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Isolate variability in Phytophthora tentaculata aggression and potential for biological control

Timothy Widmer: USDA ARS FDWSRU

<div><em>Phytophthora tentaculata</em> (Pt) is a pathogen first described in 1993 from Germany on <em>Origanum vulgare (</em>oregano) and has been reported from the Netherlands, Spain, Germany, and Italy. It has recently been reported in California in 2012 on native plants including <em>Diplacus aurantiacus </em>subsp.<em> aurantiacus</em> (orange sticky monkey flower). Very little is known about this species and its behavior. The purpose of this study was to investigate the differences in aggression of Pt isolates and whether <em>Trichoderma asperellum</em> has the potential as a biological control agent (BCA) against it. Oregano seedlings were transplanted in potting mix amended with millet seed colonized by Pt isolates from Germany (2 isolates), Netherlands (1), Spain (1), and the USA (3) at rates of 0.25 or 0.5% (v/v). After 8 weeks, seedling survival among the isolates was significant and varied from 96% for one of the Germany isolates to 0% for the Spain isolate, which compared to 96% for the non-infested controls. The average top dry weight for the controls was 0.33 g and ranged from 0.45 g for one of the Germany isolates to 0.02 g for one of the USA isolates. There was no difference between the two rates. Incorporation of wheat bran colonized by <em>T. asperellum</em> isolate 04-22 into potting mix infested with a Pt isolate from the USA not only improved survival (from 42 to 100%) but significantly improved the top dry weight (1.36 g) compared to non-amended controls (0.65 g). This study shows that there is a difference in aggression among isolates and that <em>T. asperellum</em> has the potential as a BCA. This study will help in understanding and mitigating the threat that <em>P. tentaculata</em> has towards the native plant industry in CA and other potential hosts around the world.</div>