A model for black rot disease on grapevine

Guignardia bidwellii (Ellis) Viala & Ravaz (anamorph: Phyllosticta ampelicida (Englem.) van der Aa) is the causal agent of the black-rot disease on grapevine. In the recent years, there was a recrudescence of the disease across Europe with severe epidemics. Black rot poses a serious threat to both yield, with losses from 5 to 80%, and wine quality. Black-rot requires fungicide control between bud break and berry touch or behind, which is the period in which vineyards need to be protected against downy and powdery mildews. In recent years, innovative model-based fungicide applications have been introduced which lead to reduce the use of fungicides against mildews by 30-40% but, at the same time, keep vines unprotected against black-rot when there is no risk for mildews. A model for black-rot should be then useful to decide the need of specific sprays or the necessity to control mildews by using fungicides with either preventative or curative activity against G. bidwelii. Simple models have been developed for black-rot based on the length of the wetness period and the respective temperature conditions required for an infection to occur. These models have limitations in which they do not consider neither the presence and abundance of inoculum nor variations in host susceptibility; in addition, these models do not provide quantitative estimates of risk. A prototype of a new, mechanistic, weather- and plant-driven model for black-rot was developed according to systems analysis. The relational diagram of the model considers the main stages of the life cycle of G. bidwelii which represent the state variables of the model. The model starts with three state variables which correspond to the three overwintering forms (pycnidia and pseudothecia on mummified bunches and pycnidia on infected lesion on canes and spurs) where ascospores and conidia progressively form and mature. Then, air-borne ascospores and splash-borne conidia are dispersed and deposited on vines where they can cause infection under favorable weather conditions, survive and cause infection later, or dye under unfavorable conditions. Black-rot symptoms appear after an incubation period as cohorts of visible lesions, which produce pycnidia after a latency period. Both lesions and pycnidia of a cohort do not appear simultaneously but are distributed over a time period whose length mainly depends on temperature. Pycnidia of a cohort continue to produce conidia throughout an infectious period; conidia exudate from pycnidia under specific moisture conditions, are dispersed by rain splashes and can infect the host causing new (secondary) infections. The model was evaluated for its ability to represent the real system and its usefulness in helping understanding black-rot epidemics over three representative epidemics in North Italy. Results are very promising and encourage a further validation in multiple years and vineyards.