Water Requirements for Sporangium Formation by Phytophthora parasitica in Relation to Bioassay in Soil. N. Ioannou, Department of Plant Pathology, University of California, Davis 95616. R. G. Grogan, Department of Plant Pathology, University of California, Davis 95616. Plant Dis. 68:1043-1048. Accepted for publication 4 May 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/PD-68-1043.

Formation of sporangia by Phytophthora parasitica from tomato was examined in unsterilized soil at constant matric potentials (φ_m) between 0 and –300 mb (soil water content between saturation and field capacity) using three types of inocula: mycelium, chlamydospore-bearing mycelium, and infected tomato tissue. Maximum numbers of sporangia were produced between –25 and –300 mb from the first two inocula and between –50 and –300 mb with infected tomato tissue. Production of sporangia was reduced significantly at φ_m = –10 mb and reduced to nearly nil at φ_m = 0 with all inocula. Artificially and naturally infested soil samples, air-dried for 10–45 days and rewetted to either –300 or 0 mb for 4–5 days, were flooded and assayed by green tomato fruit baiting. Detection of P. parasitica was more efficient from dried soil samples that had been wetted to –300 mb before saturation than from those saturated immediately after collection. Soil samples collected during the summer months from two infested, dry-fallowed tomato fields usually failed to yield P. parasitica when flooded and assayed immediately after collection. The fungus was readily detected, however, when comparable soil samples were wetted to –300 mb for 4–5 days, then flooded and assayed. Soil samples collected during the growing season from 37 processing tomato fields in the Sacramento Valley were assayed by three baiting procedures, which (before flooding) involved the following treatments: no treatment (direct flooding), prewetting to –300 mb, and air-drying for 3 wk followed by wetting to –300 mb. The three procedures were about equally efficient for qualitative detection of the pathogen, but only the third resulted in significantly different bait infection levels among samples from fields with obviously diseased and apparently no diseased plants, respectively. Some fields with no obviously diseased plants were infested, but significantly higher inoculum levels were demonstrated in soil from fields with evident disease. Green tomato fruits facilitated the selective trapping of Phytophthora spp. pathogenic to tomato plants.