I recently received a call from a PCA who had questions about nematodes that infect alfalfa. He had observed stunted crop regrowth after the first cutting and had reason to think that nematodes could be the cause.
There are three nematodes that tend to be the most problematic in alfalfa. Stem nematode (Ditylenchus dipsaci) is a nematode that feeds on above-ground plant parts, unlike most plant-parasitic nematodes that feed on roots. To recap a blog by Rachael Long and Dan Putnam, stem nematode symptoms include stunting, shortened internodes, and even plant death when infections are severe. It is a fairly easy pest to diagnose if you have a dissecting microscope available. Cut up some crowns and shortened stems in a petri dish; add water; and the nematodes can be seen emerging from the tissue, swimming in the water. Stem nematode prefers heavier soils and cooler, wetter conditions. As the weather warms, they go dormant, and hence, it tends to be a spring pest in the Central Valley. While stem nematode has a wide host range, the alfalfa stem nematode, as a race, has a limited host range. For that reason, rotation with crops like corn, small grains, and beans can help to alleviate their infection. Planting with resistant varieties, however, is probably the best management practice. Chemical controls tend to be cost prohibitive.
Several species of root knot nematode (Meloidogyne spp.) may infect alfalfa. Symptoms of root knot nematode include excessive root branching and possibly stunted growth of the stems and leaves. This is because the pest prunes feeder roots and inhibits the plant’s ability to take up water. Signs of the pest are lateral root galls. These galls are not usually as large as nitrogen-fixing bacteria nodules. The galls and nodules can be distinguished by rubbing the roots with your fingers – nodules are easy to dislodge, but galls are not. Infection with root knot nematode may increase the susceptibility of alfalfa to other pathogens, including bacterial wilt, Phytophthora root rot, Fusarium wilt, and stem nematode. The pest prefers sandy loam soils and warmer soil temperatures. It has a wide host range, which means that crop rotation is not usually a successful means of control. Fumigation can be effective but is expensive, and currently, there are no non-fumigant nematicides registered for alfalfa. The best management practice is planting resistant varieties.
Root lesion nematode (Pratylenchus spp.) infection may be difficult to diagnose because the symptoms include stunting and nutrient deficiencies. Brown or black lesions on the roots may also be visible. If root lesion nematode is a suspected pest, the best way to have it diagnosed is to take soil samples to a commercial laboratory for nematode identification. Root-feeding nematodes are identified by their stylet, or probe-like feeding structure, which cannot be seen under a dissecting microscope. The UC Integrated Pest Management website has a description of how to collect soil samples when root-feeding nematodes are suspected pests. (See the “Field Evaluation” section.) Root lesion nematodes are found worldwide in temperate and tropical environments, preferring sandy and sandy loam soils. Their presence can break alfalfa resistance to fungal and bacterial pathogens, and they have been implicated in replant disease in tree crops. Because they have a wide host range, crop rotation generally does not help manage this pest, but fallowing a field and then treating it with a nematicide may reduce root lesion nematode populations. Also, resistant varieties have become commercially available.
A common theme with these three pests is that chemical controls may not be worth the expense, but resistant varieties are available. Consider this when planning your future rotations. The National Alfalfa Variety Review Board and the Association of Official Seed Certifying Agencies produce a guide for alfalfa varieties and their pest resistance. Also, remember that these pests are moved with soil and water, so good sanitation practices can help to reduce the spread of these nematodes.