovered from unheated control soils. Heat treatment of soils collected during the summer months yielded population densities up to twice as large as control populations. There was no evidence that heat caused active growth of the fungus in sieved soils. We have selected 32 C for 5 days as a routine treatment for winter soils before isolations for P. parasitica are attempted.

Additional keywords: root rot.