Etiology, epidemiology and pathogen biology of Esca disease of grapevines in California

Esca, also called black or Spanish measles, is a significant disease of table, raisin and wine grape cultivars worldwide. Phaeomoniella chlamydospora and Phaeoacremonium aleophilum are considered the primary pathogens responsible for the disease. In this study, the teleomorph of P. aleophilum was discovered using in vitro pairings of compatible California isolates and identified as Togninia minima, an Ascomycete in the Diaporthales. Ascospores of T. minima were capable of both forcible and passive release from perithecia after sufficient wetness. Perithecia of T. minima were also identified for the first time on standing grapevines in infected California vineyards, suggesting ascospores as a possible source of inoculum. Togninia minima ascospores and conidia were both shown to effectively colonize grapevine pruning wounds. Wounds of other hosts were also shown to be susceptible to T. minima and Pa. chlamydospora. Thirty months after artificial inoculation, T. minima could be recovered from apple, cherry, oak, peach, walnut, almond and maple. However, T. minima could not be recovered from willow and alder. Pa. chlamydospora was recovered from all hosts except willow, alder and walnut. Togninia minima perithecia also formed in vitro on the wood of 13 different hosts, with the highest number on apple. Additional in vitro parings showed that both temperature and light have an effect on T. minima perithecial formation. Significantly higher numbers of perithecia developed on plates placed at 20° and 25°C, while no perithecia formed at 5°, 10° and 35°C. Molecular analyses of the ITS region and the beta-tubulin gene of the entire Phaeoacremonium collection at UC Davis showed that 7 additional Phaeoacremonium species occur in grapevines in California. Many of these species have not previously been reported on grapevines in California and a few have never been reported on grapevines worldwide. Although their pathogenicity is unknown, Cadophora luteo-olivacea and species of Phialophora were also isolated from affected California grapevines.