Rootstock selection can be a daunting task for grape growers, but sorting through the pros and cons of alternatives for a particular vineyard site is essential for success, says Jennifer Hashim-Buckey, Kern County farm advisor.
To offer some guidance for growers, she reviewed the traits of a half-dozen rootstocks commonly used in California in a talk during the San Joaquin Valley Table Grape Seminar held recently in Visalia.
While rootstocks can provide varying degrees of protection from soil pests and soil problems, selecting the best overall for a certain vineyard depends on array of factors, including scion compatibility, ease of grafting, irrigation, trellis design, soil depth and texture, soil chemistry and potential vine vigor.
Hashim-Buckey said Vitis berlandieri, a native of central and southwestern Texas, is often used in hybridization with other species.
Crosses of it with V. riparia, V. rupestris, and V. vinifera have resulted in rootstocks having lime tolerance and resistance to phylloxera. It is, however, susceptible to rootknot and dagger nematodes.
Another from Texas, V. champinii, thought to be a hybrid of V. candicans and V. rupestris, goes by the common names Dog Ridge or Salt Creek and is a hybridization component in Freedom and Harmony. It has good lime tolerance and moderate phylloxera and rootknot resistance. V. champinii can be highly vigorous in deep, fertile soils and may be difficult to root.
Easy to root is V. longii, a native of Central Texas, Oklahoma, New Mexico and Kansas, typically found in dry creek beds. It has good drought tolerance and moderate phylloxera and nematode resistance.
Vitis riparia, the most widespread of the American Vitis species, occurs naturally on moist, fertile soils of riverbanks and does not tolerate calcareous soils. It has good resistance to phylloxera and roots and grafts well, but it is prey to nematodes. Hybrids of it and V. berlandieri are popular around the world.
Vitis rupestris, also known as St. George, is native to Midwestern and Southern states, where it occurs near gravelly stream beds. Its strengths of moderate resistance to phylloxera and ease in grafting and rooting are offset by a high susceptibility to nematodes and lack of tolerance to lime.
A native of the southeastern U.S., Muscadina rotundifolia is a separate species from Vitis and has high adaptability for breeding programs. In addition to its ability to withstand nematodes and phylloxera, it also resists many fungal diseases, such as powdery mildew.
Current breeding programs, Hashim-Buckey said, are generating new rootstocks intended to resist soilborne pests while doing a better job of maintaining desirable characteristics of table, raisin, and wine varieties grafted to them.
M. Andrew Walker, grapevine geneticist at the University of California, Davis, continues an effort to provide broad and durable resistance to nematodes and find a replacement for the fanleaf virus-resistant rootstock, 039-16.
The program tests rootstock materials for resistance to multiple biotypes of rootknot nematode, plus citrus, lesion, pin, and ring nematodes and combinations. At the same time, it evaluates for resistance to phylloxera and fungi, tolerance to drought and salinity, and viticultural traits.
The USDA Grape Rootstock Improvement Program at Geneva, New York, focuses on breeding and evaluation of new materials for resistance to rootknot nematodes.
The USDA breeding program headed by David Ramming at Fresno continues to search for improved rootstocks to combat rootknot nematodes along with control of excessive vigor. Among recent releases from Ramming's program are the RS-3 and RS-9, the hybrids of Ramsey and Schwarzmann that are becoming available in limited supply from nurseries.
RS-3 is moderately vigorous and has resistance to all common rootknot nematodes, including the known resistance-breaking biotypes. It also has resistance to ring, dagger, root lesion and citrus nematodes. RS-9 has lower vigor and has similar nematode resistance, with the exception of defense against ring nematode.
In describing her Kern County trials in recent seasons with various rootstocks and the midseason, seedless Princess table grape variety, Hashim-Buckey said 6-19B, one of the products of the USDA program at Fresno, has insufficient vigor for use with table grapes.
Two others from the program, 10-23B and 10-17A, showed low to moderate vigor and performed well in terms of yield and quality when grafted with the vigorous Princess variety.
She added that “the devigorating effect (in the absence of soil pests) may not be true for other scion varieties and since the trial was established in 2002, different trends may arise as the vines mature.”
Mike McKenry, UC, Riverside nematologist stationed at the Kearney Ag Center, Parlier, said results in his trial on Chardonnay at Gilroy suggest additional research on rootstocks might be an answer to part of the “replant problem” that challenges replaced vineyards where old roots persist.
Rootstock resistance, preplant fumigation, or postplant nematicide applications can manage certain nematode species but not all. Dagger nematodes return in a few years in many vineyards.
“It is technically difficult to deliver postplant nematicides to the site of nematode attack when the nematicides available must not be overly persistent, overly volatile, or overly expensive,” McKenry said.
Chemigation is another means of managing root pests, but the chemicals must move through massive volumes of soil to reach the nematodes.
Given these facts, McKenry said, “the use of a rootstock to deliver the nematicidal agent directly to the site of nematode attack has merit.” This takes place when a susceptible nematode feeds on the roots of a rootstock that has a mechanism to destroy the pest.
In the replanted Chardonnay vineyard at Gilroy, McKenry and Larry Bettiga, Monterey County farm advisor, observed that two rootstocks, 10-17A and 039-16, grew well in either fumigated or non-fumigated plots.
That raised the question whether either of the two rootstocks might be a way to avoid the rejection component of the replant problem.
McKenry said 10-17A, which has resistance to common and aggressive rootknot nematodes and some resistance to dagger species, “may offer relief from a diversity of nematodes as well as the rejection component of the replant problem.”