First Report of Calosphaeria pulchella Associated with Branch Dieback of Sweet Cherry Trees in California
Authors: Trouillas F. P. , Lorber J. D. , Peduto F. , Grant J. , Coates W. W. , Anderson K. K. , Caprile J. , Gubler W. D. 
Published: August 2010

California is the second largest sweet cherry producer in the United States with approximately 10,800 ha and an average annual crop value of approximately $150 million. Perennial canker diseases constitute major threats to the cherry industry productivity by reducing tree health, longevity, and yields. During the course of summer 2006, we observed severe limb and branch dieback of sweet cherry (Prunus avium L.) in San Joaquin, San Benito, Contra Costa, and Stanislaus counties of California. Isolation from diseased branches repeatedly yielded the fungus Calosphaeria pulchella (Pers.: Fr.) J. Schröt. (1,2). Cankers and vascular necroses had developed in tree limbs and branches, generally initiating from the heart wood and later spreading into the sapwood. External symptoms of disease may be unapparent throughout the early stages of infection, particularly in large diameter shoots. Older infections often appeared as wilted leaves. Branches and trunks affected with cankers from which C. pulchella was isolated also generally bore perithecia of C. pulchella beneath the periderm. Perithecia were nonstromatic and arranged in dense, circinate groups, with elongated necks converging radially and fissuring the periderm. Asci were unitunicate, clavate, and 45 to 55 × 5 to 5.5 μm. Ascospores were allantoid to suballantoid, hyaline, and 5 to 6 × 1 μm. Colonies on potato dextrose agar (PDA) were dark pink to red in their center with a white margin. Conidia were hyaline, allantoid to oblong-ellipsoidal, and (3–) 4 to 6 (–9) × 1.5 to 2 (–2.5) μm. Identification of C. pulchella isolates also was confirmed by sequence comparison in GenBank database using the internal transcribed spacer region (ITS1-5.8S-ITS2) of the rDNA. Sequences of California isolates shared 100% similarity with C. pulchella reference isolate CBS 115999 (EU367451) (2). ITS sequences of the California isolates used in this study were deposited into GenBank (Nos. HM237297 to HM237300). Pathogenicity of four isolates recovered from the margin of active cankers was determined by branch inoculations. In December 2006, 2- to 4-year-old twigs of P. avium cv. Bing were inoculated with a 5-mm cork borer to remove bark and by placing an agar plug from the growing margin of 8-day-old colonies directly into the fresh wound, mycelium side down. Ten branches per isolate were inoculated. Ten control shoots were inoculated with noncolonized agar plugs. Inoculations were covered with vaseline and wrapped with Parafilm to retain moisture. Branches were harvested in July 2007 and taken to the laboratory to be examined for canker development, and the extent of vascular discoloration in each branch was assessed. Isolations from the edge of discolored tissue were conducted to fulfill Koch's postulates. After 8 months, C. pulchella was reisolated from 100% of the inoculated branches. Length of vascular discoloration averaged 62.5 mm in branches inoculated with the four C. pulchella isolates and 16.5 mm in the control twigs. No fungi were reisolated from the slightly discolored tissue of the controls. To our knowledge, this study constitutes the first report of C. pulchella as a pathogen of sweet cherry trees in California.

References: (1) M. E. Barr. Mycologia 77:549, 1985. (2) U. Damm et al. Persoonia 20:39, 2008.