2017 ASHS Annual Conference

Rhododendron root rot, caused by several Phytophthora species, can cause devastating losses in nursery-grown plants. Most research on chemical control of root rot has focused on Phytophthora cinnamomi. However, it is unknown whether treatments recommended for P. cinnamomi are also effective for other Phytophthora species causing root rot, such as P. plurivora. Chemical control options for P. cinnamomi and P. plurivora were investigated in two canyard studies to assess whether two fungicides (phosphorous acid, mefenoxam) and different application methods (drench, foliar) altered disease in Rhododendron catawbiense ‘Album’. Visual symptoms of disease (wilting, death) and plant health (leaf color, stomatal conductance) were measured during the growing season, and plant quality (root rot, biomass) was determined at 13 weeks. When no fungicide was applied, plants started to die within 4 weeks after inoculation and mortality progressed more rapidly with P. cinnamomi (95% mortality, 11 weeks) than P. plurivora (65% mortality, 11 weeks). For plants inoculated with P. cinnamomi, soil drenches of either phosphorous acid or mefenoxam decreased root rot and mortality compared to those receiving no fungicide. Foliar applications of these two fungicides were less effective than soil drenches against P. cinnamomi. In contrast, neither phosphorous acid nor mefenoxam decreased root rot or mortality of plants inoculated with P. plurivora, regardless of application method. All inoculated plants grew less than noninoculated controls regardless of whether or not they were treated with fungicide. For P. cinnamomi, plants drenched with phosphorous acid had greater root and stem growth compared to inoculated plants without fungicide. In contrast, fungicides had no influence on growth in plants inoculated with P. plurivora. The ability to re-isolate the pathogen from inoculated plants was positively correlated to root rot ratings – the more effective the fungicide was in reducing root rot, the less likely the pathogen would be re-isolated. Our results indicate that soil drenches with phosphorous acid can decrease root rot and mortality in rhododendron caused by P. cinnamomi. However, surviving plants were less healthy than controls (greater root rot and chlorosis, lower stomatal conductance, and less growth) and were still infected by the pathogen (P. cinnamomi was recovered from 40% to 60% of surviving plants). Unfortunately, no treatments were effective for root rot caused by P. plurivora. Future research will evaluate whether increasing the number of fungicide applications can increase root rot control for both pathogens.