First report of Verticillium isaacii causing wilting and vascular blackening on wasabi (Wasabia japonica) plants

Abstract

Wasabi (Wasabia japonica (Miq.) Matsum) plants are grown over a period of 12–18 months for their valuable rhizomes, and pathogens infecting the rhizome are of economic importance for producers. During 2016 and 2017, symptoms of wilting and internal vascular blackening of rhizomes were observed in two greenhouses in British Columbia, with disease incidence in the range of 5–10%. Isolates recovered from diseased tissues were examined morphologically and a majority (70%) resembled a Verticillium species. Molecular analysis using PCR of the ITS1-ITS2 region of rDNA and a portion of the actin gene identified the isolates as V. isaacii Inderb. R.M. Bostock, R.M. Davis & Subbarao. The optimal temperature for growth of V. isaacii on potato dextrose agar was 20°C and a distinct yellow pigment was produced, which is characteristic of species in the clade Flavexudans. Inoculation of detached wounded wasabi tissues with V. isaacii produced blackening symptoms and chlorosis on leaves. Black streaks developed on petioles and internal blackening of rhizomes was also observed. Plants inoculated in the greenhouse by immersing wounded roots in a spore suspension of V. isaacii developed a reduced root system compared with control plants. Blackening of the rhizome tissue was observed after three months. Differences were observed in pathogenicity between V. isaacii isolates. We demonstrate for the first time that V. isaacii is the cause of wilting and vascular blackening on wasabi plants. It is considered a weak pathogen, only affecting wounded tissues with symptoms developing after an extended time following inoculation.