<p style="text-align: justify;"><strong>Aims</strong>:</p><p class="Tabelle" style="text-align: justify;">Under Central European climatic conditions, bunch rot caused by <em>Botrytis cinerea</em> occurs virtually every season on <em>Vitis vinifera</em> 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.</p><p class="Tabelle" style="text-align: justify;"><strong>Methods and results</strong>:</p><p class="Tabelle" style="text-align: justify;">Sigmoidal regression models were used to describe the disease progress as function of thermal time. Coefficients of determination were &gt; 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.</p><p class="Tabelle" style="text-align: justify;"><strong>Conclusions</strong>:</p><p class="Tabelle" style="text-align: justify;">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.</p><p class="Tabelle" style="text-align: justify;"><strong>Significance and impact of the study</strong>:</p><p class="Tabelle" style="text-align: justify;">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.</p>

Molitor, D., Fruehauf, C, Baus, O., and Berkelmann-Loehnertz, B. 2012. A cumulative degree-day-based model to calculate the duration of the incubation period oí Guignardia bidwellii. Plant Dis. 96:1054-1059.The duration of the incubation period of Guignardia bidwellii on leaves and clusters of Vitis vinifera strongly correlates to temperature. To describe this relationship mathematically, a new, cumulative degreeday-based model was developed. According to this model, first symptoms on leaves appear after reaching a threshold of 175 cumulative degree-days (calculated as the sum of average daily temperatures between 6 and 24°C starting on the day after the infection). On clusters, the duration of the incubation period is additionally affected by their respective developmental stages. For 'Riesling', the duration of the incubation period on clusters corresponds to the duration on leaves until reaching £he phenological stage "berries beginning to touch" but extends continuously with ongoing phenological development. Therefore, a correction factor recognizing cluster phenology was derived to calculate the cumulative degree-day thresholds for the occurrence of first symptoms on clusters after reaching "majority of berries touching". Hence, this present model allows the estimation of fungal development and forecasts the appearance of new symptoms on leaves as wel as on clusters, enabling growers to more precisely schedule curative as well as protective fungicide applications against grape black rot.

Strobilurin or triazole type fungicides have proven to be very effective in strategies to control grape black rot (causal agent: Guignardia bidwellii) due to their combined protective and curative activities. However, differences in their long-lasting protective capacity or in the limitation of the time frame for curative treatments have not yet been clearly described regarding this pathogen. To address these issues, trials with artificial inoculation were conducted using one representative of each group, pyraclostrobin and myclobutanil on leaves of potted vines as well as on berries of field grown vines. It could be demonstrated that pyraclostrobin possesses excellent pre-infection as well as post-infection activities on both, leaves and berries. Post-bloom, pyraclostrobin was able to completely inhibit infections on berries up to three weeks after application. Full curative control of developing black rot infections was achieved for up to 85% of the total incubation period. Myclobutanil turned out to be of limited protective activity. If applied the day prior to inoculation, the efficacy level reached around 90% but decreased continuously with increasing temporal distance between application and inoculation. However, curative applications of myclobutanil in the first half of the incubation period completely prevented symptom development on leaves as well as on berries. Due to their combined reach-back and forward activity to control black rot, both, pyraclostrobin as well as myclobutanil can be recommended in practical viticulture, especially in the most susceptible period between flowering and berries beginning to touch. Pyraclostrobin offers an exceptional protective and long-lasting curative potential, whereas myclobutanil is most effective in post-infection use.

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