Dual Fumigant and Herbicide Use Optimizes Replanting Preparation in a Virus- and Nematode-Affected Vineyard

Goal: An exploratory study evaluated best-management techniques for replanting a commercial vineyard suffering from both a high incidence of Grapevine leafroll associated virus-3 and high densities of northern root-knot nematode (Meloidogyne hapla). The grower had two goals: to reduce the density of plant-parasitic nematodes, which can delay establishment of young vines, using a pre-plant fumigant, and to reduce survival of virus-infected vine tissue by applying an herbicide to the foliage prior to vine removal.

Key Findings:

• Fall-applied metam sodium reduced densities of M. hapla in the following growing season. Its herbicidal properties also resulted in high root mortality. In cases where vines survived, only a few shoots (and suckers) emerged, displaying typical morphology.

• Fall-applied foliar glyphosate significantly reduced shoot and sucker regrowth, but did not kill root tissue. In the few instances where shoots and suckers grew the following spring, that tissue expressed typical symptoms of glyphosate phytotoxicity. While low rates of abnormal growth would eliminate much of the aerial reservoir of virus, the presence of live roots indicates that rogue vines and living root debris could contribute to infection of new, virus-free planting stock.

• Combining fumigant and foliar glyphosate was additive: nematode control, root mortality, and a lack of healthy shoot/crown sucker regrowth.

Impact and Significance: This preliminary work showed that the combination of fall fumigation and foliar glyphosate application reduced nematode populations and vine survival. In situations where plant-parasitic nematodes are the sole issue facing replanting, fumigation alone suffices. Where virus alone is the impetus for replanting, fall-applied foliar glyphosate may provide sufficient overall vine mortality, with a caveat of low root mortality and its attendant risk of root-feeding vectors spreading the virus to new planting stock.