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Point of Rocks, MD, United States

Widmer T.L.,Foreign Disease and Weed Science Research Unit | Dodge S.C.,Foreign Disease and Weed Science Research Unit
Plant Disease | Year: 2015

Phytophthora pinifolia is known to cause a devastating disease on Monterey pine in Chile. Although this pathogen is not yet present in the United States, there is reason for concern. The main source of Monterey pine genetic material is found in California and there is potential for other important tree species to be hosts. The study presented here was conducted to develop a method to produce zoospores and determine optimal conditions for infection to be used in future host range studies. Abundant zoospores were produced when agar plugs containing P. pinifolia mycelia were ground into suspension prior to transfer in a solution of carrot broth. These zoospores then were used to inoculate Monterey pine seedlings under various conditions. Infected plants displayed necrotic crowns and stems, often resulting in wilting of the seedling. Consistent infection occurred when seedlings were wounded by trimming needles prior to inoculation and exposure of inoculated seedlings to constant dew for 5 days. Dew chamber temperature (15, 20, or 25°C) did not affect the infection rate. Information obtained from this study will be useful in screening other hosts for susceptibility to P. pinifolia infection. © 2015 The American Phytopathological Society.

Widmer T.L.,Foreign Disease and Weed Science Research Unit | Shishkoff N.,Foreign Disease and Weed Science Research Unit | Dodge S.C.,Foreign Disease and Weed Science Research Unit
Plant Disease | Year: 2012

Little is known about colonization of roots of trees by Phytophthora ramorum. We examined zoospore concentration and exposure time needed to infect six Quercus (oak) species and the inoculum produced from their roots. Sprouted acorns, exposed to zoospores (3,000/ml) for different times and transplanted to potting soil, were susceptible to infection within 1 h of exposure but root weights were not impacted after 4 weeks (P = 0.952). Roots of Quercus prinus seedlings, inoculated with sporangia, had 0.6 to 3.2% colonization of the total root mass after 5 months. Neither root lesions nor obvious root sloughing were observed. Inoculum threshold levels were tested by exposing radicles to varying zoospore concentrations for 24 h. Results showed that radicle infection occurred even at 1 zoospore/ml. To test inoculum production, roots were inoculated with sporangia and transplanted into pots. Periodically, samples of runoff were collected and plated on selective medium. Afterward, root segments were plated to calculate percent colonization. After 16 and 35 days, root colonization and inoculum production from oak was lower than that of Viburnum tinus, a positive control. This study shows that P. ramorum is able to infect sprouted oak acorns and produce secondary inoculum, which may be important epidemiologically. © 2012 The American Phytopathological Society.

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