When dealing with nematodes, Washington State vineyardists need two things they currently lack: more effective control strategies and meaningful economic thresholds to guide them in knowing when treatment is necessary. Answers for both issues should be forthcoming from work being conducted by Washington State University nematologist Dr. Ekaterini Riga.
Since losing the use of Nemacur (fenamiphos) almost two years ago when Bayer Corporation voluntarily pulled its U.S. registration due to association with bird kills, grape growers have not had an effective nematicide for vineyards already established. Soil fumigation is an effective pre-plant strategy, but that is little consolation if your vines are already planted and nematodes are causing vine decline.
Of the 2,000 species of plant parasitic nematodes, only a few have been found to be important in Washington vineyards. Dagger and root knot nematodes are the biggest concern and were found in nearly 75 percent and 63 percent, respectively, of the vineyard sites surveyed in 2003 by Dr. Jack Pinkerton of the U.S. Department of Agriculture. Of lesser concern are ring and lesion nematodes, also found in Washington vineyards. The survey found nematodes in just about all wine grape varieties sampled.
Riga believes the survey numbers are conservative because they were taken several years ago, and since then, grapes have been planted in many more areas of the state.
"We are in need of a new survey," she said, during educational talks at the Washington Association of Wine Grape Growers convention in February. "Pretty much every site surveyed then had nematodes, with many sites at levels above economic thresholds."
Prevention still best
Keeping nematodes out of your vineyard is the best control. Always start with clean plant material and clean soils, advised Riga. Fields should be sampled for nematodes before planting to determine if nematodes are present, what species, and at what population levels. Knowing the field history can help in deciding if soil fumigation is necessary. "A lot of cherry orchards have dagger nematodes," she said, adding that if grapes follow cherries, fumigation is likely warranted.
"We have almost no postplant control choices that are effective and we have only a few preplant materials that are moderately effective," Riga emphasized. "It’s best to get rid of the nematodes before you plant with a fumigant like Telone or Vapam (metam sodium) or a combination."
Green manures by themselves or in combination with Vapam may be an option if the nematode numbers are low, she noted. Enzone is a postplant, water-soluble fumigant registered in several states, including Washington, Oregon, and California. The product has shown some promise in reducing nematode levels, she said, but added, "it has not been as good as we were hoping."
Since 2002, Riga’s research has involved testing a range of synthetic and biological nematicides for their efficacy in a commercial vineyard with high nematode levels. She is looking for different control options that would work preplant or in the field with vines already established, without interfering with management practices.
Thus far, trap crops planted in the perimeter or in-row to attract nematodes have not worked well. The biocontrol fungus muscodor albus is another way to control nematodes, but there are practical problems in getting the fungus established in the field, she said.
She is encouraged with results from a few chemicals, and will be analyzing pruning weights to learn about vine response to the different compounds. The chemical fosthiazate initially looked very good in trials, but was recently pulled, because it was found to translocate to the berries. Two Stoller products for the drip system show promise and there are three Dow Chemical materials of interest. She is also experimenting with green manures (Braco mustard, arugula, and Caliente mustard), in hopes of reducing high dagger nematode numbers. When the green manures are chopped up and incorporated into the soil, they act as a biofumigant and release a chemical similar to that of Vapam.
"Green manures work if they are incorporated at the right time and if there is moisture in the soil," Riga said. "So, there are a lot of ifs with green manures, but if done properly, they can be effective. The trick with planting green manures in the spring is that they can go from flower to seed very quickly. That’s why I recommend planting in the fall."
Results from two seasons of growing green manures in a vineyard reduced dagger nematode levels from up to 400 per 250 cubic centimeters of soil down to below 25, she noted. The manures were as effective as the control treatment, which was soil fumigation with Vapam.
Riga has also been working to develop nematode economic threshold densities for vineyards that will be meaningful to Washington growers.
"Right now, what we have are theoretical thresholds," she said. Most of the thresholds that have been established for grapes come from California or don’t consider the age or variety of the vines.
"We don’t know exactly how many is too many or too few," she said, adding that she has a trial underway to find the minimum number of nematodes that plants will tolerate in a new vineyard. "If you have 20-year-old vines, 100 nematodes per 250 cubic centimeters of soil is nothing. But if you have a young vine, that number could be a problem."
In some samples collected in Washington, nematode densities were as high as 3,000 per 250 cc of soil. Riga pointed out that yield loss can occur at densities as low as 50 per 250 cubic centimeters of soil, depending on the age of the vines.
To learn what nematode densities during the vine establishment period can result in economic damage, she is infecting Chardonnay and Cabernet Sauvignon rooted cuttings planted in one-gallon pots with low, medium, and high densities of two species of dagger nematodes and root knot nematodes. Soil used in the pots has been fumigated and the field where the pots are buried was fumigated.
Preliminary results show that root knot nematodes have become established inside the roots and are beginning to reproduce. The data shows significant difference amongst the three densities of root knot in the Chardonnay grapes in both years, 2007 and 2008. The project will conclude by 2011.