Survival of Phytophthora kernoviae oospores, sporangia, and mycelium

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Interpretive Summary: A recently discovered plant pathogen has been found to attack many different hosts including rhododendrons and beech trees in the United Kingdom and New Zealand. Although not currently known to exist in the United States, it is a pathogen of concern due to its destructive nature and threat to valuable hosts in the U.S. This pathogen produces different propagules for spread and survival of which very little is known. This study was done to learn how long these propagules can survive at different temperatures. Results show that one propagule type can survive buried in sand for up to a year at moderate temperatures but declines rapidly when exposed for increased periods of time as temperatures increase. In addition, this pathogen is able to persist and produce survival propagules in sand with very minimal nutrients. These results provide information to regulatory agency personnel and scientists to develop guidelines that help in limiting the spread of this pathogen. Technical Abstract: Phytophthora kernoviae is a pathogen recently found only in the U.K. and New Zealand. Phytophthora kernoviae, not known to produce chlamydospores, is homothallic and produces abundant oospores and sporangia. This study was conducted to examine long-term survival of oospores, sporangia, and mycelium buried in sand at different temperatures. Viability of oospores embedded onto 20-µm-mesh screens and buried in sand at 4, 10, 20, and 30C was measured over time by staining with tetrazolium bromide solution. For one isolate, viability was 82, 81, 79, and 58% and 86, 75, 82, and 78% for the other isolate at 4, 10, 20, and 30C, respectively, after 1 year. The infection potential of the oospores was checked by removing additional embedded screens and placing on rhododendron leaf disks. No necrosis was observed on leaf disks exposed to oospores buried for 1 year at 30C. However, necrosis was observed on leaf disks at the other temperatures after the same time. Oospores exposed 6 h at 50C and 24 h at 40 and 50C were less viable than controls and did not germinate. To study survival of sporangia and mycelium, sand was infested only with sporangia or mycelium from four isolates, incubated at different temperatures, and plated on Phytophthora-selective medium over time. All isolates reacted in the same manner with populations generally declining within 1 week but then remaining steady over time. Soil extractions of the infested sand showed that over time sporangia and oospores formed at all temperatures except at 30C. This demonstrates that P. kernoviae oospores can persist in sand for long periods of time at different temperatures, which could be a significant factor in spread of this pathogen.

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