UCD GRN-4
| Variety Passport | |
| Country of origin: | USA |
| Breeder: | University of California, Davis |
| Variety number VIVC: | 25636 |
| Prime name: | UCD GRN 4 |
| Breeding number: | 9365-85 |
| Patent: | US PP21,358 P3, USA |
| Principal Synonyms: | 9365-85, UC DAVIS GRN 4 |
| Year of obtaining / registration: | The breeding program commenced in 1990, the parental cross was made in 1993–1994, and the patent was issued on October 5, 2010. |
| Pedigree given by breeder: | Davis L 514-10 x Vitis Champinii C 9038 |
| Pedigree confirmed by markers: | Davis L 514-10 x Vitis Champinii C 9038 |
| Rootstock type: | Phylloxera-resistant, Nematode-resistant |
| Data Sources: | Vitis International Variety Catalogue (VIVC), University of California, Davis (UC Davis), Google Patents |
| Agrobiology | |
| Lateral shoot formation: | weak |
| Flower type: | male |
| Nematode susceptibility: | 1 point (highly resistant) |
| Root phylloxera susceptibility: | 1 point (highly resistant) |
| Rooting ability: | high |
| Graft compatibility with Vitis vinifera: | high |
| Recommended soil types: | Recommended for vineyard sites with severe and chronic nematode infestations. |
| Ampelographic overview |
History of Origin and Agricultural Importance
The grapevine rootstock variety UCD GRN-4 was developed by American geneticist Andrew Walker at the University of California, Davis. A targeted breeding program was initiated in 1990 to provide a group of rootstocks with broad and durable resistance to soil-borne pests. The parental cross to develop this hybrid line, which was experimentally tested under the selection number 9365-85, was performed in 1993–1994, and vineyard-level evaluation of the progeny began in 1996. The official plant patent for this rootstock was registered on October 5, 2010.
The primary agricultural value of this hybrid lies in its comprehensive protection of the root system against a wide range of parasitic worms in vineyard sites with severe pest pressure. The grapevine rootstock UCD GRN-4 exhibits strong resistance to root-knot nematodes, including the highly aggressive HarmA and HarmC strains, as well as to dagger nematodes (Xiphinema index), citrus nematodes, and lesion nematodes. Long-term greenhouse testing under optimized feeding conditions and field trials in Fresno and Santa Maria counties demonstrated minimal root gall formation. However, this variety is moderately susceptible to ring nematodes.
In addition to its nematode defense traits, the UCD GRN-4 rootstock possesses strong resistance to grape phylloxera. In laboratory bioassays on young root pieces, the average population increase rate of the aphid on root nodosities was only 1.99, confirming a reliable biological barrier. The variety UCD GRN-4 shows excellent grafting compatibility with popular wine and seedless cultivars of Vitis vinifera, such as Chardonnay and Fiesta Seedless, causing no scion suppression and ensuring balanced growth and stable productivity.
Morphological Description of the Rootstock Variety UCD GRN-4
Over years of observation, scientists compiled the following morphological description of the variety:
- Flowers: Functionally staminate (male type). Pollen is completely non-viable, and the anthers are poorly developed. Flower clusters are small, and the vine does not set fruit or produce berries.
- Mature Leaves: Leaves are medium-sized, broad cuneiform-shaped, with slight three-lobed differentiation. The leaf blade is relatively thin and broad-rugose, with characteristic indented puckering near the petiole. The upper surface is glabrous and dark green, while the lower surface features scattered arachnose tomentum, fine bristles along the main veins, and small tufted bristles (domatia) at the vein intersections. The petiolar sinus is a narrow U to lyre shape. The leaf margin is distinctly and uniformly dentate with long, sharply pointed teeth, closely resembling the Vitis riparia type.
- Young Shoots and Apex: Vigorously growing shoot tips of the UCD GRN-4 rootstock are enclosed in expanding young leaves, featuring a typical green color without distinct anthocyanin coloration. The young three-lobed leaves are mostly glabrous with a fine arachnose tomentum.
- Tendrils: Well-formed, long, and slender, often exceeding the length of the leaf blade. Structurally, the tendrils are bifid, completely glabrous, and follow a discontinuous arrangement on the shoot.
- Petioles: Typically measure 4 to 5 cm in length with a diameter of 3 to 4 mm. The coloration of the petiole matches the upper surface of the leaf blade.
- Canes (One-Year-Old Wood): Mature canes are oval in cross-section and dark brown in color. The internodes are long, typically measuring 15 to 20 cm. Lateral shoot development on UCD GRN-4 vines is highly limited. The nodes bear short, pointed buds, the pith is medium-sized, and the dividing septum (diaphragm) is thin.
Propagation and Nursery Management Practices
In practical nursery management, the UCD GRN-4 rootstock is evaluated as a highly efficient variety with excellent operational characteristics in mother blocks. Due to its long, straight canes with optimal internode lengths and limited lateral development, the vines require significantly fewer manual pruning and desuckering operations during the growing season. Preliminary trials indicate that the harvested cuttings possess good rooting ability, establishing a well-developed root system with moderately deep rooting angles, and deliver a high yield of top-tier grafted saplings from the nursery bed.