Evaluation of Phytophthora ramorum in Nursery Crop Tissue Culture Propagation

Abstract

Phytophthora ramorum, cause of Ramorum blight on numerous woody ornamental shrubs, is a regulated pathogen in the US and internationally. Currently, nurseries are inspected to detect infected plants; however, many plants are propagated by tissue culture nurseries and the behavior of P. ramorum in this system is unknown. Pathogen growth and sporulation in propagation vessels containing different multiplication and rooting media, with a range of plants and without plants, was evaluated with regard to pathogen visibility and induction of disease symptoms. Within 2 weeks, the pathogen colonies were visible to the naked eye on all 26 multiplication media and on 9 of 11 rooting media tested (without plants). The appearance of colonies on different media was variable and no sporangia but occasional chlamydospores were produced. The pathogen growth was very visible on multiplication media containing susceptible plants, inoculated plants exhibiting obvious discoloration and mortality. The pathogen was reisolated from terminal shoot tissue and roots of symptomatic plants. Variability occurred in susceptibility of different cultivars of a plant species, in virulence of the two isolates of the pathogen, and in recovery from shoot tissue. We conclude that fungal growth on the media, with or without plants, and symptoms of disease were apparent enough that contaminated vessels would be destroyed. Accepted for publication 9 April 2007. Published 22 August 2007.