Assessing Status of and Resistance to Phytophthora Root Rot on True Fir (Abies spp.)
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Many fir species (Abies) are susceptible to Phytophthora Root Rot (PRR). Poor drainage and standing water facilitate pathogen survival, proliferation, and spore dispersal. Disease is caused by numerous Phytophthora spp. that constitute regionally-adapted communities. Survey results of diseased Abies in U.S. Christmas tree farms demonstrated a nearly uniform community profile dominated by P. cambivora on noble fir in the Pacific Northwest, and a more diverse array of Phytophthora species collected from Fraser, Turkish, balsam, and Canaan firs in the Great Lakes and Eastern states. It was concluded that Phytophthora community compositions vary in response to host availability, environment, and anthropogenic distribution of infested host material. Aside from short-term chemical and cultural individual-tree protection, there are few feasible methods for altering the environment to provide perennial disease abatement. An alternative approach is to reform host conduciveness to disease by identifying resistant members of a population through the application of molecular tools. As a precursor to molecular marker development, a multifactorial study was performed to characterize host phenotypes in response to multiple species of Phytophthora under variable environmental conditions. It was demonstrated that P. cinnamomi and P. taxon kelmania cause greater root damage and more mortality than P. pini or P. cambivora and that disease is more severe at warmer temperatures. Furthermore, it was established that there is a spectrum of resistances among the various species of Abies, ranging from highly susceptible noble and Fraser firs to more resistant Turkish and Nordmann firs. Previous observations of PRR-resistant Turkish fir plantings have indicated that the ability to resist disease is not uniform among seed sources. To test variability among Turkish fir families, a collection of 36 seed sources was inoculated with 5 Phytophthora species under saturated field conditions. Chi square analysis indicated that 4 seed sources were significantly more susceptible; however, mortality of less than 10% in the most susceptible Turkish fir families in comparison to 100% mortality in Fraser and noble fir suggested that Turkish fir is a PRR-resistant Abies species. Significantly more disease caused by P. cryptogea in this field study confirmed previous observations of variability in Phytophthora species aggressiveness.