Applying Phytophthora ramorum Inoculum to Hosts: A New Method That Simulates Overhead Irrigation

Abstract

The inoculum threshold for Phytophthora ramorum in irrigation water required for infection of plant material was investigated using a novel pressurized device designed to deliver zoospore inoculum in a way that simulated certain aspects of overhead irrigation. The measured-inoculum spray applicator (MISA) was made from plastic plumbing parts and worked by spraying measured volumes of pressurized zoospore inoculum onto plant material through an adjustable misting nozzle attached to the bottom of the device. Pressurization and spraying of P. ramorum zoospores through the MISA did not significantly affect zoospore viability or infectivity on wounded and non-wounded detached Rhododendron x ‘Nova Zembla’ leaves under controlled laboratory conditions. An inoculum threshold of 51 zoospores/ml was found for infection of Rhododendron leaves by P. ramorum using regression analysis. The MISA can potentially be used to simulate overhead irrigation in research involving pathogenic Phytophthora spp., and the results of the current research may assist nursery managers, property owners, and regulatory agencies in assessing the risk of using P. ramorum infested water for irrigation within nurseries and private landscapes. Accepted for publication 19 June 2015. Published 26 June 2015.