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Until recently, commercial grape producers in Alabama and the Southeast have been restricted to growing native muscadines and hybrid bunch grape cultivars due to the looming presence of Pierce's Disease (PD), caused by Xylella fastidiosa, an endemic xylem clogging bacterium that is deadly to susceptible European (Vitis vinifera) grapevines.
In response to the spreading threat of PD in California vineyards, the U.C. Davis grape breeding program is developing high quality PD resistant European grape selections. Three of their advanced selections were obtained and planted in 2010 at an experimental vineyard at the Chilton Research and Extension Center (CREC) located in Chilton County, AL to examine the feasibility of cultivation within Alabama's high Pierce's disease pressure environment. Detailed research is being conducted to assess the phenological development, vine physiological responses, and fruit quality characteristics for the three experimental selections, namely: '502-10', '502-01', and '501-12'.
A new crop for Alabama, European grapevines are trained in a different manner than muscadine grapes. The training system facilitates the upright growing habit of European grape cultivars. Rather than allowing fruiting canes trained to a single-wire bi-lateral cordon to grow downwards, V. vinifera fruiting canes are instead trained upwards and periodically directed within a vertical shoot positioning (VSP) system. This training system facilitates efficient pest management practices, while concentrating the crop load within a compact fruiting zone (Fig. 1).
Figure 1. Ripening clusters of V. vinifera selection '502-01' within fruiting zone of a VSP trained vine at the Chilton REC, AL, 2017.
Studying the vines' development throughout the growing season allows for development of proper management techniques in a given set of environmental conditions. For the three selections, bud break usually occurs by the first week of April and canopy formation is completed by late April. Flowering is initiated in the first days of May, and full bloom occurs within a week.
Veraison is the stage associated with grape ripening. It starts when the berries start turning their color from green to black or fully colored berry, and sugar accumulation increases. Veraison takes about 40-50 days depending on the particular cultivar and continues until harvest. The three selections studied vary in color accumulation timing, with '502-10' being fully colored early season, usually in July, and attains maturity in mid-August. The remaining two selections complete veraison stage in late September and mid-October.
Figure 2. 2012-2017 cumulative yield of PD resistant European grape selections grown at the CREC, Clanton, AL.
For the period of 2012-2017, the total cumulative yield per vine was 85.7 lb for early ripening selection '502-10', with an average per season yield per vine of 14.3 lb (Figure 2). The highest cumulative yield of 94.9 lb/vine was recorded for the late ripening '501-12' with an average per season crop of 15.8 lb. Yield data indicates all of the studied PD resistant European grape selections are highly productive in Alabama environment, producing well above the suggested optimal crop level for vinifera grapes of between 8 and 12 lb/vine.
None of the experimental vines studied under Alabama's high PD pressure conditions have exhibited PD symptoms and there have been no vine losses resulting from other pathogens. The results for vine growth, performance, and fruit quality are very promising demonstrating the PD resistant European grape selections have the potential to diversify and improve the grape production sustainability in the southeastern region and enhance the local food systems.
Ext. Fruit Crops Specialist, ACES
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