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Host-to-host transmission rate of Phytophthora ramorum is highest during a relatively short period in mid-winter in California.

Wolfgang Schweigkofler: Dominican University of California

<div>The invasive forest pathogen<em> Phytophthora ramorum</em>, causal agent of Sudden Oak Death, spreads via sporangia produced on leaves of woody host plants. We tested the short-distance vertical transmission of <em>Phytophthora ramorum</em> from bay laurel to rhododendrons during the winter 2016/17. Healthy rhododendrons were placed under symptomatic bay laurels and monitored regularly for symptom appearance. Symptomatic rhododendrons were exchanged with healthy ones and exposure time recorded for each transmission event. <em>Phytophthora ramorum</em> was detected from rain water in the canopy of the Bay laurel from November 21 onwards with increasing concentrations. A total of 17 transmission events were detected from December 19, 2016 to February 10, 2017. The average exposure time was 14 to 25 days (N: 15; 88.2 %), but two transmission events (11.8 %) with very short exposure times (four days) were also detected. No transmission was detected after February 10, although favorable climate conditions (including higher than normal rainfall and mild temperatures) persisted for several more weeks, and <em>Phytophthora ramorum</em> propagules were detected regularly from rainwater until April. To our knowledge, this is the first field experiment to study the precise timing of <em>Phytophthora ramorum</em> transmission from a host plant to another. The results will help to better understand the epidemiology of this important plant disease and potentially improve CA forecast models.</div>