Challenges: Perspectives of Expert Growers

Cover Cropping and Nematode Management in Walnut Orchards

(Based on grower interviews conducted in 2021-22)

Nematode infestations can seriously impair tree health, especially in young orchards. Cover crops may potentially serve as one tool to keep nematode populations in check. Here, three walnut orchard managers in California’s Sacramento Valley provide insight into how they have tried using cover crops, especially species in the Brassica family, to control nematodes, with varying success.  

Plant-parasitic nematodes are microscopic roundworms that feed on plant tissues. In California orchards, the most common are lesion, ring, and root-knot nematodes. Nematode damage is highly dependent on the density of the nematode population and the age of the trees. Young trees can be severely stunted and weakened by nematodes if planted into an infested field. Cover crops are known to influence population dynamics of nematodes in orchards (Meyer et al., 1992; Gruver et al., 2010). A general concern with cover crops is that certain species can host plant-parasitic nematodes and can thereby increase their populations. However, some species are also used to reduce nematode populations, as illustrated by these three walnut growers. 

Alan Siegle of Double A Walnuts in Colusa has a 35-acre block of walnuts that he knows has nematode issues and has been doing trials with the Community Alliance with Family Farmers to see if there are cover crops that will reduce the nematode numbers. He planted a mix of grasses with mustard and tillage radish. So far, the trials are inconclusive, with Alan saying, "I'm not convinced that that mix is really having an impact on the field level and the soil testing we've done…[is] not conclusive. In fact, it's very suspect because in the tree strips, we did nematode testing, and we did nematode testing where the cover crops [are]. The levels were lower in the tree strips, which doesn't make any sense. I'm not convinced about the testing". Nematodes are known to have very patchy populations, so sampling design is critical to accurately reflect field numbers (Ferris et al., 1976).  

Rod Bradford also farms walnuts in Colusa County and planted arugula as a cover crop before planting walnuts where tomatoes had previously been cultivated. He says, "whether or not it worked, I don’t know." Several cover crop species are known to decrease nematode populations. One of the most effective is sunn hemp, Crotalaria juncea (Wang et al., 2002), and some of the brassicas (Fourie et al., 2016). Most studies on using brassica cover crops to control plant-parasitic nematode populations are investigating the potential of biofumigation, which involves the incorporation of the terminated cover crop back into the soil with a disc harrow, sealing the soil and keeping it moist (Kruger er al., 2013). It is imperative to understand what species of nematodes are causing the problem, however, because even some brassica cover crops can host nematodes, such as certain species of root knot nematodes, in their roots and contribute to population increases (Edwards & Ploeg, 2014).         

Daniel Unruh farms 193 acres of walnuts in Colusa County, near the Sacramento River. Control of root lesion nematodes was his main aim when he first started cover cropping. After trying some chemical products against nematodes without success, Unruh realized that some products use a mustard derivative, so he decided to plant a mustard-dominated cover crop mix, utilizing primarily Nemagone mustard, supplemented with brown and yellow mustard. He samples every June and carefully marks the area where soil samples are taken so he can return to the same places for subsequent samples. He manages his orchards under no-till, so he does not incorporate the terminated cover crop material back into the soil, instead using a roller-crimper or shredder, However, he has still seen tremendous reductions in nematode numbers, to the point of none being detected in the last few years. 

Overall, the science of plant-parasitic nematode control using cover crops, and what mechanisms are at play, are not yet well understood (for an overview of potential mechanisms, see http://nemaplex.ucdavis.edu/Mangmnt/Covercrops.htm).