UCD GRN 1

Origin History and Economic Importance

The grapevine rootstock UCD GRN 1 was developed as part of a breeding program initiated in 1990 at the University of California, Davis (UC Davis) by Andrew Walker and Harold P. Olmo. The parental cross was performed in 1993–1994, and field evaluation of the progeny for nematode resistance began in 1996. The primary economic importance of this rootstock lies in providing extremely strong and broad resistance against soil-borne pests. The rootstock variety UCD GRN 1 was designed specifically for vineyard sites with severe and chronic infestations of nematodes and grape phylloxera. It serves as one of the rare and unique sources of genetic resistance to the ring nematode (Mesocriconema xenoplax) and demonstrates complete stability against aggressive strains of root-knot, dagger, citrus, and lesion nematodes. Furthermore, this grapevine rootstock possesses the potential ability to induce scion tolerance to fanleaf degeneration (grapevine fanleaf virus), a disease vectored by nematodes, thereby preventing disease expression in the grafted scion. The rootstock UCD GRN 1 positively impacts scion combinations, delivering an 80% success rate for both rooting and grafting from dormant cuttings.

Morphological Description of the Rootstock Variety UCD GRN 1

The morphological characteristics of this hybrid combine traits from both parental species, though the variety displays a prominent resemblance to Vitis rupestris in its shoots and leaf blades.

• Shoot tips: Exposed, featuring light green expanding leaves with large, broad, angular teeth.
• Young leaves: Round to reniform, entirely glabrous (lacking hair), light green in color, with margins covered in large, broad, angular teeth.
• Mature leaves: Small to medium in size (average blade measurements are 6.5 cm in length and 8.8 cm in width). The leaf blade shape is reniform to round. The upper surface is glabrous, waxy, and folds slightly on itself along the main vein, visually resembling the morphology of Vitis rupestris. The lower leaf surface is also glabrous but less waxy. The teeth are relatively large and broad, varying from angular to convex. The petiole is relatively short. The petiolar sinus is a broad V-shape with a slight V-notch directly at the petiolar attachment.
• Canes (vines): Mature canes are brown, round in cross-section, and covered with lenticels and tightly adhering bark. They are characterized by relatively short internodes and extensive lateral shoot development. A diaphragm is absent at the nodes. The wood is green, and the pith is green, small, and dense (not spongy).
• Flowers: Morphologically appear to be hermaphroditic; however, mother vines bear sterile flowers due to a triploid chromosome number (2n=39).

Nursery Management and Cultivation Features

In nursery conditions, the rootstock UCD GRN 1 demonstrates high technical feasibility, disproving initial breeder concerns regarding poor rooting performance typically associated with distinct Muscadinia rotundifolia phenotypic traits. The standard output of first-grade vines during rooting and bench grafting reaches an 80% success rate. However, nursery management must account for winter temperature factors: extreme mid-winter drops (down to -5 °C or below) can completely block the rooting and grafting capacity of cuttings collected in February. Plants of this rootstock variety develop moderately deep rooting angles. In mother blocks, the rootstock UCD GRN 1 produces moderately long canes with short internodes, requiring timely canopy management to control leaf mass due to its tendency for strong and extensive lateral shoot production.