Evaluation of Grapevine Rootstocks on Slightly Acidic and Strongly Alkaline Texas Hill Country Soils

Goals: Rootstock selection is usually dictated by vineyard site conditions such as pest and disease pressures, soil, and climate. This work evaluated the effect of rootstock on Sangiovese vine size, nutritional status, and fruit composition at two sites in the Texas Hill Country, one with a slightly acidic (pH 6.58 ± 0.05) loamy fine sand soil, and the other with a moderately to strongly alkaline (pH 8.48 ± 0.06) clay loam soil. Eleven rootstocks representing common rootstocks in Texas (Paulsen 1103, Kober 5BB, Teleki 5C), nematode resistant rootstocks (UCD GRN-1, UCD GRN-2, UCD GRN-3, UCD GRN-4, and UCD GRN-5), and rootstocks with possible tolerance to cotton root rot (Phymatotricopsis omnivora) (Ramsey, Dog Ridge, Florilush) were studied over a five-year period.

Key Findings:

• At the Real County test site, in an alkaline clay loam soil, the rootstocks Dog Ridge and Teleki 5C maintained the most favorable nutrient status, resulting in higher vine vigor and dormant cane pruning weights with pruning weights averaging 0.52 kg/vine and 0.37 kg/vine, respectively, versus 0.09 kg/vine for own-rooted Sangiovese.

• The rootstocks Florilush, UCD GRN-1, UCD GRN-5, and own-rooted Sangiovese performed poorly at the Real County test site as a result of nutritional deficiency.

• At the Gillespie County site, in a slightly acidic loamy fine sand soil, all rootstocks performed acceptably with respect to vine size, nutrition, and fruit composition, although average dormant pruning weights over five years ranged across rootstocks from 0.52 to 0.87 kg/vine.

• Petiole potassium and magnesium were inversely correlated across rootstocks, with the strongest relationship (r² = 0.58) observed in late summer at the Gillespie County site.

• Own-rooted Sangiovese maintained the lowest petiole K and highest petiole Mg concentrations over the study, as well as the lowest juice K and pH at the Gillespie County site.

• Juice K and pH were positively correlated (r² = 0.57 and 0.76) across rootstocks in two out of three years of study at the Gillespie County site.

Impact and Significance: This study demonstrated the importance of rootstock selection on a moderately to strongly alkaline soil versus a slightly acidic soil. Out of the 11 rootstocks tested, only Dog Ridge and Teleki 5C produced an acceptably sized (0.25 and 0.16 kg/m canopy) mature vine (fourth leaf), likely as a result of nutrition. All rootstocks under study resulted in petiole iron concentrations below recommended values, but the severity of shoot tip chlorosis, which is associated with Fe deficiency, was highly rootstock dependent. The rootstocks with the lowest pruning weights and vigor ratings at the Real County site (own-rooted Sangiovese, Florilush, UCD GRN-1, and UCD GRN-5) generally expressed the most shoot tip chlorosis. Addressing this apparent nutritional deficiency with targeted fertilizer applications may have improved vine performance, but this research suggests that proper rootstock selection is a more viable option.

The effect of rootstock was less apparent on slightly acidic loamy fine sand soil. All rootstocks and own-rooted Sangiovese conferred acceptable pruning weights, nutritional status, and fruit composition. Basic juice chemistry did vary by rootstock, with own-rooted Sangiovese having the lowest juice K and pH, but the risks of growing own-rooted Vitis vinifera likely outweigh this potential benefit on sites where climate, soil conditions, pests, or disease may be limiting.