Root-knot nematodes and herbaceous perennials

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.  

Root-knot nematodes are important pathogens of herbaceous perennials. In Michigan, the most common species is the northern root-knot nematode, Meloidogyne hapla. Where most root-knot nematodes are subtropical or tropical in their distributions, M. hapla is often the dominant species in temperate areas. Another species also found in temperate areas is the Columbia root-knot nematode, M. chitwoodi. This species has yet to be detected in Michigan, but does occur in the Netherlands where many of our landscape plant species are propagated. Efforts should be made to keep Michigan free from this species of nematode for as long as possible.

Feeding by root-knot nematodes results in swellings or knots on roots called galls. Some species, such as the southern root-knot nematode, induce very large galls but those that develop as a result of northern root-knot nematode feeding are quite small. On some common herbaceous perennials such as daylilies and Hosta that have large, fleshy roots, the galls associated with northern root-knot nematode are inconspicuous and often difficult to discern by untrained eyes.

All root-knot nematodes are attracted initially to the zone of elongation of roots where the vascular tissue has yet to differentiate. As the roots grow and vascular tissue forms, the heads of root-knot nematodes are found within the phloem and xylem thereby disrupting the function of the vascular system. Due to their feeding behavior, root-knot nematodes rob their hosts of many of their nutrients and also disrupt water flow. It has often been noted with heavy infestations that root-knot nematode-infected plants appears starved for water.

Sites should always be sampled from nematodes prior to establishment of herbaceous perennials. Preplant sampling is even more important since the removal of fenamiphos (Nemacur) from the market. For years, in Michigan, nematode-infested plants could be treated with oxamyl (Vydate), until it was not re-labeled for use on landscape plants, and then Nemacur. Whereas, neither product provided complete control (100 percent mortality) of nematodes, especially root-knots, they were quite effective. However, currently, there are no post-plant chemical options, so most, if not all, nematode management strategies and tactics must be implemented prior to planting.

Field managers in some of Michigan’s nurseries have reported that symptoms due to northern root-knot nematode feeding are not evident until the second or third year plants are in the ground. There are really only two plausible explanations for this observation barring alien nematodes. Either sites were infested with northern root-knot nematodes at the time of planting or northern root-knot nematode-infested planting stock was purchased and planted. If a site was heavily infested with northern root-knot nematodes, any plants previously harvested from the field would have significant galling indicating population reduction of the nematode was critical before replanting. If very few plants were infected, these would enter the market but probably no tactics would be utilized to control northern root-knot nematodes.

For new plantings, roots are often inspected prior to transplanting and hopefully any exhibiting symptoms are culled. Therefore, it is assumed, initially, the population density of northern root-knot nematodes are usually low, possibly even at a non-detectable level. Time is therefore required for the nematode to build to damaging levels, hence resulting in seemingly healthy plants the first year of growth.

Fields with nematode infestations are often fumigated prior to growing herbaceous perennials. However, plants eventually harvested from fumigated fields are often infested with northern root-knot nematodes. If infested planting stock was used, the money spent for fumigation was wasted as a severe infestation may result as a consequence of eliminating biological control agents or competing organisms from the soil. Even if nematode-free planting material was used, infestations may still result as nematodes that were outside the fumigant’s kill zone eventually invade the roots. Water can passively move nematodes deeper into the soil in channels produced by earthworms, plant roots or just bulk flow. As roots grow, the nematodes move vertically to attack them. It should also be noted the rooting depth of plants plays a serious role in the vertical distribution of nematodes in soil. The deeper the roots grow, the deeper the worms are found. Nematode control may also be compromised in the top inch or two of soil depending on the fumigant and mode of application.

The fumigation kill zone can be increased by using a higher rate or by simply applying the fumigant deeper into the soil. Shallow and deep fumigation is often the best approach to control nematodes after replacing old vineyards but really isn’t necessary in fields with histories of herbaceous perennials. However, root growth is an important factor to consider. Alfalfa, for instance, can grow deep into the soil and is a very good host for northern root-knot nematodes. Therefore, you should expect these nematodes to be distributed deeper in the soil following two or three years of alfalfa growth than a much shallower rooted plant species.

Plant-parasitic nematodes are typically disseminated over long distances in plants or in soil adhering to equipment. Even if low population densities are relocated, if left unchecked, major nematode problems can develop. To reduce the spread and impact of these microscopic plant pathogens, Michigan’s nurseries should all have nematode management programs. Successful nematode management programs typically have a strong sampling component, but very few nurseries sample routinely for nematodes. Early detection goes a long way toward avoiding serious issues with nematodes.

Nematode samples can be sent to MSU Diagnostic Services. Please consult our web site for address, sampling instructions, forms and other pertinent information. There is a $25.00 fee for a nematode analysis. Typically, results will be returned within seven to 14 following sample submission.

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