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

Overview

Grapevine leafroll disease (GLRaV) contributes to an ∼10% yield loss in winegrape production in Washington State.¹ The disease also negatively affects growth and development, delays ripening, and reduces sugar while increasing titratable acidity in both grapes and must.^1,2 Depending on incidence, GLRaV can significantly affect the profitability of a vineyard.³ At economic thresholds that vary by grower and region, virus-infected vineyard blocks are removed and replanted.

The commercial vineyard used in this study was undergoing sequential replanting of blocks that were heavily infected with GLRaV. In the study block, 92% of sampled vines tested positive for Grapevine leafroll associated virus-3 (GLRaV-3). A vector for GLRaV-3, grape mealybug (Pseudococcus maritimus), was present and created the potential for virus spread from rogue vines to newly-planted, certified virus-free vines (Washington State Department of Agriculture Plant Certification Program). Reducing virus inoculum in the presence of a vector is of utmost importance for the vineyard’s long-term economic viability and to slow disease spread in both new and established blocks.

This 25-year-old vineyard also had a high density of plant-parasitic nematodes: in particular, northern root-knot (Meloidogyne hapla), which causes direct damage to roots through feeding and the formation of root galls. This feeding habit can delay establishment of newly-planted and young, own-rooted Vitis vinifera vines. In Washington State, the majority of grape production is own-rooted, due to a lack of established phylloxera populations.⁴ This practice allows rapid vineyard retraining following damaging winter cold events that can occur in the area. Thus, using rootstocks is not a currently accepted commercial practice for the region, regardless of any benefits it might have in mitigating nematode damage when replanting.

Given these circumstances, we performed an exploratory study in a commercial vineyard undergoing replanting to evaluate potentially effective practices that address these two separate issues. The goal was to reduce plant-parasitic nematodes prior to replanting while simultaneously reducing the likelihood that virus-infected vine tissue survived the rogueing process. Thus, two compounds, a soil fumigant and a systemic herbicide, were evaluated for their efficacy in reducing nematode density and killing mature vines, respectively. The two compounds, alone and in combination, were selected for the following reasons:

1. Glyphosate was selected for its systemic properties^5,6 and broad-spectrum efficacy in perennial plants.^5,7 Glyphosate is frequently used to kill woody plants. In other studies, its efficacy in killing grapevines when applied directly to the trunk stump immediately after the trunk was severed has been assessed. ⁸ This “painting” approach is commonly used when a limited number of plants are to be removed, but is economically unfeasible for large-scale vineyard removal due to the extensive labor required and the timing of application (between harvest and leaf fall) is not a trivial factor in cooler climates. We used foliarly-applied glyphosate, similar to past studies,⁹ to achieve vine mortality. In comparison to hand application, this process was less expensive and reduced risk of worker exposure, as only one handler/tractor operator was exposed, rather than a large hand crew.

2. The fumigant, metam sodium (Vapam) was selected for its efficacy against plant-parasitic nematodes and for labeling that allows targeted application to the drip zone, necessary in the study vineyard because the trellis was being retained intact. The site had known populations of M. hapla that were above-average densities for Washington vineyards.¹⁰ Soil fumigation was necessary to reduce nematode populations prior to replanting because M. hapla is known to impede establishment of young vines. Metam sodium is labeled and recommended for nematode management in grapes in Washington.¹¹ Metam sodium also controls certain pathogenic fungi and select weed species.^12,13,14,15

3. Justification for combination use: Because both virus and high nematode populations were present, products were applied in concert. It was not known whether the herbicidal activity of metam sodium would create additive effects in the combination treatment.

From fall 2015 to fall 2016, we evaluated the efficacy of these two products, independently and in combination, at inducing vine mortality and (for fumigation) reducing nematode density. The production section abutting the experimental area was removed about six weeks after fumigation, consistent with the regional norm of removing the vines within a few months postharvest. For the purposes of this investigation, treated vines were left in the ground for 12 months posttreatment to allow for periodic assessment of vine regrowth. The resulting information could be used to develop best-management practices for vineyard replanting at virus- and nematode-affected sites in regions where certified, own-rooted vines are the industry standard. Because the vineyard was slated for removal in fall 2016, and was the last block in the replanting cycle, this study could not be repeated at the same site.