Breaking Winter Dormancy of Phytophthora Parasitica Propagules Using Heat Shock

Abstract

Phytophthora parasitica Dast. is a common fungus in citrus soils in California and causes feeder root rot of citrus (2). Population fluctuations during the year correlate with soil temper? ature (3). The pathogen population reaches its peak during the hottest summer months of July through September and declines to virtually zero (less than one propagule per g rhizosphere soil) during the winter (3), when the soil temperature is 9-12 C (4). Similar results have been reported by Kuske and Benson for P. parasitica dieback of Rhododendron (8). It was not determined whether most propagules had died, leaving a re? sidual population that was too low to detect using standard assay procedures, or if the propagules were present, but in a cool temperature-induced dormancy. Preliminary studies by Holdaway indicated that low-level recovery of propagules from infested soil was possible when artificially inoculated soil that had been incubated at 1 C for 7 wk was then reincubated at higher temperatures for one wk (5). Kuske and Benson found that P. parasitica could not be recovered from naturally infested soil or Rhododendron tissues when winter tem? peratures were below 0 C, but could be recovered readily once the temperature rose in the spring (8). Based on these studies, we hypothesized that at least residual spore populations survived dur? ing the winter in a dormant state induced by cool temperatures. Exposing soil to short periods of heat (heat shock) was tested as a way to break this dormancy. During January and February, several field soil samples were taken from the edge of the furrow irrigation zone on the north side of individual citrus trees [Citrus sinensis (L.) Osbeck]. Ten pairs of trees were sampled. Pairs were assigned based on tree-to-tree proximity and similarity of foliar disease severity symptoms of Phytophthora root rot. Samples were obtained with a bucket auger up to a depth of 23 cm. The top 7.5 cm soil was discarded. Soil adhering to the feeder roots (rhizosphere soil) was collected, and soil from the two trees in each pair was combined. Samples were dug from each pair of trees until approxi? mately 60 g rhizosphere soil was collected. Each rhizosphere soil sample was divided into three subsamples, and these were incubated at 8, 24 or 34 C for 48 h (pre-incubation). A 1:10 dilution of soil to water was made based on the dry weight of each soil sample, and one ml of solution was pipetted into each of 40 Petri dishes to which molten PVPH selective medium (9) was added. Eight of the 40 soil dilution plates from each subsample were incubated at 20, 24, 28,31, and 34 C for 3 da (post-incubation). The number of P. parasitica colonies was counted, and the number of propagules per g dry weight of rhi? zosphere soil was calculated. The effect of soil moisture content on breaking temperature-induced dormancy by heat was tested in a separate experiment. Soil samples were divided into subsamples and the moisture con? tent adjusted to 14,12, or 9% by weight. Samples with these different moisture contents were heat-