Nematodes Threaten Vineyards

While problem grows, UC Davis plans research cutbacks

by Paul Franson
nematode vineyard
When nematodes invade a vineyard, they can strangle vines from
the roots up.

Napa, Calif. -- Nematodes are becoming an increasingly vexing problem for grapegrowers in California. No clear remedies exist, even with synthesized chemicals, much less biological controls or cultural practices. And the University of California, Davis, intends to shut the nematode department that has been working to find solutions.

To address the pesky issue, the Association of Applied IPM Ecologists held a roundtable discussion Monday in Napa, attended by more than 50 academics, suppliers and growers. Laura Breyer was the moderator.

They shared their experiences, starting with speculation about why nematodes are more of a problem than in the past.

Most agreed that at least three changes in North Coast grapegrowing have exacerbated the problem. The practice of replacing deep-rooted, nematode-resistant grapestocks like St. George and AXR-1 with modern rootstocks is one problem -- although it’s well known that AXR-1 is susceptible to phylloxera. In addition to having less resistance to the parasites, newer rootstocks are typically more shallow-rooted, a tendency heightened by drip irrigation.

In the past, fields were often fumigated with methyl bromide before planting, a practice now prohibited.

In addition, the mustard that was once the ubiquitous cover crop in Napa has been supplanted by other crops in much of the valley. Mustard, like other brassicas, can act as a biofumigant. Plants in the brassica family contain glucosinolates that are transformed into toxic iso-thiocyanides when the plants decompose in the soil.

Meadowfoam seed meal also was mentioned as having similar properties. These chemicals are similar to the active ingredients of some synthetic fumigants such as Vapam and Basamid. The effect is increased if the plants are chipped or ground. In effect, they can take on part of the role of the now-banned methyl bromide.

Unfortunately, many brassicas are hosts for root-knot nematodes, one of the most troublesome species. Others aren’t. Scientists are now researching the best brassicas to use.

Apparently another factor increasing nematode problems is growing practices that weaken the vines -- including deficient irrigation and leaving grapes to hang too long.

Root knot nematodes, shown here with egg masses, are among the most damaging to grapevines.

Whatever causes nematode infestations, participants at the seminar suggested that planting and management practices, “natural” and synthetic chemicals, and choice of plant materials can help to overcome them. None of these, however, appears to provide a complete solution.

For new plantings, careful three-way ripping of the soil to remove any dormant roots that could harbor nematodes is important, ideally followed by fumigation.

In 2008, UC Davis released five nematode-resistant rootstocks developed by Prof. Andy Walker labeled GRN-1 through 5. They are also resistant to phylloxera. They are now being multiplied at nurseries, so few are yet planted in the field. Other rootstocks differ in susceptibility, too.

Some speakers suggested that scions could also affect resistance.

For existing vines, suggested cultural practices include more dispersed irrigation -- perhaps every two feet along an irrigation line, instead of one per vine -- microsprinklers or inter-row irrigation.

Planting and tilling-under certain brassica, ideally after shredding, can also help. By contrast, legumes can host ring nematodes. Pacific Gold canola plants are especially promising, said some growers.

Suggested natural nematode control applications are Dragon Fire, a sesame oil compound that requires emulsifying, shrimp meal (chitin), DiTera, Melo-Con (paecilomyces, a fungus that feeds on nematodes) and Nema-Q. Some pesticides not specifically approved for nematodes are also being tried.

Predatory nematodes are also in the soil and can be encouraged. Many natural predators and fungi feed on nematodes.

Becky Westerdahl, the continuing education nematode specialist at UC Davis, warned, however, “They don’t work as well as hard chemicals.” She suggested that a combination of treatments might be necessary.

Conventional chemicals include Enzone (sodium tetrathiocarbonate) and Moventa (a foliar-applied systemic pesticide not approved yet for nematodes).

Speakers warned also that application timing is critical. One speaker noted that soil temperature is vital, and recommended 56-58°F, when the nematodes are most active and reproducing. This is typically late May and early June in Northern California. This is important because most of the nematicides are short-lived. Water application can be critical to avoid washing the substances below the roots.

Experts also recommended power-washing equipment before moving from one field to another, to remove dirt and clinging nematodes.

The conclusion of the speakers was that much more work is needed. The AAIE is scheduling additional seminars for local growers and others. In addition, it is sponsoring an Ecological Pest Management Conference in Napa from Jan. 31 to Feb. 2. Get more information at aaie.net or phone (559) 761-1064.

Future uncertain
At a time when nematodes seem to be a growing problem, UC Davis stated its intention to eliminate its Department of Nematology. Chairman Steven A. Nadler said the department has seven staff members who may be dispersed through the university -- if they’re fortunate. Overall, 30 to 50 positions in the agricultural sciences school will be lost -- that’s 35% of the faculty. “What’s the future of this study if this happens?” Nadler asked. “Many of these projects are long-term in nature. Will it be able to continue?”

UC Riverside also has a Department of Nematology.


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