An important component for the sustainable production of red raspberry in the Pacific northwestern United States [PNW (includes Washington and Oregon)] is the use of winter cover crops before establishing the next planting. Because of the limited land suitable for red raspberry production, PNW growers commonly replant in the same location after removing the previous planting. After the last summer harvest, all trellising infrastructure is removed, canes are mowed, and the plant material is chopped and incorporated into the soil. Soil fumigation commonly occurs in the summer or early fall shortly after the incorporation of the plant material when soil temperatures are still above 50 °F. Heavy and frequent precipitation is common during winter in the PNW. A winter cover crop is often overseeded before fumigation to provide soil coverage and protection during this time. In the early spring, beds are shaped and bare-root or tissue culture red raspberry is planted. Trellising infrastructure, including posts and wire, is then put in place.
One of the primary production challenges red raspberry growers confront is soilborne diseases, which includes root lesion nematodes (Zasada et al., 2015). This migratory endoparasite moves between soil and plant roots and has a broad host range of more than 400 plant species (Castillo and Vovlas, 2007). A common strategy to manage RLN in red raspberry is through preplant soil fumigation, primarily with fumigants containing 1,3-dichloropropene (1,3-D), chloropicrin, or methyl isothiocyanates (e.g., metam sodium) (Walters et al., 2017). Depending on the grower’s needs, soil fumigation can occur in the late summer/early fall or spring before planting. The efficacy of this application depends on land preparation, soil type, and temperature. In the red raspberry production system, fumigants are applied without covering the soil with a polyethylene or impermeable tarp, which can leave the top 4–6 inches of soil untreated, making it a reservoir for RLN in sandy soils (Kroese et al., 2016). Application of post-plant nematicides is an additional option for red raspberry growers attempting to manage RLN. However, only oxamyl (Vydate® L; DuPont Agricultural Products, Wilmington, DE) is presently being used, with application restricted to nonbearing acreage (Zasada and Walters, 2016).
Crop rotation has been suggested as a cultural management practice to reduce the severity of RLN and other soilborne diseases in the red raspberry system. Although crop rotation has many agronomic benefits, its efficacy for reducing RLN population densities is limited given the broad host range of this nematode. Furthermore, land suitable for red raspberry production is limited in the PNW and the justification to rotate to another crop species is often not economically viable. Red raspberry is a high-value crop and there are few crop alternatives of equal value to which PNW growers can rotate. For this reason, PNW growers practice a very limited, short rotation with a winter cover crop (Pacific Northwest Extension, 2007). The winter cover crop most commonly used by red raspberry growers is winter wheat, which is grown in between late summer/early fall fumigation and spring planting the following year. A winter wheat cover crop is normally seeded before fumigation because seeds are able to survive exposure to fumigants at the shallow depth of seeding. Although this cover crop helps maintain soil health and limits erosion during the winter months, it may also be an unintentional green bridge for RLN into the next red raspberry crop.
Because winter wheat may serve as a green bridge for RLN to infect the following crop, managing RLN populations while maintaining a cover crop is a potentially effective compromise. Therefore, applying a nematicidal product to the cover crop may be a viable option. Although oxamyl is used as a post-plant nematicide in nonbearing raspberry (Zasada and Walters, 2016), it is not labeled for use on wheat. Methomyl (Lannate® LV, DuPont Agricultural Products) is a broad-spectrum insecticide that has also been shown to be effective against root-knot nematodes [Meloidogyne incognita (Desaeger et al., 2011)] and is registered for use in wheat. However, it has not yet been evaluated against RLN.
The objectives of this project were 2-fold: 1) to determine if winter wheat provides a green bridge for RLN in continuous red raspberry production systems, and 2) to determine if modified winter cover cropping practices can be used to reduce population densities of RLN before replanting red raspberry. Cover cropping practices under investigation included modification of planting and destruction times, as well as methods of destruction (tillage or herbicide), which included the additional application of a systemic insecticide with nematicidal properties.
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