Nematodes
Cotton Nematodes in the Fall - Don't Neglect Them
I recently read an informative article in a magazine about the
effects of record-breaking rainfall this summer on cotton root
development. Heavy rains and saturated soil in much of the mid-South
kept the plants from developing a normal root system. The take-home
message of the article was that without full root development, all
facets of plant development can be affected. The article went on to
stress the importance of developing a management strategy for the crop
that considered the fact that the plants were not functioning with a
root system that was one hundred percent.
In reading the story, it dawned on me that this same problem faces some of
our farmers every year—although not because of excessive rain. In fields all
across the mid-South, nematodes keep plants from developing normal root systems
EVERY year. We just don't notice this like we do when it is caused by
water standing in the row middles for weeks at a time.
In fact, both major economic nematodes of cotton in the mid-South, root-knot
and reniform, can cause extreme damage to cotton roots. Sometimes the damage is
obvious but other times it is much more subtle. There is no mistaking the damage
to the root system that occurs in a cotton plant that is infected by the
root-knot nematode. The galls that are formed seriously change both the
appearance and the functionality of the root. Severely infected plants may have
damaged tap- and lateral roots, or taproot development may be poor to almost
non-existent. These root systems will never perform normally, and the problem
gets progressively worse throughout the season as more and more nematodes infect
the plant. Water absorption is significantly decreased in infected plants, and
translocation of nutrients is inefficient.
Reniform nematodes affects plants similarly, but reniform damage is less
obvious--and even downright sneaky. Reniform nematodes do not cause galls to
form on the roots. There are no obvious symptoms of infection by this nematode
other than the fact that infected plants have much smaller root systems
(including the taproot). The problem for a cotton producer is the same, however,
because small root systems mean less water and nutrients moving up the plant to
leaves and bolls. As with root-knot, root damage gets progressively worse
throughout the season.
How much damage is caused and what does it mean in terms of yield loss? It
varies according to several things including the number of nematodes in the
field at the beginning of the season, the soil type, and the weather during the
growing season. In heavily infested fields, studies in Arkansas during the last
five years using soil fumigation to eliminate most of the nematodes prior to
planting, resulted in yield increases ranging from 150 to 220 pounds of lint per
acre. The magnitude of the response depended on the year. Last year when growing
conditions were very good, responses were on the lower end of this range. The
year before that, responses were higher. It did not seem to matter whether we
were working with root-knot or reniform nematodes, the responses were pretty
much the same within a given year. Keep in mind, however, that the fields we
chose for our tests had very severe nematode problems. Fields with lower
nematode pressure might have shown less dramatic yield improvements, but these
studies illustrate how much a severe nematode problem can cost a producer.
The point of all this is not to sell nematicides to cotton producers. There
are lots of fields out there that don't need nematode control. And, there are
also lots of fields with low nematode pressure where the cost of the control
would be more than the cost of living with the problem another year. BUT - there
are a large number of fields throughout the mid-South, that need nematode
management. This fall would be a really good time to check some fields to see
what their nematode status is.
Obtaining a nematode assay is as simple as collecting a soil sample
(following some simple guidelines) and having the sample assayed for nematodes
by a laboratory. Arkansas , Louisiana , Mississippi , and Tennessee all have
public nematology laboratories that can provide assays for a small fee. There
are also several private labs that can provide this service. In cotton, the best
time to sample fields for nematodes is immediately after harvest and stalk
destruction . Nematode population levels will be at their highest point of the
season at this time of year and nematodes are most easily detected. Information
on the proper techniques for collecting and handling nematode samples is
available through all county extension offices and local county agents will be
more than happy to provide some general guidelines and specific information.
Anything that reduces root function can adversely affect the growth,
development, and yield of a cotton crop. Whether the problem is too much water
like this summer or root-knot or reniform nematodes - lack of a healthy,
vigorous root system can turn a potential profit into a serious disappointment.
It pays to know what is in your fields before you plant again.
And it can pay off big .
Terry Kirkpatrick
Professor and Extension Nematologist
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