Release of Guignardia bidwellii ascospores and conidia from overwintered grape berry mummies in the vineyard

Background and Aims: Black rot, caused by Guignardia bidwellii, is a polycyclic grape disease with repeated primary and secondary infections. In this work, the release of primary inoculum (both ascospores and conidia) from overwintered grape mummies was studied over a 3-year period. Methods and Results: A spore sampler was designed consisting of: (i) a funnel for collecting spores washed off from grape mummies by rain; and (ii) microscope slides for spores being airborne in the proximity of mummies. Ascospores and conidia were frequently found in the runoff water; Gompertz equations were developed describing their seasonal cumulative numbers as a function of degree-days, with R2≥0.92. The number of both spore types in the runoff water significantly increased as the amount of rain and its duration and intensity increased; >1 and >3mm of rain were the best cut-off points for predicting the release of conidia and ascospores, respectively. No ascospores were found on the microscope slides, but conidia were found on rainy days, clustered in circular areas, probably due to droplets splashing from mummies to slides during rainfall. Conclusions: This work provides new information on the inoculum dynamics of G. bidwellii in vineyards that can be used in refining disease prediction models for improved disease control. Significance of the Study: New equations were developed to describe the dynamics of G. bidwellii spore release as a function of degree-days, and 1 and 3mm of rain have been proposed as cut-off values to predict the release of conidia and ascospores, respectively. This information could be incorporated into the mathematical models supporting the decision-making for disease control.