Plant-parasitic nematodes are major pests of red raspberry, reducing yield and cane growth, and leading to economic losses in many production regions (Bélair, 1991; McElroy, 1991; Szczygiel and Rebandel, 1988; Trudgill, 1986). Plant-parasitic nematodes were first reported in Rubus species in North America in the 1930s, when root lesion nematodes were associated with declining red raspberries (McElroy, 1992). Growers have been able to manage plant-parasitic nematodes in raspberry crops by pre-plant fumigation with methyl bromide and other soil fumigants and/or post-plant treatment with fenamiphos. However, in recent years, many of the most effective nematode management tools are no longer available, with fenamiphos being removed from the U.S. market in 2007, and the phasing-out of methyl bromide use in all U.S. commodities in 1995. In addition, new U.S. Environmental Protection Agency (USEPA) reregistration eligibility requirements (REDs) for common soil fumigants will put restrictions on their use (USEPA, 2009). Therefore, the red raspberry industry is at a time when long-term, economically viable plant-parasitic nematode management strategies must be developed.
The root lesion nematode (RLN) is a migratory endoparasite that moves between soil and roots, but feeds on and migrates in root cortical cells. Feeding by the nematode kills tissues in the root cortex, which appear as necrotic lesions or spots on roots. On raspberry, RLN feeding on feeder roots can reduce the capacity of the plant to uptake nutrients and water. The RLN was shown to cause 24% mortality of red raspberry plants after 2 years (McElroy, 1975). The rate of raspberry decline depends upon the nematode population density but usually occurs over a 3- to 4-year period (McElroy, 1992). The rate of decline will also depend upon the variety and environment in which a plant is grown, but clearly, when this nematode is left unchecked and population densities increase in established raspberry plantings, significant yield loss can occur.
Currently, there are few post-plant nematicides labeled for use in red raspberry. For example, a recent search of the Crop Data Management Systems Inc. (CDMS) pesticide label database identified only two commercially available products, Nema-Q (saponins of soapbark; Monterey AgResources, Fresno, CA) and Ecozin (growth regulator containing azadirachtin; AMVAC Chemical Corp., Los Angeles), labeled in Washington State (CDMS, 2009), neither of which has been tested for managing RLN in raspberry crops. However, there are other insecticides/nematicides labeled for use in the United States that could potentially be used to control plant-parasitic nematodes in red raspberry.
Traditional nematicides that are registered for use in the United States include fosthiazate, oxamyl, and 1,3-dichloropropene (1,3-D). Fosthiazate has been shown to effectively control plant-parasitic nematodes in a diversity of crops, including potato [Solanum tuberosum (Ingham et al., 2000)] and strawberry [Fragaria ×ananassa (Gilreath et al., 2008)]. Oxamyl is labeled for use in perennial crops such as apple (Malus spp.), banana (Musa spp.), citrus (Citrus spp.), and pear (Pyrus spp.), and is labeled for use on raspberry in Canada. While 1,3-D is known to be a very effective nematicide when applied as a soil fumigant (Gilreath et al., 2008; Schneider et al., 2008), it is also marketed for application by drip irrigation. A relatively new nematicide that belongs to the fluoralkenyl group is MCW-2 [5-chloro-2-(3,4,4-trifluorobut-3-enylsulfonyl)-1,3-thiazole; Makhteshim Agan, Airport City, Israel]. This compound has a far lower mammalian toxicity than organophosphate or carbamate nematicides, exhibits low leaching potential in soil, and has a relatively short half-life in soil (Oka et al., 2008a).
Another grouping of nematicides includes plant-based products, including furfural (2-furancarboxaldehyde), saponins of soapbark, and extracts of walnut (Juglans regia). Furfural is a liquid found in many essential oils from plants, and is present in fruit juices, alcoholic beverages, and bread (Rodríguez-Kábana et al., 1993). The mode of action of furfural against nematodes has been described as the destruction of the nematode cuticle (Burger, 2005). A product based on the saponins of soapbark is labeled for use on red raspberry and is approved for use in organically managed cropping systems. Plants in the family Juglandaceae (including walnut) produce juglone, which is known to be active against a range of organisms, including weeds (Shrestha, 2009) and nematodes (McKenry and Anwar, 2003).
Two insecticides that have received attention as potential nematicides are methomyl and spirotetramat. Methomyl, a carbamate, was effective as a nematicide against the cereal cyst nematode (Heterodera avenae) applied to wheat (Triticum sp.) as a broadcast application (Brown, 1973). Spirotetramat is systemic in the plant and is registered for use in several perennial cropping systems, including grape (Vitis spp.), cherry (Prunus spp.), and citrus for the control of insects, including aphids (Aphidoidea), phylloxera (Viteus vitifoliae), and thrips (Thysanoptera). Recently, spirotetramat has shown the potential to control plant-parasitic nematodes in perennial crops (M. McKenry, personal communication). There are also products that are not marketed as nematicides per se, but as plant performance products based upon the use of plant hormones, supporting nutrients, and other hormone cofactors designed to ensure optimum hormone balance and activity to withstand stress, including plant-parasitic nematodes.
The goal of our research program is to identify nematicides and management practices that can be labeled and used to minimize the impact of plant-parasitic nematodes on red raspberry crops. The specific objectives of this research were to identify nematicides that are directly toxic to RLN in soil, to identify nematicides that suppress RLN population densities in established red raspberry plants, and to determine if there are any phytotoxic effects of the nematicides on raspberry plants.
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