Issue link: https://read.dmtmag.com/i/298602
www.goodfruit.com GOOD FRUIT GROWER MAY 1, 2014 39 vineyards by additional irrigation and nutrients, but in the long term, it's a growing problem." Crosby believes there is potential to develop a nematode degree-day model that could help growers time postplant nematicide applications when juvenile nematodes are most active. Research from the potato industry indicates that this approach might be possible in grapes. The main four plant-parasitic nematodes of concern in Washington are northern root knot (Meloidogyne hapla Chitwood), dagger (Xiphinema americanum), root lesion (Pratylenchus spp.), and ring (Mesocriconema xenoplax). A statewide survey conducted several years ago found about 60 percent of the surveyed vineyards had high popu- lations of root knot and dagger nematodes and 45 percent of the vineyards had root lesion nematodes, but at low populations. Ring nematode was found in 14 percent of the vineyards. Biology Life cycles of plant-parasitic nematodes include the egg, four juvenile stages, and one adult stage, Zasada said. Most nematodes complete their life cycle in 20 to 40 days. For some nematodes, two to three generations are possible per year. Knowing when nem- atodes are most active in the soil can help growers collect more accurate samples and better time nematicide applications, she said. Root knot nematode is a sedentary endoparasite that lives most of its life within plant tissue. Some 300 to 400 eggs are laid on the root surface or in the root. Migration in the soil occurs at the second juvenile stage. More than 550 host plants are known, including potatoes, alfalfa, dandelion, and lamb's quarters. In their research, Zasada and a team of scientists found root knot nematodes concentrated in the top 18 inches of soil in clearly defined areas that follow water (drip lines) and roots. Dagger nematode is a migratory ectoparasite, spending all of its time in the soil liv- ing on the exterior of roots. Dagger nematodes can transmit tomato ringspot virus and grape fanleaf virus to grapes. The species that transmits tomato ringspot virus, X. index, has not been found in Washington vineyards. The list of host plants is extensive and includes broadleaf weeds, grasses, potatoes, and more. Dagger nematodes were found indiscriminately in vineyards at various depths in the soil. NEMATODE management strategies P replant soil fumigation has long been an effective way to control nema- todes, but fumigation in the future may be limited, says Dr. Inga Zasada, U.S. Department of Agriculture scientist. She offers the following suggestions to help vineyardists manage nematodes in the event fumigants are unavailable. 1. Planting material—Use certified plant material to avoid bringing nema- todes into your field. Choose a grape variety based on the kinds of nematodes you have. "If you sample and have northern root knot and can't fumigate, you may want to stay away from white varieties," she said. Consider using nematode- resistant rootstock, matching rootstock resistance to the nematode species in your field (see "Nematode- resistant rootstocks available"). 2. Fumigation options—If you can fumigate, remember that fumigants are not as effective if plant residue remains in the field so it's important to remove as much root material as possible. Sedentary nematodes, like root knot nematode, reside in root material. Nematodes inside nondecomposed plant material will not be controlled by fumigation. By applying knowledge about nematode biology, she sees an opportunity for spot treatment instead of fumigating the entire field. For example, depending on the species, fumigation may only be needed under the vine row that follows the old drip irrigation line. 3. Cover crops—Mustards, Sudan grass, and arugula have been grown as a cover crop in fallow fields to reduce nematode densities. However, growing cover crops in an existing vineyard in the row alleyway may not be very effective if roots and nematodes don't reside there. Additionally, it may be difficult to find a cover crop that is not a host of the nematode. For example, mustards are hosts of root knot nematodes. 4. Postplant grape nematicides—Seventeen postplant nematicides are regis- tered in Washington for grapes, Zasada said, but there is little efficacy data from Washington vineyards. Also, the mode of action on some nematicides is not well understood. "We don't yet have a good handle on the economic impact of pop- ulation densities or a lot of information on the efficacy of products." Economic threshold numbers (population numbers which cause economic impact) used in California and Australia aren't applicable in Washington, but she noted that a sample of 100 root knot nematodes per 250 grams of soil is a level that warrants action. Data has shown that active nematode developmental stages, such as juve- nile stages, are more susceptible than adult resting stages. In a trial with Cordon (1,3-dichloropropene) applied to an existing vineyard through drip irrigation, fall applications reduced the nematode populations better than applications made in spring when most of the population is in the egg stage. "So if you apply in spring when there aren't that many active juveniles, the chemical won't be as effective as fall applications when populations are peaking." —M. Hansen