Adapted from the article “General Guidelines for Sacramento Valley Orchardists Wishing to select Rotation Crops and Intercrops that do not increase Nematode Pests” by Michael McKenry, Extension Nematologist, Kearney Agricultural Center in the July 2011 Sacramento Valley Almond News.
The two nematodes of major concern to Sacramento Valley orchardists are root lesion nematode, Pratylenchus vulnus, and ring nematode, Criconemoides xenoplax. Any crops considered in the following discussion as intercrops in young orchards or as rotation crops should not be planted two years in succession if used as alternatives.
Root Lesion Nematodes
There are eight species of root-lesion nematode in California. The single species currently of concern is Pratylenchus vulnus. Almost every woody perennial will host this nematode but to varying degrees. English walnut, Paradox hybrids and Black walnut are all highly susceptible. Cherry and rose are similarly great hosts of P. vulnus. These nematodes can slowly spread across an orchard via tillage and irrigation.
Most of the Prunus rootstocks are a notch lower as a host but can still be considered good hosts of P. vulnus. Populations of P. vulnus can flourish in any soil type and (on deep rooted perennials) at any rooting depth. Across a list of potential intercrops, none will host this nematode as well or as deep as woody perennials.
There are annual crops such as sudan grass and milo that have a negative impact on P. vulnus. One year of fallow can reduce P. vulnus soil populations by 50%. Five years of fallow can reduce P. vulnus by 95%. One year of growing sudan grass instead of simply fallowing can reduce P. vulnus by 60%. For P. vulnus sudan grass and milo are among the best choices for a rotation crop.
Annual crops that host P. vulnus make a much shorter list but whatever crop is chosen; do not replant it again down the same rows in two consecutive years. The list of annual crops that will host P. vulnus is best determined by referring to NEMAPLEX. This compiling effort by Howard Ferris, Nematologist at UC Davis, does not list some crops that are not consistent hosts across all cultivars. For example is some cultivars of strawberry, field corn and sweet are hosts for P. vulnus but are not listed.
Ring Nematodes. There are numerous species of ring nematode in the Sacramento Valley. There is one that prefers to feed on roots of perennials including almond and walnut. This nematode can feed near roots that leak some rather nasty substances, such as cyanide from almond roots. Mesocriconema xenoplax has a lengthy host list. Some of its favorite foods are legumes but it will be difficult to accurately determine which crops it does not feed upon.
M. xenoplax builds to its highest population levels within soils that are highly porous. Fine sandy loams can support several hundred per soil sample but do not usually exceed that population level over the life span of a perennial. Higher population levels can be achieved in coarser sands or well-structured clay loam soils. Weedy orchards can be a good source of ring nematode build up and their presence can be every bit as important as intercropping choices.
Root-knot Nematodes. Meloidogyne spp. are of lesser concern in the Sacramento Valley but can become noteworthy if the orchard crops roots are also a great root-knot host such as Lovell or Krymsk 86. Second, root-knot nematodes present in the Sacramento Valley tend to be Meloidogyne hapla. This species produces barely visible root galls and is not damaging to perennials unless their population levels exceed at least a thousand or more per soil sample.
How good of a nematode host are vegetables or seed crops intercropped in young orchards? These are all great hosts for Meloidogyne spp. (root knot) but the Sacramento Valley is generally safe from the really harmful species of this nematode. These intercrop choices are also not of concern for P. vulnus (lesion) but could easily be as much of a host for ring nematode as are common weeds.
How good of a nematode host are oats, barley, alfalfa and sunflowers? Barley and oats are good intercrop choices because of their timing, their cost, and their nematode host status. First, winter time crops (mid November to mid April) do not increase nematode populations because soil temperatures are generally below 60F. Use NEMAPLEX to determine if some cultivars have been reported as a good host.
California selected alfalfa tends to be a good host for M. hapla (root knot) only. This species is not damaging to perennials unless their population levels exceed at least a thousand per soil sample. Our California selections of alfalfa are not a good host for P. vulnus. Alfalfa cultivars from some northern states (e.g. Minnesota) can host Meloidogyne spp. other than M. hapla. However, all legumes such as alfalfa are a good host for ring nematode. Avoid intercropping almonds and other Prunus spp. with alfalfa.
Sunflowers and safflower are of minimal concern with Meloidogyne spp. and P. vulnus but they will probably grow ring nematode. Crops like safflower render soils so dry that during the spring warming periods nematodes cannot find enough soil moisture to migrate to a root.
In summary, there are multiple factors to contend with when answering questions about intercropping. The safest answer north of Sacramento is do not replant the same intercrop or rotation crop two years in succession.