First record of natural infection by Chondrostereum purpureum on Rhododendron in North America.

Abstract

Chondrostereum purpureum (Pers. ex Fr.) Pouzar infects many woody plants, including fruit trees, ornamentals, and forest species (Setliff, 2002). Its broad host range and ability to cause silver leaf disease make it a significant concern in forestry and horticulture. Although C. purpureum was found on Rhododendron spp. in the UK, New Zealand, and Nepal, it was unreported in North America (Farr et al. 1996). Rhododendron is the largest genus in the Ericaceae family, comprising over 1,000 species and more than 25,000 cultivars. Introduced to Britain in 1763, R. ponticum naturalized and became invasive in some woodlands (Cross 1975). These shrubs are susceptible to infection by Phytophthora ramorum, which can cause severe disease (Willoughby et al. 2015). Mechanical cutting removes infected material, but stumps frequently resprout. C. purpureum was evaluated to control R. ponticum resprouts. Though less effective than synthetic herbicides, it may serve as an alternative where herbicides are restricted (Willoughby et al. 2015). In January 2010, a C. purpureum specimen was collected on white-flowering Rhododendron in an urban forest (Victoria, BC, Canada; GPS: 48.477244, 123.383991). Thin, undulating, woody bracket-like basidiocarps were found on stem sides and branch crooks. Upper surfaces were brown-grey with whitish hairs; new growth on edges and smooth undersides was purple. Symptoms included discoloured stem lesions and wilting leaves on infected stems. A pure culture was obtained by plating surface-sterilized, discoloured stem wood on acidified PDA. Within 7 days at 20°C, flat, white, cottony mycelia with clamp connections emerged. Guaiacol testing confirmed laccase activity (Stalpers 1978). Purified DNA was amplified with basidiomycete-specific rDNA primers (Gardes & Bruns 1993) and compared via BLASTn (core_nt). Sequences showed 99% identity to C. purpureum voucher strains LE-BIN 2764 (OR475683.1, 631/635 nt) and PDD:91629 (GQ411519.1, 634/641 nt), and were deposited as GenBank KM652470.1. To fulfill Koch's postulates, 3-year-old R. caucasicum × ponticum 'Cunningham's White' stems were cut and inoculated with either C. purpureum (4 plants) or PDA (2 plants) as a control. Two months later, inoculated plants developed symptoms including darkening stems, yellowing and wilting leaves, and browning of leaf tips. By six months after inoculation, infected plants had wilted and died, while control plants remained symptomfree. Subsequent PCR with 'APN1' C. purpureum-specific primers (Becker et al. 1999) confirmed the presence of the pathogen only in infected plants, and not in control plants. This is the first verified North American occurrence of C. purpureum naturally infecting Rhododendron. Following verification of identity and pathogenicity, the culture and an infected stem were deposited at the Pacific Forestry Centre-Forest Pathology Herbarium (DAVFP #29828). This finding expands the known host range of C. purpureum and has important implications for Rhododendron management.