Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide

Abstract

Phytophthora gonapodyides causes decline and death of English (Persian) walnut (Juglans regia) in Italy.

Abstract

In late spring 2015, several commercially grown Persian walnut (Juglans regia) cv. Lara trees in northeastern Italy (Udine) exhibited extensive foliar wilt and canopy decline associated with collar and root rot. Sudden collapse was recorded in ∼80% of trees facing an irrigation canal. Symptomatic tissues excised from roots and collars of affected plants were surface disinfested for one minutes in a 1% NaOCl solution, rinsed for 5 minutes in sterile distilled water, and placed onto P5ARPH selective medium. A Phytophthora-like organism was consistently isolated. Pure cultures, with a typical rosette pattern, were obtained by single-hyphal transfers onto potato dextrose agar. Based on morphological characteristics, molecular data (sequence analysis of the internal transcribed spacer and cytochrome-c oxidase, subunit II gene) and pathogenicity tests, the causal agent was identified as P. gonapodyides. This fungus is ubiquitous in streams and ephemeral water pools, and is capable of saprophytic existence. Although it is mainly known as a minor pathogen, there are reports indicating that some isolates can be highly virulent as in the present study where well developed 7-year-old walnut trees were killed by the pathogen. In the current study, P. gonapodyides aggressiveness was most likely sustained by the prolonged presence of flooding water at the root level, and a cool soil environment. This is thought to be the first report of P. gonapodyides on Persian walnut in Italy or elsewhere.