First Report of Bot Canker Caused by Diplodia corticola on Coast Live Oak (Quercus agrifolia) in California
Authors: Lynch S. C. , Eskalen A. , Zambino P. , Scott T. 
Published: October 2010

Sharp decline and mortality of coast live oak (Quercus agrifolia) has been observed in San Diego County, CA since 2002. Much of this decline has been attributed to a new pest in California, the goldspotted oak borer (GSOB, Agrilus coxalis) (1). Symptoms include crown thinning, bark cracking and/or peeling, patches of stain (1 to 10 cm in diameter), bleeding on the bole, and tree death and are most often observed on trees with a diameter at breast height (DBH) >30 cm. In 2008, a Botryosphaeria sp. was recovered from necrotic tissue of bleeding bole cankers from GSOB-affected trees in Jamul, CA. Zone lines separated dead and live tissue in affected phloem and xylem. Pycnidia were observed on the bark surface of the infected host. Fifty conidia averaging 32 × 18 μm, one-septate with age, and morphologically similar to conidia described by Úrbez-Torres et al. were observed (4). Oak stands with tree mortality were surveyed in GSOB-infested and -uninfested sites over eight locations throughout San Diego and Riverside counties in 2009 and 2010. Symptomatic tissue or conidia from pycnidia of affected trees, plated onto potato dextrose agar amended with 0.01% tetracycline and incubated at 25°C for 1 week, consistently produced cultures with dense, wooly, olive-green mycelium. Mycelia fit the description of Botryosphaeria corticola A.J.L. Phillips, Alves et Luque (anamorph Diplodia corticola) (2). The resulting amplified ITS4/5 region of two sequences matched 100% to published D. corticola sequences (GU799472 and GU799460) (4). These sequences were deposited with NCBI GenBank (HM104176 and HM104177). Koch's postulates were conducted by inoculating 2-mm-diameter holes on five coast live oak trees with D. corticola. Holes were drilled to the cambium at 2 to 4 locations per tree within 1 to 2 m up the bole using a 0.157-cm portable electric drill. Trees ranged from 3.7- to 32.4-cm DBH. Either single agar plugs from two isolates each of a 7-day-old culture (UCR454 and UCR793) or noncolonized agar plugs as uninoculated controls were inserted into the holes and then covered with petroleum jelly and Parafilm. Average temperature was 10°C, relative humidity of 64%, and no precipitation during inoculation. Inoculations were conducted at a location in San Diego County uninfested by GSOB and repeated twice. After 3.5 months, bark was removed from inoculation sites. Average lesion length was not significantly different between inoculations, thus data were combined (one way analysis of variance [ANOVA]; P = 0.05). Lesions averaged 13.9 × 2.3 cm and were significantly different (n = 30; one way ANOVA; P = 0.05) from controls that measured 0.31 × 0.3 cm. Staining was observed around the inoculation points on all trees and three trees exhibited bleeding. Necrotic tissue was observed in the phloem and 3 mm into the xylem tissue, where the lesion had extended up and down the grain. D. corticola was consistently reisolated from necrotic tissue but not from control treatments. B. corticola was originally described as a canker pathogen on Quercus spp. in the western Mediterranean (2), and is known to contribute to the decline of cork oak (Q. suber) in the region (3). To our knowledge, this is the first report of D. corticola causing bot canker on coast live oak in California.

References: (1) T. W. Coleman and S. J. Seybold. U. S. For. Serv. R5-PR-08, 2008. (2) A. Correia et al. Mycologia 96:598, 2004. (3) J. Luque et al. For. Pathol. 38:147, 2008. (4) J. R. Úrbez-Torres et al. Plant Dis. 94:785, 2010.