You may not be able to see them with the naked eye, but if you were to pick up a handful of soil, it would probably contain 100 or more very small roundworms called nematodes.
Most soil nematodes are not harmful to plants, as they feed off bacteria, fungi, and other nematodes. However, there is a small percentage of nematodes that are plant pathogens. They attack and feed on plant tissues, typically roots. In general, plant parasitic nematodes reduce water and nutrient uptake, which subsequently affects plant yield.
They can also cause physical/mechanical damage to the host that can serve as entry points for other pathogens. In the case of small grains in the Pacific Northwest, the major plant-parasitic nematodes capable of causing economic damage are root lesion nematodes (Pratylenchus), cereal cyst nematodes (Heterodera), and root-knot nematodes (Meloidogyne).
Root lesion nematodes are commonly detected in small grain fields. These nematodes can feed on the outside of plant roots on the root hairs, but they can also completely enter plant roots.
Root-lesion nematodes cause a lot of damage as they migrate and feed on plants. In some plants, the nematode infections will lead to obvious necrosis (dark, dead patches) on the roots.
They can also cause retard root growth. It may be difficult to see the belowground symptoms, but looking above ground, the symptoms will be patches of unthrifty and/or yellowed plants, similar in appearance to other root rot pathogens like Rhizoctonia. In the PNW, the two most common species of root lesion nematodes are Pratylenchus thornei and P. neglectus.
These species have broad host ranges, infecting cereals and many rotation crops, such as pulses, as well as common weeds. Management of root-lesion nematodes includes crop rotation with non-hosts, using varieties with genetic resistance, and using good sanitation practices on farm equipment to help prevent the further spread of this nematode.
Cereal cyst nematodes, like root lesion nematodes, are root-infecting nematodes. There are two species of cereal cyst nematodes of concern in the PWN, H. avenae and H. filipjevi.
They have a slightly different lifestyle compared to root lesion nematodes in that once they are completely inside the roots, they establish permanent feeding sites and will not move again; they simply sit still and feed. As with root lesion nematode infections, it may be difficult to know if you have a cyst nematode infection because finding tiny cysts in the soil or on the roots may be difficult if you are not carefully looking.
Above ground, symptoms are non-specific. You may see patches of yellowed and stunted plants in the fields. Unlike root-lesion nematodes, cyst nematodes found in small grain fields have a much narrower host range, infecting only grasses. The narrow host range of these nematodes can be exploited for their control; crop rotation with non-hosts is a possible control strategy.
Root-knot nematodes are another group of nematodes that may cause problems in small grain fields. Root-knot nematodes get their name because infections cause the formation of root knots or galls on the root systems. Like cyst nematodes, they completely enter the plant roots and turn surrounding plant cells into feeding sites.
Once they start feeding, the nematode stops moving and just sits still to feed. The cereal root-knot nematode Meloidogyne naasi can infect barley, wheat, sorghum, and some grasses. M. chitwoodi is a root-knot nematode endemic to the region and it has also been shown to infect small grains and cause small galls on the roots. In general, root-knot nematodes are more common in irrigated soils.
There are general management strategies for all three nematodes. Avoid the movement of soil from infested fields to un-infested fields as nematodes move easily in the soil left on equipment, tools, and even boots. Identification of the nematode to species, such as which species of cereal cyst nematode is present (H. avenae or H. filipjevi), can help inform decisions regarding which variety should be planted.
Winter varieties tolerate damage from root-knot and cereal cyst better than spring varieties as the winter varieties have a larger root system when these nematodes become active in the spring.
Reduction of plant stressors, including soil compaction, pH, and nutrient deficiency encourages plant growth, therefore, tolerance to nematode infection. No chemical fumigants are currently registered for use in small grains; fumigants are also unlikely to be economical unless there is a high-value crop in rotation.