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Population Fluctuations and the Numbers and Types of Propagules of Phytophthora parasitica That Occur in Irrigated Citrus Groves. A. L. Lutz, Department of Plant Pathology, University of California, Riverside 92521. J. A. Menge, Department of Plant Pathology, University of California, Riverside 92521. Plant Dis. 75:173-179. Accepted for publication 22 July 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0173.

Citrus trees were irrigated via furrows or low-volume emitters, and populations of Phytophthora parasitica from the rhizosphere were monitored over a period of 12 days after irrigation by soil dilution plate assay on a selective medium. The types of propagules present under long- and short-interval irrigation regimes were quantified from November through August. Populations of P. parasitica increased from 17 propagules per gram (ppg) the day before irrigation to 70 ppg 2 days after long-interval furrow irrigation. Populations ranged from 71 to 93 ppg with short-interval drip irrigation. Low levels of a unique propagule of P. parasitica that resembled a dark, multipapillate sporangium were observed consistently. Low numbers of viable oospores were observed throughout the year. In November and from May through August, sporangia and zoospore cysts accounted for the majority of the population. From December through April, chlamydospores were the most frequently observed propagules.