Temperature and fungicide sensitivity in three prevalent Phytophthora species causing Phytophthora root rot in rhododendron

Temperature is an important environmental variable affecting species 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 species 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 species 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 (0-120 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 between the two (25.3°C). and had the lowest minimum temperatures (~2.4°C) compared to (6.5°C), while all three species had a similar maximum temperature (~35°C). When tested against mefenoxam, all three species were generally more sensitive to mefenoxam at cool temperatures (6-14°C) than at warmer temperatures (22-30°C). was also more sensitive to phosphorous acid at cool temperatures 6-14°C. However, both and tended to be more sensitive to phosphorous acid at warmer temperatures (22-30°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.