Disease Note.

First Report of Pleurotus ostreatus Causing a Wood Rot of Grapevines in California Vineyards. M. E. Vail, Vino Farms, Inc., 1377 E. Lodi Ave., Lodi, CA 95240. W. D. Gubler and J. E. Adaskaveg, Department of Plant Pathology, University of California, Davis 95616. Plant Dis. 79:1187. Accepted for publication 13 September 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-1187C.

In 1994, cream-colored, agaricoid basidiocarps were observed on grapevines in two adjacent 20-year-old commercial vineyards in Lodi, San Joaquin County, California. Based on macroscopic and microscopic fruiting body characteristics, the fungus was identified as Pleurotus ostreatus (Jacq.:Fr.) P. Kumm. (sensu lato). The white spore print, small cheilocystidia, hardwood substrate, and geographical distribution probably indicate that the species is P. ostreatus (sensu stricto); however, Vilgalys et al. (1) have indicated that mating compatibility tests are needed for reliable identification of species in the P. ostreatus complex. In one vineyard (11.5 ha), the grape (Vitus vinifera L.) cultivar was Cabernet Sauvi-gnon whereas, in (he other vineyard (13.6 ha), the cultivar Napa Gamay was grafted or "top-worked" in 1989 to Cabernet Sauvignon with a modified whip-graft procedure. For this, a transverse "bench cut" of the grapevine trunk is made, the scion wood is inserted into splits made in the wood, cambial tissues of rootstock and scion are aligned, and the exposed wood surface is coated with a tree sealant. During the mild (average temperatures 8.3°C), wet (4.6 cm rainfall) month of January 1994, basidiocarps of the fungus commonly developed immediately below the transverse, grafting cut of vines in the "top-worked" vineyard. Subsequently, a survey of 1,448 vines (1.1 ha) indicated that 183, or 12.6%, of the vines were observed to have basidiocarps. In (he noncon-verted vineyard, however, only one, or 0.1%, of the vines had a fruiting body of the fungus in a survey of 1,052 (0.8 ha) vines. Diseased vines had a punky, white rot of the secondary xylem. The relatively high incidence of this fungus in the "top-worked" vineyard indicated that the modified whip-graft procedure described above may provide an ideal infection court and that the tree sealant used in the grafting procedure apparently had no effect in preventing the establishment of this species of Pleurotus. Although affected vines showed no symptoms of stress and remain economically productive, the wood-decayed vines are predisposed for structural failure under environmental stress or crop load.