Susceptibility of New Zealand flora to Phytophthora ramorum and pathogen sporulation potential: an approach based on the precautionary principle

Abstract

Phytophthora ramorum, the cause of sudden oak death in the westernUSAand a damaging pathogen in Europe, is a biosecurity threat of unknown magnitude to New Zealand and Australasia because of its presence in traded ornamental plants. Knowledge of potential hosts acting as carriers and of symptoms caused by the pathogen on such hosts will strengthen precautionary quarantine regulations to prevent inadvertent introductions of P. ramorum into the region. Also, the identification of potential hosts will permit determination of areas at risk within countries that do not have P. ramorum. Susceptibility of New Zealand plants, including 17 endemic and three commercial species (Eucalyptus globulus, Pinus radiata and Acacia melanoxylon), as well as two known Rhododendron cultivar hosts, was determined by analysing the size of lesions on inoculated excised leaves and branches, while infectivity was determined by counting sporangia produced on leaves. In order to identify extremely susceptible hosts, seven species were inoculated using three concentrations of zoospores ranging from low (1×10² zoospores/mL) to high (5×10³ zoospores/mL). In branch inoculations, P. radiata and Nothofagus fusca were as susceptible as the Rhododendron cultivars. Pseudopanax arboreus, Fuchsia excorticata and one Rhododendron cultivar were equally susceptible in leaf inoculations. However, F. excorticata was the only species with 100% infected leaves, high foliar sporulation and was highly susceptible at all three zoospore concentrations. Leptospermum scoparium was the only asymptomatic foliar host that had high reisolations of the pathogen. F. excorticata, P. radiata, N. fusca, P. arboreus and L. scoparium should be added to the potential host list for P. ramorum and monitored for symptoms and sporulation in gardens and nurseries in theUSAand Europe. As part of a precautionary strategy, these species are suitable candidates for targeted surveillance programs in high-risk incursion areas of New Zealand. Furthermore, the sympatry of foliar hosts with high infectivity and of highly susceptible stem hosts was identified: these areas may be at risk for the development of a forest epidemic.