Monitoring Using a Sentinel Plant System Reveals Very Limited Aerial Spread of Phytophthora ramorum From Infected Ornamental Plants in a Quarantine Research Nursery

Abstract

The potential aerial spread of Phytophthora ramorum, causal agent of sudden oak death and Ramorum blight, from infected plants in a quarantine research nursery at the National Ornamentals Research Site at Dominican University of California (NORS-DUC) to the environment was monitored weekly for five years (2011 to 2016) using a sentinel system. Phytophthora ramorum was never detected on any of the sentinel plants (Rhododendron, Viburnum, and Loropetalum spp), indicating very limited aerial spread under suboptimal meteorological and environmental conditions. An infection experiment with host plants placed in the immediate vicinity of symptomatic plants proved the potential for short-distance (1 to 2 m) aerial transmission of P. ramorum. Other Phytophthora spp. causing symptoms similar to P. ramorum were detected during the rainy season (January to May) on the sentinel plants, among them potentially two novel species. These data reveal how sentinel monitoring at NORS-DUC allows for seasonal assessments of disease incidence and provide longitudinal data to assess the threat of P. ramorum movement in nurseries.