In October 2010, containerized Acer palmatum Thunb. trees (‘Red Select’, ‘Viridis’, and ‘Hubb's Red Willow’) with crown cankers and root rot were observed at a central Virginia nursery. Sections of lower stem cankers and rootlets were sampled from 14 trees by surface-sterilizing tissue and plating on V8 juice agar. A species of Cylindrocarpon was isolated from 72% of sampled trees and recovered from 57% of 50 cankers and 74% of 50 sections of symptomatic rootlets. On potato dextrose agar (PDA), macroconidia were cylindrical, straight, and rounded at both ends. Chlamydospores were common and intercalary or terminal in the mycelium. To identify to species, the internal transcribed spacer of ribosomal DNA and partial sequences of the beta tubulin gene were sequenced and compared with published sequences (GenBank Accession Nos. AM419110.1 and AM419109.1), which exactly matched sequences of Cylindrocarpon macrodidymum, a recently described species (1,2). Neighbor-joining analyses using Blast Tree View placed C. macrodidymum and the unknown isolate in a closely related but distinct clade from C. pauciseptatum and C. liriodendri. Species-specific primers also confirmed the isolates to be C. macrodidymum (2). A. palmatum seedlings (generic root stock) were inoculated by placing a colonized PDA plug over 1-cm scalpel incision wounds (n = 12) and wrapping treated sections of stem with Parafilm to prevent desiccation. Twelve root systems were inoculated by mixing a colonized PDA plate with 1 liter of water and drenching the soil of each plant. Twelve wounds and root systems were inoculated with uncolonized PDA for controls. After 4 weeks, inoculated wounded seedlings developed brown lesions extending 1 to 3 cm from incision wounds. Inoculated roots developed root rot. Controls produced no visible symptoms. A Cylindrocarpon sp. was recovered from 85% of surface-sterilized sections of inoculated wounded stem and 100% inoculated symptomatic roots. No Cylindrocarpon sp. was isolated from stem or root controls. To our knowledge, this is the first report of this pathogen on A. palmatum. Other reported hosts include grapevines (Vitis spp.) and apple (Malus spp.) (3). This finding demonstrates the ability of C. macrodidymum to impact an ornamental host and the potential for this pathogen to impact the ornamental nursery industry.
References: (1) S. Alaniz et al. Plant Dis. 93:821, 2009. (2) J. Auger et al. Plant Dis. 91:470, 2007. (3) Y. T. Tewoldemedhin et al. Eur. J. Plant Pathol. 129:637, 2011.
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