Recovery of Phytophthora ramorum Following Exposure to Temperature Extremes

Phytophthora ramorum Werres, De Cock & Man in ‘t Veld, a recent introduction into the United States, has proven to be a threat to hardwood forests on the west coast, specifically tanoak (Lithocarpus densiflorus (Hook & Arn.) Rehder) and Quercus spp. (34). This pathogen has a broad host range which is ever-expanding and includes understory plants and commonly grown nursery plants (19,25). Some native eastern plants in the Ericaceous family such as mountain laurel, blueberry, bearberry, and huckleberry are susceptible to infection by P. ramorum (43), as well as white oak, chestnut oak, and northern red oak that developed disease following stem inoculations (42). This raises concerns for the health of the hardwood forests in the eastern United States should P. ramorum become introduced. Attempts have been made to determine the risk posed by P. ramorum to the eastern United States using weather data (39,45). However, the utility of efforts to determine the risk of P. ramorum spread is restricted by our limited knowledge of the pathogen’s survival ability. Previous work has shown that temperatures supporting mycelial growth of P. ramorum in vitro were 2 to 28°C, chlamydospore production 8 to 28°C, and sporangia production 10 to 30°C (12,47). In spite of the wide temperature range for growth shown by P. ramorum, harsh winter temperatures encountered in the northern regions of the country may limit its establishment. Because P. ramorum is primarily a foliar pathogen, it is likely to overwinter as hyphae or chlamydospores in stems and leaves on the plant or in abscised leaves found on the soil surface, in leaf litter, or in shallow depths of soil. Therefore, the fungus will be subjected to freezing ambient temperatures, freeze and thaw fluctuations, and desiccation. Similarly, high temperatures and dry conditions encountered in the summers in various regions may limit growth and establishment of P. ramorum. Davidson et al. (8) reported that P. ramorum survived the warm, dry summer months in California in attached bay laurel (Umbellularia californica) leaves, although levels declined over time. These workers were not able to reisolate P. ramorum from abscised leaves collected from leaf litter. Davidson et al (10) also established a strong link between sporulation on bay laurel and transmission of P. ramorum in California forest ecosystems. It was demonstrated (9) that P. ramorum chlamydospores could survive a 30-day treatment at 15°C in water or on moist filter paper, but did not survive when initially dried at room temperature and 30% relative humidity for 30 min. Studies by other workers (5,13,29,37,38) have demonstrated that P. ramorum chlamydospores are long-lived and can be recovered following placement in potting media after many months. Knowledge of the impact of extreme temperatures on P. ramorum survival both as chlamydospores and within host tissue is necessary to predict the ability of the pathogen to overwinter and provide inoculum for continuing epidemics in forest ecosystems. Such knowledge also would help identify constraints to oversummering and overwintering survival, and allow predictions of how widely the pathogen may become established in new regions. This information would be of use to regulators, inspectors, and Forest Service personnel who seek to determine the limits to P. ramorum establishment. We assessed the germinability of P. ramorum chlamydospores following exposure to extreme constant temperatures, both high and low, for up to 7 days. We also assessed the recovery of P. ramorum from plant host tissue (Rhododendron ‘Cunningham’s White’) at extreme constant temperatures for up to 7 days. Leaf disks were incubated in three different soils (sand, sandy loam, and loam) and at two different water holding capacities (WHCs) in each soil type. In addition, we monitored recovery of P. ramorum in plant host tissue held under previously mentioned soil and moisture regimes, but subjected to ambient temperatures based on historical weather data for a period of 12 weeks.