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Shijian Zhuang, Letizia Tozzini, Alan Green, Dana Acimovic, G. Stanley Howell, Simone D. Castellarin and Paolo Sabbatini

Achieving desired fruit quality at harvest in cool climate conditions is a challenge, especially for red varieties, and the typical inability of fruit to reach technological maturity is a critical contributing factor requiring examination. To probe this issue, this research investigated the impact of two levels of crop thinning and of basal leaf removal at three phenological stages in the 2011 and 2012 growing seasons in Michigan. Experiments were conducted at the Southwest Michigan Research and Extension Center (SWMREC) in Benton Harbor. Using ‘Cabernet franc’ (Vitis vinifera L.) vines, yield components (yield per vine, pruning weight, and cluster and berry weight) and basic fruit composition traits [total soluble solids (TSS), pH, titratable acidity, anthocyanins, and phenolics) were studied to investigate the effect of cluster thinning and basal leaf removal on vine performance and fruit quality at harvest. Neither of the treatments significantly impacted TSS in either of the two seasons. Cluster thinning treatment successfully altered cropload ratio, indexed as Ravaz Index (RI), independently of the time of application. Basal leaf removal increased exposed berry temperature, cluster light exposure, and subsequent anthocyanin and phenolic content of the berry in both seasons, again independent of application date, whereas cluster thinning was effective only in 2012. Crop thinning coupled with basal leaf removal resulted in an increased efficiency in anthocyanin accumulation in relation to TSS accumulation, expressed as anthocyanin:sugar, in both years. This is significant because it offers potential for vineyard management practices aiming to improve fruit quality in cool climates where the onset of anthocyanin accumulation could be reduced and decoupled from sugar accumulation.

Open access

S. Kaan Kurtural, Andrew E. Beebe, Johann Martínez-Lüscher, Shijian Zhuang, Karl T. Lund, Glenn McGourty and Larry J. Bettiga

A field study was conducted for three consecutive seasons in the hot climate of central California to assess the performance of ‘Merlot’ grapevine (Vitis vinifera) grafted onto ‘Freedom’ [Fresno 1613-59 × Dog Ridge 5 (27% V. vinifera hybrid)] during training system conversion to facilitate mechanization. The traditional head-trained and cane-pruned (CP) system was either retained or converted either to a bilateral cordon-trained, spur-pruned California sprawl training system (HP), or to a bilateral cordon-trained, mechanically box-pruned single high-wire sprawling system (SHMP). After the conversion, SHMP sustained greater yield with more clusters per vine and smaller berries without affecting the canopy microclimate. This was due to a higher number of nodes retained after dormant pruning. The SHMP canopies, compared with CP and HP; filled allotted canopy space earlier based on photosynthetically active radiation (PAR) transmitted through the canopies, populating the space allotted per vine, favoring higher production efficiency. There were no adverse effects of training systems on berry composition or flavonoid concentration, during or after conversion to mechanical management. However, experimental year effect was obvious on anthocyanin composition of ‘Merlot’ berries, increasing trihydroxylated (i.e., delphinidin-based) anthocyanins in the latter years of the experiment. Our results also provided evidence that earlier canopy growth coupled with sufficient reproductive compensating responses allowed for increased yields while reaching commercial maturity without a decline in anthocyanin content with the SHMP. Converting CP to SHMP reduced labor operations costs by 90%. Furthermore, the SHMP had greater gross revenue and resulted in greater net income per acre even when the conversion year was taken into account. Therefore, SHMP is recommended for growers within the hot climate of the central San Joaquin Valley as a means to maintain productivity of vineyards while not sacrificing berry composition at the farm gate.