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 www.pestid.msu.edu
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.