Using nematode-resistant rootstocks is one of the most effective ways to manage the parasite in grape vineyards, says a former U.S. Department of Agriculture grape breeder.
Several new nematode-resistant rootstocks have recently been released and more are coming.
Root knot nematodes are among the most damaging pests of grapevines in the United States, said Dr. Peter Cousins, former grape breeder and geneticist for USDA in Geneva, New York, who now is in charge of grape plant improvement at E & J Gallo in Modesto, California.
Nematodes are microscopic worms that feed on roots and cause damage by reducing vine yields and limiting the vine’s water and nutrient uptake. Root knot nematodes cause more than $1 billion in damage annually to U.S. viticulture. Vitis vinifera cultivars are particularly sensitive.
In the past, rootstocks have not been used much in eastern Washington vineyards because of the periodic need to retrain vines up from the ground after winter damage.
But a growing nematode problem and other replant issues are forcing Washington growers to consider nematode-resistant rootstocks as an alternative to soil fumigants and other chemical nematicides.
Thanks to a refocus of several grape-breeding programs, improved nematode-resistant grape plant material is now available, giving growers several choices, said Cousins, who spoke at the annual meeting of the Washington Association of Wine Grape Growers.
There are six grape-breeding programs in the world working on nematode resistance, and three or four programs in the United States, according to Cousins. After developing several phylloxera-resistant rootstocks, grape-breeding programs in North America have shifted their focus from phylloxera resistance to nematode resistance.
Grape growers in California’s San Joaquin Valley now face a bigger problem from root knot nematodes than phylloxera, he said.
“But be aware that all rootstocks are not created equally and come in varying levels of vigor and nematode resistance,” he said. In the past, a few rootstocks offered root knot resistance, though not against virulent populations, and they had other problems or were difficult to propagate.
The shift in breeding focus has led to several selections with improved nematode resistance.
Of nematode-resistant rootstocks recently developed by USDA, he identified three that might have potential for Washington growers—Kingfisher, Matador, and 10-17A. Matador and Kingfisher, which were released in 2010 and are just now becoming available from commercial nurseries, offer better root knot resistance than Freedom and Harmony rootstocks.
The newer 10-17A, released in 2012, is highly vigorous and is recommended for the dagger nematode (Xiphinema index), but it hasn’t yet been tested for resistance to root knot (Meloidognye hapla) or northern root knot.
The grape-breeding program led by Dr. Andy Walker at University of California, Davis, has developed five new nematode-resistant selections, GRN1 to 5. According to literature, all five are resistant to root knot and dagger, though they vary in resistance to other nematode species. The five rootstocks have a range of vine vigor, from low to medium.
“We’re just now learning how they do under commercial settings,” Cousins said.
RS 2 and RS 9 were bred by USDA’s Dr. David Ramming at Parlier, California, and selected by UC’s Dr. Michael McKenry. Both have shown resistance to root knot nematodes and are rated as medium vigor, based on evaluations in California’s San Joaquin Valley.
Nemadex Alain Bouquet, a rootstock developed in France, is resistant to the dagger nematode X. index. About three-quarters of the vineyards in France have X. index populations, Cousins said. “This rootstock is only for the most vigorous sites because it is very slow growing.”
For those needing a more moderate vigor rootstock that is resistant to dagger nematodes, he suggested another UC rootstock, 039-16.
“Breeding programs are coming up with new resistant rootstock, but it takes time to test new selections for disease, get material to commercial nurseries, ramp up propagation, and field test in different settings,” he said. “It’s a very detailed process.”
He cautioned Washington growers who have little rootstock experience to be aware of interaction between the scion and rootstock. Most nematode-resistant rootstocks are very intolerant of virus-infected scions. “Rootstocks are less tolerant of virus compared to own roots. It’s not well understood why, but you need to be aware of the interaction of disease with rootstocks,” he said.
A variety of high and low vigor rootstocks, each resistant to different nematode populations, is available, he said. Growers have to match rootstocks to their soil conditions and nematode species in their particular site.
Reviewing research data of rootstock resistance can be “partly useful,” he said. “It can vaguely point you in the right direction, but data usually don’t tell you the whole story. There are hundreds of nematode species, and most research charts don’t tell you what species or where the rootstocks were tested. The choice of rootstock for individual growers depends on pest pressure and soil type.”
He encourages growers to start by planting a few rootstocks to see how they perform.
Growers in dry soil locations will probably want to use vigorous rootstocks because they are better at taking up water and supplying it to the scion, giving young plants a boost. Those with a high water table and deep soils should consider a devigorating rootstock.
“The differences among rootstocks in deep soils with a lot of water are much less than those growing in a trial with limited soils and water,” Cousins said.
Vine productivity is also a factor. Pruning weight, fruit yield, and fruit quality are influenced by rootstock choice. In a California table grape rootstock trial, vines on Ramsey rootstocks produced the most fruit but also had the most cull fruit. Freedom had the most premium fruit while Harmony produced the fewest culls.
“Your rootstock choice must factor in your needs of vine size, water relations, and nematodes,” he said. “It’s not just about the nematodes.” •