Meteorological conditions determine the thermal-temporal position of the annual Botrytis bunch rot epidemic on Vitis vinifera L. cv. Riesling grapes

摘要

Aims:Under Central European climatic conditions, bunch rot caused by Botrytis cinerea occurs virtually every season on Vitis vinifera L. cv. Riesling grapes. Statistical investigations based on at least three annual disease severity assessments in 7 seasons (2007-2013) aimed at (i) simulating the disease progress and (ii) identifying meteorological conditions with predictive value for epidemics.Methods and results:Sigmoidal regression models were used to describe the disease progress as function of thermal time. Coefficients of determination were > 0.97. The thermal time adjusted pace of the epidemic was almost constant in all seasons while the point of time when 5% disease severity was reached varied among years. Window pane analyses showed that relatively low temperatures and wet conditions during bloom as well as relatively high temperatures and low precipitation sums around/after veraison were associated with thermal-temporally late epidemics.Conclusions:Environmental conditions determine the timing of annual bunch rot epidemics. Analyses indicate a strong link between meteorological conditions around grape bloom (probably affecting fruit set and cluster structure) and the predisposition of the grape clusters to bunch rot.Significance and impact of the study:The enhanced understanding of the effect of environmental conditions on the bunch rot epidemics supports growers to optimize control measures and is supposed to result in a Botrytis bunch rot model. IntroductionBunch rot also referred to as Botrytis bunch rot or grey mould caused by Botrytis cinerea Pers.:Fr. (teleomorph: Botryotinia fuckeliana (de Bary) Whetzel) is a major fungal disease on more than 200 mainly dicotyledonous plant species (Williamson et al., 2007) including grapevine (Vitis vinifera L.), where it causes severe economic damage worldwide (Kassemeyer and Berkelmann-L?hnertz, 2009). Besides the loss of yield, B. cinerea can reduce wine quality in terms of off-flavors, unstable color, oxidative damages, premature aging and difficulties in clarification (Ribéreau-Gayon, 1983; Smart and Robinson, 1991; Ky et al., 2012).B. cinerea is considered as an opportunistic pathogen infecting plant tissues primarily via wounds (Elmer and Michailides, 2007; Evans, 2008). Grape flowers are susceptible to infections since they represent natural apertures and provide sugars, which foster the colonization (Keller et al., 2003; Viret et al., 2004). Between fruit set and veraison, young, immature grape berries are highly resistant to B. cinerea (Hill et al., 1981). Here, constitutive and inducible defense mechanisms (Elmer and Michailides, 2007) leading to a stop of the infection in the penetration stage (Kelloniemi et al., 2015) account for a low susceptibility. Since the levels of fungistatic substances decline with maturity and micro-cracks in the berry skin occur more frequently after veraison, host defense progressively breaks down with grape maturation (Kretschmer et al., 2007). Hence, infections of ripe berries are most common and destructive (Shtienberg, 2007).Under the climatic conditions of many traditional (non-irrigated) grapegrowing regions, grape bunch rot occurs virtually every season. The time course of the B. cinerea disease severity was shown to be well described by logit, exponential or sigmoidal functions (Beresford et al., 2006; Evers et al., 2010; Hill and Beresford, 2010; Molitor et al., 2015a). However, the starting point of bunch rot outbreaks is considered as highly unpredictable (Redl et al., 2014), and forecasting the severity of bunch rot at harvest continues to represent a challenge (Evans, 2008).Since the development of grape bunch rot on grapes cannot be suppressed completely under the climatic conditions of many viticultural regions, bunch rot control strategies primarily aim at delaying the epidemic as much as possible to take benefit of a long-lasting maturation period prior to enforced (due to the declining grape health status) harvest date (Molitor et al., 2015a). In the past, control strategies were mainly based on routine applications of fungicides at pre-determined intervals (Shtienberg, 2007). Since pesticide use shall be minimized in Integrated Pest Management, excessive chemical treatments are becoming increasingly criticized and restricted (Elmer and Michailides, 2007; Shtienberg, 2007). A reduction of fungicide use in integrated bunch rot control strategies could be realized by (i) crop cultural measures that suppress fungal infections and spread and (ii) more targeted timing of botryticide treatments based on reliable forecast and decision support systems. For instance, the decision for botryticide applications could be guided by a warning system, which attempts to recognize weather conditions highly conductive for spore germination and to schedule fungicide applications accordingly (Shtienberg, 2007). Such efficient decision support systems for targeted control strategies need to incorporate the (potentially