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Use of Hot Water for Nematode Control: A Research Summary
Joseph W. Noling, nematologist, University of Florida, Institute
of Food and
Agricultural Sciences, Citrus Research and Education Center, Lake
Alfred, FL 33850.
The use of hot water is not a new concept to nematode management. Belwey
(1923) found that it took two million gallons of hot water per acre by a
surface drench method to achieve nematode control. Compton (1936)
devised
a portable hot water sterilizer to be used at the end of a steam
line for
killing soil nematodes. Since the 1930's, most research has
focused on procedural development of hot water dips for nematode
disinfestation of plant materials. Only more recently have studies been
reinitiated to evaluate soil applications of hot water for nematode
control (Noling et al; 1994). This report attempts to summarize Florida
research efforts on the use of hot water for nematode control utilizing a
prototype hot water machine.
During the Fall of 1992, the first experiment with hot water was
conducted and demonstrated that drip irrigation system delivery of hot
water (104oF) could not provide effective
nematode control, particularly at soil depths in excess of 8 inches. A
second experiment in the spring of 1993 indicated that a
"bottoms-up" approach, where a majority of total hot water soil
input was delivered 16-18 inches below the finished plant bed, did not
uniformly heat soil or provide nematode control within the surface 6
inches of soil. Since then, field experiments have focused on evaluating
modifications to soil incorporation and hot water delivery systems. In
some studies, hot water was applied as a surface drench or injected into
the soil directly at a depth of 8-10 inches via 10-12 steel chisels.
Rototilling and rotovation soil incorporation methods have been evaluated.
Tractor speeds were varied between 0.2 and 1.2 mph to examine the
influence of dosage and total volume of hot water delivery per unit length
of plant row. Water temperature and flow rates were held constant at
temperatures between 220-230oF and 75-90
gpm. Soil temperatures were usually monitored at 3 or 4 depths, ranging
between 2 and 18 inches, and compared with equivalent measurements in an
untreated control.
The overall results from hot water experiments performed in Florida
since 1994 indicate that irrespective of soil depth, maximum soil
temperature elevations above that of the untreated control increase
linearly with temperature treatment. The soil is generally heated very
rapidly and in most cases, does not return to ambient conditions for many
hours following treatment. The data also suggest that threshold levels of
total hot water dosage required to elevate soil temperatures of a fine
sandy soil (96% sand, <2% silt, clay, organic matter) to achieve
nematode control under a plastic mulch covered plant bed is in the range
of 30,000 to 70,000 gallons per treated acre. The wide range in water
requirements is due to heating inefficiencies caused by differences in
soil type and moisture content, as well as initial, seasonally defined,
soil temperature conditions. For example, comparisons of field trials
performed during the spring, summer, fall, and winter months showed that
up to twice as much hot water may be required during the winter months
when soil temperatures of 60oF occur. The
method of soil incorporation also appears to be very important in
determining control. For example, rototiller mixing of soil in a vertical
plane tends to increase heat losses by allowing cool air to intrude the
soil and allowing heated water vapor to escape with each revolution of the
rototiller blade. But, rotovator incorporation, mixing hot water into soil
in a horizontal plane, minimizes these losses by embedding the heated soil
layer at the depth in which hot water is injected into soil. Other studies
have also confirmed that irrigation water (79oF), introduced as simulated rainfall immediately
after a hot water soil treatment, reduces maximum temperature development
and increases the rate of heat loss, thereby reducing cumulative exposures
of nematodes to elevated soil temperatures.
The depth at which lethal temperatures have been achieved (8-10
inches) also appears to be dependent upon soil incorporation depth. For
example, in sandy soils, it is not possible to escape significant heat
losses occurring via downward percolation of hot water into deeper, cooler
nontarget soil profiles. In contrast, due to the slow downward percolation
of water within heavier textured soils, water tends to pond at the depth
of soil incorporation, and heat losses to deeper soil layers appear to be
significantly reduced. Soil temperature gradients are immediate and
transition zones between hot and cold soil narrow. To date, the most
promising use of hot water soil treatments appears to occur in heavier
textured soils or in soils where a compacted or impermeable layer
restricts and delays downward, gravitational movement of hot water. The
fear exists, however, that regardless of soil type, lack of pest control
in soil horizons below the incorporation depth will allow subsequent pest
recolonization and only delay pest impacts to crop growth.
New technological advances in hot water generation, delivery,
distribution, and soil incorporation must still be developed to adapt hot
water methods for broad scale, commercial field use. Further research is
also needed to determine, in real time, hot water volume requirements for
efficacious field soil treatment regimes. It also appears that commercial
development and expanded use of hot water soil treatments for nematode
control will also depend on overcoming other technical, environmental, and
economic constraints. Because hot water alone is unlikely to substitute
directly for methyl bromide soil fumigation, an integrated system of
combining hot water with other approaches, must also be considered. These
integrated approaches have not been intensively studied and additional
research will be required to maximize pest-specific efficacy, consistency,
and geographical adaptability.
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Last Updated: October 16, 1996
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