2017 ASHS Annual Conference

Plant pathogens in the genus Phytophthora cause root rot that decrease product quality and result in plant death and economic losses to the nursery industry. Recently, we found Phytophthora plurivora prevalent on rhododendron in nurseries in the Pacific Northwest, USA, but there is little information available to compare its pathogenicity in different environments to P. cinnamomi, a more well-studied rhododendron pathogen. Substrate moisture in container production can influence plant vigor as well as pathogen survival and inoculum build-up. We evaluated how irrigation frequency and volume affected the ability of P. plurivora and P. cinnamomi to cause disease on Rhododendron catawbiense ‘Boursault’ and ‘Roseum Elegans’. Plants were grown for 26 weeks in non-infested media (controls) or media infested with 1 propagule per gram (ppg) or 100 ppg of P. plurivora or P. cinnamomi. During the growing season, plants were irrigated daily to maintain a substrate moisture (SM) of >75% container capacity (CC) (1.0X), irrigated daily with one-half volume of the 1.0X treatment (0.5X), or irrigated only after SM was <50% CC (BB). Visual symptoms of plant health (leaf color, stomatal conductance) and disease (wilting, death) were measured during the growing season, and plant quality (root rot, biomass, nutrient uptake) was determined at 26 weeks. Each species caused similar mortality at 100 ppg (50-80%), but visual disease symptoms occurred earlier on plants inoculated with P. cinnamomi than with P. plurivora. Mortality at 1 ppg was < 10% for both species. However, compared to controls, plants inoculated with 1 ppg Phytophthora exhibited decreased plant health (lower stomatal conductance, less growth, lower nutrient uptake, more chlorosis and root rot). In general, disease was less severe and occurred slower in ‘Roseum Elegans’ than in ‘Boursault’. Irrigation treatments had no influence on plant quality of controls. The effects of irrigation regime on disease differed between cultivars. Compared to ‘Boursault’ in the 1.0X irrigation treatment, decreasing irrigation frequency (BB) or volume (0.5X) decreased disease incidence and severity. In contrast, Phytophthora had least impact on disease in ‘Roseum Elegans’ in the 1.0X irrigation treatment. Our results indicate that (1) P. plurivora can cause similar losses in rhododendron plants as P. cinnamomi; (2) low substrate levels of Phytophthora can decrease plant health; (3) plants grown in media free of Phytophthora can adapt to a broader range of irrigation regimes than plants grown in infested media; and (4) irrigation management alters the incidence and severity of Phytophthora on rhododendron.