New Phylloxera Strain Poses Threat Primarily To Grape Nurseries

November 25, 1998

DAVIS -- A new form of phylloxera -- related to the subterranean insect that cost California's premium winegrape industry more than $1 billion in replanting costs during the last decade -- has appeared in three grape nurseries over the last two years. One of the University of California's experts on the pest says the new type of phylloxera does not pose a major threat to growers who have switched to resistant rootstock.

"Our friend phylloxera is back," Andrew Walker, a UC Davis viticulture and enology professor, told a large crowd of perhaps 500 concerned growers during the recent Napa Valley Viticultural Fair. "I hope we can calm some of the hysteria."

Walker explained in some detail during his presentation at the fair and in a follow-up interview what scientists know about the pest. The new form of phylloxera is a foliar or leaf-feeding pest. Tell-tale galls form on the underside of fresh grape leaves in addition to attacking roots like the typical California strains of the pest. The finds thus far have all been on the leaves of rootstock varieties. To the relief of the grape growing community, however, the pest does not appear to have an affinity for vinifera varieties such as merlot, chardonnay and cabernet sauvignon.

Although cases of the foliar phylloxera have been noted on wild grape plants in Southern California and elsewhere in the Southwest, the foliar form of this pest has only very rarely appeared in California's wine country before and never stayed more than a portion of a season. This form of phylloxera is common on the East Coast and in Europe, where the combination of high humidity with summertime rainfall is thought to be ideal for its survival. "It is possible that nurseries may have brought rootstock in from the East Coast and the foliar phylloxera may have come along," Walker said. "That's all we know at this point about where it came from and how it got here."

The earlier strains of phylloxera that caused growers to spend millions on new, more resistant rootstocks does its damage by attacking roots. The orange-colored, aphid-like louse sucks nutrients out of the roots until they crack and split and become vulnerable to soil fungi and bacteria, eventually killing the vine.

The foliar-feeding phylloxera, on the other hand, burrows itself into the leaf, where one female lays a couple hundred eggs. As the young emerge from small bumps or galls, they crawl toward the shoot tip, and begin feeding on new growth. Feeding can be severe enough to defoliate part of the vine. "They don't really fly very well," Walker says, "but they can be blown in the wind. It's mainly propagation wood and people that move phylloxera around long distances. The root form is commonly spread on infested vine roots, but we're not quite sure how the foliar forms could move so quickly. They could be inside bud scales or under the bark near the buds."

Walker is currently researching the genetics and behavioral habits of foliar phylloxera from Europe and the eastern U.S. One of his graduate students is comparing DNA fingerprints of 150 populations from across Europe to foliar types from the Southwest and New York. They've also tried to force feed the leaf gall phylloxera on roots. So far, the pest won't feed on mature roots but will -- slowly -- on young roots. "Somehow they're adapting slowly," Walker said. "That's the part that's interesting because they're asexual, clonal insects... We're trying to figure out why they've been able to adapt and how they've managed to change their behavior quickly over five to 10 generations.

On the East Coast and in Europe the foliar phylloxera is commonly found and creates limited problems for vinifera grafted on rootstock varieties. "It probably has very limited viticultural importance in California," Walker said. "I think the issue is that it has big implications for the nursery industry. If the foliar form of phylloxera becomes widespread, it will be difficult to prevent low levels of phylloxera from establishing on resistant rootstock roots."

Walker believes the real danger is that the pest could spread through rootstock nurseries. Nurseries will have to spray several times per season to control the foliar form if it spreads, although pesticides do not control it completely. Growers who have already switched from non-resistant rootstock such as AXR will not be affected. Growers who have yet to switch are at most risk -- the Sierra Nevada foothills, Monterey Valley and portions of Mendocino and Sonoma counties -- as they may inadvertently bring phylloxera into these areas on rootstocks from nurseries with low level infestations due to foliar phylloxera.

"Foliar forms of phylloxera are more of a nursery issue than a grower issue," Walker summarized. "But we may now have these additional strains in the state. These new strains may not have a direct impact on resistant rootstock roots but they are likely to hasten the decline of non-resistant plantings, AXR plantings or own-rooted plantings."

University of California - Davis

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