Production of Guignardia bidwellii conidia on grape leaf lesions is influenced by repeated washing events and by alternation of dry and wet periods

The ascomycete Guignardia bidwellii is an economically important pathogen in many grapevine-growing areas. Primary infections are caused by ascospores and conidia produced in mummified berries and in cane lesions. Secondary infections are caused by the conidia produced by pycnidia formed in leaf lesions and, in later season, in rotted berries. Environment-controlled experiments were conducted to study the production dynamics of G. bidwellii conidia on grape leaf lesions as influenced by: i) repeated washing events, and ii) alternate dry and wet periods. Under optimal environmental conditions (25 °C, 100 % relative humidity), production of conidia declined over washings and was almost completely depleted after four washings. When pycnidia were kept in a low humidity environment (average of 54 % relative humidity) between two successive washings, the production of conidia progressively diminished as the time between washings increased, with few conidia being still produced after 87 days. This decline in conidial production was faster at 29 °C than at 20 °C. This information is relevant in that it determines the potential of black-rot lesions to produce conidia along the grape-growing season and, therefore, their contribution to epidemic development.