VR O39-16

History of Origin and Viticultural Significance

The rootstock VR O39-16 is a unique complex interspecific hybrid of American breeding, developed in response to one of the most pressing challenges in Californian viticulture: infectious degeneration (grapevine fanleaf disease) induced by the grapevine fanleaf virus (GFLV). The virus is vector-transmitted by the soil-borne dagger nematode, Xiphinema index; consequently, nematode management simultaneously prevents the dissemination of the fanleaf virus. Standard rootstocks previously utilized in the United States failed to provide adequate protection for Vitis vinifera scions in infested areas, leading to rapid vine degeneration and yield losses exceeding 80%. The solution to this issue was the development of fundamentally novel genetic material at the intersection of European grapevines and American muscadine grapes.

The history of the cultivar began in 1948 at the University of California, Davis, when researchers Lloyd A. Lider and Harold P. Olmo conducted a controlled cross between the Vitis vinifera cultivar Almeria and Vitis rotundifolia male No. 2. Initial selection tests for phylloxera resistance demonstrated successful results as early as 1964; however, due to propagation difficulties, the VR O39-16 rootstock was not immediately released for commercial use. Interest in this vine was renewed when researcher Austin C. Goheen observed differential tolerance to infectious degeneration and X. index feeding. These findings led to extensive experimental trials confirmed by enzyme-linked immunosorbent assay (ELISA) serological testing. The patent for the VR O39-16 rootstock was officially granted on May 3, 1988, to the Regents of the University of California.

The primary viticultural significance that distinguishes the VR O39-16 grapevine rootstock on the global nursery market is its proven immunity to feeding by the nematode vector Xiphinema index, effectively preventing the spread of grapevine fanleaf virus. Long-term field trials in the premium wine grape areas of Napa and San Joaquin Counties demonstrated that susceptible vinifera cultivars such as Cabernet Sauvignon, Chenin Blanc, and Carignane grafted onto this stock remained disease-free, highly vigorous, and fully productive with excellent yields for at least seven years. An additional advantage of the hybrid grapevine rootstock VR O39-16 is its complete root immunity to phylloxera, distinguishing it from its companion selection VR O43-43 (which shows only tolerance), as well as its high resistance to most strains of root-knot nematodes, to which standard stocks like Rupestris St. George and Ganzin 1 are completely susceptible.

Ampelographic and Botanical Description of the VR O39-16 Rootstock

The morphological characteristics of the rootstock combine distinct traits of European Vitis vinifera and the wild muscadine species Vitis rotundifolia, expressed in deeply lobed leaves and a highly specialized woody structure.

• Young Shoot (Growing Tip): Vigorously growing shoot tips are almost woolly white, heavily covered with a dense white tomentum. The young shoots exhibit exceptionally high growth energy.
• Mature Leaf: The leaf blade is deeply three to five lobed. Both the inferior and superior lateral sinuses are distinctly open. The margin is uniformly and sharply dentate. Older mature leaves are light to light yellow-green, showing a slightly tomentose texture with tufted hairs that tend to dehisce over time. Younger leaves feature soft, long hairs distributed on both the upper and lower surfaces, with a significantly higher density on the upper surface.
• Tendrils: Long, slender, and well-formed. The tendril length often exceeds that of the leaf blades. Anatomically, the tendrils are bifid and exhibit a discontinuous arrangement along the shoot, fully resembling the Vitis vinifera type.
• Flower: The flower type is male (staminate). The flower cluster form is small, measuring 4 to 6 cm in length. The stamens are moderately reflexed with a strong morphological tendency towards perfect flowering. The ovary is small but structurally well-formed. The pollen is completely sterile; the flower is infertile and results in absolutely no fruit set or crop production.
• Woody Cane (Mature Wood): Mature wood features a distinct tan color with persistent bark and prominent, highly visible lenticels. Internodes and nodes are formed with a clear internal separating septum, which serves as a definitive botanical marker for the variety. Time of bloom is late, with the rootstock blooming 12 to 14 days later than the reference cultivar Semillon (with 10% bloom recorded at Davis on May 25).

Nursery Management and Agrotechnical Features

The hybrid grapevine rootstock VR O39-16 is categorized as a technologically demanding variety under motherblock and nursery conditions, requiring specific propagation protocols. Due to the genetic influence of Vitis rotundifolia, the primary agrotechnical drawback is that the variety is naturally difficult to propagate by standard cuttings. In commercial nursery operations, achieving adequate root development and successful rooting of the VR O39-16 rootstock requires the mandatory application of synthetic rooting hormones.

However, when grafted with standard Vitis vinifera cultivars, the variety demonstrates excellent anatomical and physiological graft compatibility, ensures an optimal circular union, and completely prevents structural degradation at the graft junction. Thanks to its complete genetic immunity, mother plants of the VR O39-16 grapevine rootstock do not require insecticide applications for root phylloxera or soil chemical fumigations against Xiphinema index vectors. The multiplication and asexual propagation of the rooted cuttings are conducted under strict phytosanitary supervision, utilizing ELISA serological assays to verify and maintain the virus-free elite status of the mother blocks.