Occurrence of Togninia fraxinopennsylvanica on Esca-Diseased Grapevines (Vitis vinifera) and Declining Ash Trees (Fraxinus latifolia) in California

Abstract

Esca (black measles) and Petri disease (young esca) are two of the most destructive diseases of grapevines in California and other grape-producing countries. This disease is now known to be caused by multiple species of Phaeoacremonium including P. aleophilum, P. angustius, P. parasiticum, P. rubrigenum, and P. mortoniae. The teleomorph of P. aleophilum was confirmed recently as Togninia minima (Tul. & C. Tul.) Berl. (2), but the teleomorph for the other Phaeoacremonium species are not known. During the summer of 2004, plant specimens were collected from declining ash trees surrounding vineyards as well as from grapevines that were showing typical symptoms of esca including the presence of purple-to-brown spots on berries and necrosis between the veins and on margins of leaves. The specimens were cut into small pieces (15 cm), placed in plastic bags, and thoroughly sprayed with 50 ml of sterile distilled water. The specimens were soaked in water for 10 min at room temperature (25 ± 2°C). Plant materials were then removed from the water and air dried under a laminar flow hood for future examination. Each wash solution was passed through 5.0- and 0.45-μm filters attached to a sterile 35-ml syringe. The 5-μm filters removed large fragments of plant tissues and larger spores, while the 0.45-μm filters trapped spores of Phaeoacremonium spp. The 0.45-μm filters were inverted onto 1.5-ml Eppendorf tubes, and 1 ml of sterile distilled water was passed through to wash out the trapped spores. Aliquots of 200 μl were spread onto potato dextrose agar amended with 0.01% tetracycline hydrochloride (PDA-tet). Colonies of P. mortoniae were identified on the plates after 10 days. Previously wetted wood pieces (grapevine and ash) were examined for fruiting bodies with a stereomicroscope. In this study, perithecia of T. fraxinopennsylvanica were discovered for the first time on diseased grapevines in California. Perithecia were embedded in decayed vascular tissue of ash branches as well as pruning wounds, cordons, and trunks of grapevine cv. Riesling from Mendocino County and cv. Thompson Seedless from Madera County. Perithecia were black, globose to subglobose, and ranged in diameter from 183 to 276 μm. They were embedded and superficial and the lengths of the perithecial necks ranged in size from 200 to 400 μm. The asci were hyaline, clavate, and ranged in size from 16.7 to 23.2 × 4.7 to 5.6 μm. Ascospores were hyaline, ellipsoid to allantoid, and ranged in size from 3.9 to 5.5 × 1.4 to 2.0 μm. When plated onto PDA-tet media, ascospores formed colonies typical of P. mortoniae. Perithecia, asci, and ascospore dimensions as well as phylogenetic analysis of sequences from the internal transcribed spacer region confirmed that the perithecia are in fact T. fraxinopennsylvanica (Hinds) Hausner et al. (1) and that this fungus is the teleomorph of P. mortoniae. Perithecia of T. fraxinopennsylvanica were also produced in vitro by crossing compatible isolates of P. mortoniae. Furthermore, spore trapping studies show that propagules of this fungus are present as airborne inoculum in infected vineyards during rainfall events throughout the growing season. References: (1) G. Hausner et al. Can. J. Bot. 70:724, 1992. (2) L. Mostert et al. Mycologia 95:646, 2003.