First Report of Twig and Branch Dieback of English Walnut (Juglans regia) Caused by Neofusicoccum mediterraneum in California
Authors: Trouillas F. P. , Úrbez-Torres J. R. , Peduto F. , Gubler W. D. 
Published: September 2010

California produces 99% of the U.S. English walnut crop with more than 30 cultivars on ~89,000 ha. Production for 2008 was ~436,000 tons with a value of $527 million. In early summer of 2009 and 2010, branch and twig dieback of English walnut (Juglans regia L.) was detected in orchards in Yolo County and submitted to our diagnostic laboratory. Disease symptoms included death of twig tips, branch dieback, wood lesions, and canker formation. Pycnidia were embedded within the bark of dead twigs. Conidia from pycnidia were hyaline, fusoid-ellipsoidal, widest usually in the middle, and 21 to 24 (–27) × 5 to 7 μm (n = 30). Isolations from cankers yielded the fungus Neofusicoccum mediterraneum Crous, M.J. Wingf. & A.J.L. Phillips (1). Fungal colonies of N. mediterraneum grew light olive green to gray on potato dextrose agar, becoming dark olive green with age. Identification of fungal isolates was confirmed by sequence comparison of Californian isolates with ex-type (CBS 121558) sequences in GenBank (3) using the internal transcribed spacer region of the rDNA, a portion of the β-tubulin gene, and part of the translation elongation factor. Sequences of Californian isolates (GenBank HM443604–HM443609) were identical to the ex-type sequences for all three genes. Previous studies in California reported the occurrence and pathogenicity of N. mediterraneum into grapevine (Vitis vinifera L.) (3) and almond trees (Prunis dulcis L.) (2). Inderbitzin et al (2) investigated the host range of N. mediterraneum in California and reported the occurrence of pycnidia on English walnut trees. However, this study did not investigate the pathogenicity of N. mediterraneum on this host. In the current study, the pathogenicity of N. mediterraneum in J. regia cvs. Hartley and Chandler was investigated in an orchard at UC Davis using two fungal isolates. Pathogenicity tests were performed by inoculating eight 2- to 4-year-old branches of mature J. regia trees. Inoculations were made in June 2009 with a 5-mm cork borer to remove bark and placing an 8-day-old 5-mm-diameter agar plug bearing fresh mycelium of the fungal isolates directly into the fresh wound, mycelium side down. An additional eight branches of each cultivar were inoculated with noncolonized agar plugs to serve as controls. Inoculated wounds were covered with petroleum jelly and wrapped with Parafilm to retain moisture. Branches were harvested after 10 months of incubation and checked for canker development. The extent of vascular discoloration was measured in each branch and isolations were made from the edge of discolored tissue to confirm Koch's postulates. Statistical analyses were performed with analysis of variance and Dunnett's t-test to assess significant differences in the extent of vascular discoloration between inoculations with N. mediterraneum and the control. Necrosis length for the two isolates averaged 131.5 mm in Hartley branches and 110 mm in the Chandler branches. Average necrosis lengths in the control branches were 18.5 mm and 16.7 mm, respectively, significantly lower (P < 0.05) than the average necrosis length found in branches inoculated with N. mediterraneum. Fungal recovery was 75% in both varieties. To our knowledge, this study is the first report of N. mediterraneum as a pathogen of J. regia trees in California.

References: (1) P. W. Crous et al. Fungal Planet 19, 2007. (2) P. Inderbitzin et al. Mycologia. Online publication. doi:10.3852/10-006, 2010. (3) J. R. Úrbez-Torres et al. Plant Dis. 94:785, 2010.