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February 2017 WINES&VINES 73 PRACTICAL WINERY & VINEYARD GRAPEGROWING HoytShepston_Monthly09 10/31/08 4:44 PM and pruning date on median EL phenological stage for any date except May 19. The lack of a significant interaction between treatment and pruning date showed that phenological de- velopment of shoots with differ- ing bud break dates on one vine behaved as if they were on sepa- rate vines. The lack of an interaction is evidence to support the hypoth- esis of phenological shoot au- t o n o m y s i n c e t h e r e w a s n o synchronization after bud break of phenology of shoots on the same grapevines. The results of this trial are similar to the obser- vation of M. Keller and J.M. Tar- ara that persistent, season-long differences in phenological shoot development are to be expected due to variable bud break. 4 There is evidence that winter temperatures can have an im- pact on variability of bud break on one vine and hence variabil- ity in flowering and véraison. D. Gadhouri showed that variabil- ity of bud break was greater in a region with warmer winter temperatures than in a grape- growing region with colder win- t e r t e m p e r a t u r e s . 3 I n s h o r t , warmer winter temperatures resulted in greater asynchronic- ity of phenological development in grapevines. The impact of climate change on winter temperatures and hence phenological asynchronicity is a concern. In addition to the poten- tial impacts of uniformity of fruit ripening, the impact of greater asynchronicity due to higher win- ter temperatures on resistance to plant diseases acquired through phenological (also called onto- genic) development such as pow- dery mildew is also a concern. 3 Shoots and clusters with delayed phenological development are, therefore, more susceptible to powdery mildew since they have not achieved the stage for onto- genic resistance to the pathogen. The bloom date (EL 23, or 50% cap fall) for shoots on early and late-pruned spurs was extrapo- lated from the weekly observa- PROBABILITY OF LOGISTIC REGRESSION Date Treat Prune Treat*Prune March 23 0.09 <0.0001 0.64 March 30 0.16 <0.0001 0.62 April 10 0.78 <0.0001 0.69 April 20 0.16 <0.0001 0.71 May 5 0.70 <0.0001 0.49 May 11 0.25 <0.0001 0.15 May 19 0.32 <0.0001 0.01 June 1 0.39 <0.0001 0.21 June 9 0.28 <0.0001 0.79 June 15 0.18 <0.0001 0.34 July 9 0.23 <0.0001 0.40 July 15 1.00 <0.0001 1.00 July 20 1.00 <0.0001 0.09 July 28 0.07 <0.0001 0.23 Aug. 5 0.16 <0.0001 0.22 Probability (p-value) of logistic regression chi square for treatment, pruning date and interaction.