A USDA grape geneticist is seeking San Joaquin Valley grower-cooperators for on-farm trials of several rootknot nematode-resistant rootstocks he is developing at Parlier, Calif.

Peter Cousins, who is based at USDA’s Agricultural Research Service at Cornell University at Geneva, N.Y., showed his most promising selections at a recent field day at the Kearney Agricultural Center. He set out a series of rootstocks grafted to Syrah in 2005 and has been making additional selections each year since.

They are being evaluated with Freedom rootstock as a standard. The nine best performers have been sent to the Foundation Plant Services center at the University of California, Davis for testing, and only those found free of viruses such as corky bark, leafroll and grapevine fanleaf will advance to further screening.

“This research is to find new rootstocks suitable for the SJV and Central California. Particularly, our goal is to develop rootstocks that have superior resistance to rootknot nematode,” said Cousins, who is a native of Ceres and graduate of UC, Davis.

The group of Meloidogyne species nematodes infest as much as 65 percent of California’s vineyards and inflict statewide losses estimated at as much as 20 percent. Their root-feeding brings on symptoms of galled roots, weak vines, shortened vine life, and reduced establishment.

Cousins noted that, although other nematodes also damage vineyards, rootknot nematodes are the chief pest of grapevines in the U.S.

Although resistant rootstocks such as Freedom and Harmony and others provided some protection against them for many years, the tiny worms have countered with more aggressive populations. These mutations, plus increasing restrictions on methyl bromide and other soil fumigants, have signaled the need for plant materials that resist the more virulent nematodes.

Cousins said he has drawn from dozens of North American wild grape species parents having excellent resistance to nematodes and other grape diseases and pests. “However,” he said, “the wild grapes do not always behave the way we would like. Our commercial varieties root well from cuttings, but wild grapes do not. Wild species also have undesirable horticultural characteristics such as flowering.” So he must find those having compatibility for breeding with materials carrying desired traits.

Using classical breeding techniques, he has sorted through crosses of Vitis vinifera and other commercial species with wild species that have the desired nematode resistance and ease of rooting, resistance to viruses and phylloxera, and suitability to California soils, climate, and management practices. He also pointed out that as water quality and availability in California become more an issue, rootstocks having greater vigor will be of increasing interest.

His selections commonly discard between 95 and 99 percent of the seedling crosses, and only the remainder is planted at the Kearney test plot. A process of five years was needed to come up with the seedlings for the selections being evaluated in the plot.

Cousins is collaborating with USDA colleagues at Parlier and Davis, as well as viticulturists at UC, Davis and elsewhere to share plant materials that contribute to the breeding process. He also maintains close liaison with commercial nurseries.

“The only way we can know what a selection will do is to graft it and put it out in trials,” Cousins said. That means that from seedling selection to release by USDA can take at least ten years of concentrated evaluations.

Interested SJV growers can reach Cousins by e-mail at peter.cousins@ars.usda.gov or by telephone at 315-787-2340.

In another presentation at the grape day, growers and PCAs learned that nematodes are not the only pests becoming harder to control by conventional means.

Anil Shrestha, UC IPM weed ecologist at Parlier, detailed his research showing how glyphosate-resident horseweed can become a runaway problem in vineyards. But he also offered some management tips on how to bridle the noxious weed.

Horseweed, a member of the Asteraceae family and easily identified by its tall stature, is a common sight along roadsides, ditch banks and cropland throughout the state.

“This weed likes to germinate in temperatures of 65 to 75 degrees Fahrenheit and needs little moisture,” Shrestha said, noting that the key to control is to prevent it from setting seeds.

“It has flushes in October and November and again in February and March. Each plant is capable of producing up to one million seeds that can be carried by wind for miles. You maybe doing a good job of weed control in your vineyard, but if your neighbors don’t do the same it will come to yours.”

Concern emerged in 2004 when Shrestha said he got the first reports of horseweed being resistant to glyphosate (the active ingredient in Roundup), a long-time herbicide used for control. Studies showed that some established, vigorous plants were glyphosate-resistant (GR) and could withstand up to four times the labeled rate of the chemical.

Later observations by him and other UC weed specialists found that even later stages of some glyphosate-susceptible (GS) plants could survive treatments of the compound at lower rates.

“Samples from several locations have shown that the GR biotype of horseweed is fairly widespread in the Central Valley with the greatest concentration being in Fresno, Tulare, and Kings counties,” Shrestha reported.

Continuing research is being done in greenhouse and vineyard test plots at the Kearney Agricultural Center. The first year of the studies indicated that stem length, number of leaves, and dry weight of the aboveground mass of young grape vines could be substantially reduced by horseweed plants.

“However,” he added, “GR and GS horseweed did not differ in their competitive ability with the young grape vines or reduce their root dry weight. Established vineyards were more tolerant of horseweed densities of up to 50 plants between two vines.”

He went on to say that horseweed control is critical in the first few years of vineyard establishment, and even though older vineyards can tolerate densities of up to 50 of the weeds between vines, steps for control should still be taken.

“It’s not a good practice to leave them because they can have a cumulative effect over the years and weaken the vines, harbor insect pests and disease vectors, add to the weed seed bank or infest neighboring areas, and reduce the aesthetic quality of the vineyard,” he said.

New research also revealed that consistent control has been obtained with Rely (glufosinate) with a tank mix of glyphosate and 2,4-D. “Our studies have repeatedly shown that for postemergence chemical control, it is very important to apply the treatment at an early growth stage, preferably the rosette stage.”

Weed specialists have also learned that horseweed seeds primarily germinate within the top quarter-inch of soil and those seeds buried deeper do not emerge. “Therefore,” Shrestha said, “fall disking or soil disturbance could be a management strategy, since we have seen that fall-disked fields in some locations in the Central Valley have lower densities of horseweed in the spring.”

Shrestha also said that growers who rely solely on postemergence herbicides such as glyphosate in environmentally-sensitive areas may have to modify practices to control or prevent the spread of GR horseweed.

“Close monitoring and an integrated weed management program will have to be implemented to manage GR horseweed biotypes in the Central Valley,” he concluded.