Temperature and Fungicide Sensitivity in Three Prevalent Species Causing Phytophthora Root Rot in Rhododendron.

Temperature is an important environmental variable affecting spp. biology. It alters the ability of species to grow, sporulate, and infect their plant host, and it is also important in mediating pathogen responses to disease control measures. Average global temperatures are increasing as a consequence of climate change, yet there are few studies that compare the effects of temperature on spp. that are important to the nursery industry. To address this, we conducted a series of experiments to evaluate how temperature affects the biology and control of three soilborne spp. prevalent in the nursery industry. In the first set of experiments, we evaluated the mycelial growth and sporulation of several , , and isolates at temperatures ranging from 4 to 42°C for different amounts of time (0 to 120 h). In the second set of experiments, we evaluated the response of three isolates of each species to the fungicides mefenoxam and phosphorous acid at temperatures ranging from 6 to 40°C. Results showed that each species responds differently to temperature, with having the greatest optimal temperature (26.6°C), the least (24.4°C), and was intermediate between the two (25.3°C). and had the lowest minimum temperatures (approximately 2.4°C) compared with (6.5°C), while all three species had a similar maximum temperature (approximately 35°C). When tested against mefenoxam, all three species were generally more sensitive to mefenoxam at cool temperatures (6 to 14°C) than at warmer temperatures (22 to 30°C). was also more sensitive to phosphorous acid at cool temperatures (6 to 14°C). However, both and tended to be more sensitive to phosphorous acid at warmer temperatures (22 to 30°C). These findings help define the temperatures at which these pathogens will be the most damaging and help delineate the temperatures at which fungicides should be applied for maximum efficacy.