Selecting and matching grapevine rootstocks and clones is always a challenge, but there is one element that should be a slam dunk — use certified plant material.

“I think the era of field selection is done,” says Jim Wolpert, viticultural specialist at UC Davis. “I don’t think you can afford to be a risk to yourself or your neighbors by planting selections that have a little bit of virus in them. I think mealybug has changed that.”

Grape leafroll virus is spread by mealybugs and has changed how researchers and the industry look at certified clones versus home-grown clones. It’s a paradigm shift, according to Wolpert, although there is still resistance to using certified plant material.

“One of the reasons growers don’t use certified clones and use home-grown clones is they don’t trust the certified clones to make wine,” he says. “I think that means we have to learn a lot more about certified clones.”

Wolpert says research has answered some of the questions, but not all, and sometimes the results beg even more questions. For example, researchers have been trying to figure out if lower-yielding clones show increased pruning weights. So far the answer is, sometimes, but not always.

Yield also is affected by various factors, depending on the clone. Yield is driven primarily depending on berries by cluster.

“However, for some clones like Pinot, all bets are off,” Wolpert says. “The low-yielding clones can compensate with higher growths, but not always.”

Growers need to keep in mind what effects a particular rootstock will have on the vigor of the vegetative and fruit development of the clone. If it’s a vigorously-growing clone, either the rootstock needs to be downsized or the grower needs to provide more canopy area for the vine to fit the growth.