The botryosphaeriaceous fungus Diplodia corticola A. J. L. Phillips, Alves & Luque was shown to be the most prevalent canker and dieback pathogen in cork oaks (Quercus suber L.) in the Iberian Peninsula causing a general decline of the trees as a consequence of canker formation in the trunks (1). In addition, D. corticola has been recently reported as a grapevine pathogen causing cankers in the vascular tissue of 1-year-old canes, spurs, and cordons in Texas (3). In 1998, Jacobs and Rehner reported one isolate of D. corticola from oak in California, but no information regarding the oak species from which the isolate was obtained and its virulence were available in the study (2). In 2009, D. corticola was isolated on potato dextrose agar (PDA) amended with 0.01% tetracycline hydrochloride from symptomatic grapevine cordons and on acidified PDA from the trunk of a canyon live oak tree from Sonoma and Plumas counties, respectively. Two grapevine isolates (UCD1260So and UCD1275So) and one oak isolate (CDFA519) were examined and morphologically compared with previously identified D. corticola isolates CBS678.88 and UCD2397TX from cork oak from Spain and grapevine in Texas, respectively. D. corticola colonies from California were characterized by moderately fast-growing, dark olivaceous, and dense aerial mycelium on PDA. Conidia were obtained from pycnidia formed on pine needles placed on 2% water agar. Conidia were hyaline, contents granular, aseptate, thick walled, ellipsoidal, sometimes becoming dark brown and septate with age. Nucleotide sequences of three genes (ITS1-5.8S-ITS2, a partial sequence of the beta-tubulin gene BT2, and part of the translation elongation factor EF1-α) from D. corticola isolates UCD1260So, UCD1275So, and CDFA519 from California were amplified. All DNA sequences from grapevine and oak tree isolates from California showed 99 to 100% homology with D. corticola isolates previously identified and deposited into GenBank. All DNA sequences obtained from Californian isolates were also deposited into GenBank. Pathogenicity tests were conducted by inoculating detached Vitis vinifera cv. Red Globe dormant canes and canyon live oak branches with agar plugs of isolates UCD1260So, UCD1275So, and CDFA519 (10 inoculations per isolate per host) as described by Úrbez-Torres et al. (3). The same number of grapevine canes and oak branches were inoculated with noncolonized agar plugs as controls. Six weeks after inoculation, the extent of vascular discoloration that developed from the point of inoculation was measured. D. corticola isolates UCD1260So, UCD1275So, and CDFA519 caused an average vascular lesion length of 30.4, 29.6, and 24 mm and 15, 13.2, and 8.6 mm in grapevine dormant canes and oak branches, respectively. Furthermore, D. corticola isolates from grapevine were pathogenic in oak branches and vice versa. Reisolation of D. corticola from discolored vascular tissue of inoculated material was 100%. The extent of vascular discoloration from inoculated grapevine canes and oak branches was significantly greater (P < 0.05) compared with the controls (1.8 and 2 mm, respectively). No fungi were reisolated from the slightly discolored tissue of the controls. To our knowledge, this is the first report of D. corticola causing grapevine cankers and oak trunk cankers in California.
References: (1) A. Alves et al. Mycologia 96:598, 2004. (2) K. A. Jacobs and S. A. Rehner. Mycologia 90:601, 1998. (3) J. R. Úrbez-Torres et al. Am. J. Enol. Vitic. 60:497, 2009.
