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NONFUMIGANT NEMATICIDES |
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CONTENTS [Return to Post-plant Options] |
Several compounds that are effective nematicides are classified as
nonfumigants and may be formulated as granules, emulsifiable concentrates or liquids. In
contrast to fumigants, these compounds have a low volatility, so they do not evaporate out
of the soil rapidly and do not require any form of soil sealing. However, this also means
that these compounds do not volatilize and therefore only control nematodes where they are
placed or where they are moved to by water or tillage. They are highly toxic, so low dose
rates are effective and there is often a considerable amount (90%) of inactive ingredient
included to help facilitate dispersal of the product. Most compounds have very low
phytotoxicity, so they can generally be used either before or after planting. However,
while they may be effective against nematodes and soil borne insects, they will not
control fungi or weeds. Their action may be to kill nematodes directly (nematicidal)
and/or to immobilize, disorient or disrupt host finding or feeding behavior
(nematistatic). Suppression of populations and/or feeding in infested mint improves vigor
and restores stand density.Characteristics
of Vydate Nematicide
Vydate (DuPont) is the only nonfumigant nematicide that is currently registered for use on mint. The active ingredient (a.i.) is oxamyl and is formulated in Vydate 2L as a 24% liquid that contains 2 lb a.i./gal. Oxamyl is completely miscible with water and moves readily in soil with soil moisture, penetrating all the soil reached by irrigation or rainfall. Persistence, as measured by half life, is greatly influenced by temperature, pH and organic matter. For example, 64% of oxamyl is lost after only six hours at pH 9 and none remains after two days. At pH 7, only 24% and 45% is lost after three and four days, respectively, and no loss occurs at pH 5. Therefore, growers should use caution when applying Vydate to a field that has recently been limed since the surface may have a much higher pH than the soil as a whole and product may be lost before it can be watered in adequately Half-life decreases from 32 days at 5C to 8 days at 25C. Soil organic matter, and to a lesser extent, heavier soil texture, increases persistence in soil and reduces the depth of movement. The loss of oxamyl is caused by hydrolysis into a nontoxic compound (Allison, 1980). Oxamyl is taken up by both roots and leaves, but movement into leaves is passive and relatively slow. The addition of adjuvants has not been shown to improve nematode control. Although the chemical moves up and down the plant, only a relatively small amount applied to leaves gets transported to the roots since most goes to growing points, leaf tips, and other leaves. Application to foliage and transportation to roots provides some protection from feeding. However, after foliar applications, the half-life is approximately one week because oxamyl rapidly breaks down into a nontoxic compound (Bunt et al, 1977, Harvey and Han, 1978). Breakdown in soil is much slower, with activity extending to 21 days. In addition, accumulation of oxamyl at the surface or outer cortex of roots has been reported to be 13 times greater after root application than after foliar application (Wright et al, 1980). With either form of application, most of the chemical activity occurs at the soil-root interface where most nematodes are found. The best protection occurs when application is made while roots are developing during the establishment phase of early root growth. The action of oxamyl in the soil is as a "contact nematistat," inhibiting movement and preventing feeding by inhibiting neuroenzymes such as acetyl cholinesterase. This may affect nematode movement, and the function of sense organs. Egg hatching, movement and feeding can be inhibited at concentrations of 5-10, 2-5, and 0.5-2 ppm, respectively (Allison, 1980). Death of nematodes can occur at high concentrations or if feeding is prevented long enough to cause starvation. However, nematodes will recover and resume normal activity if removed from the compound before a sufficient contact time. Inhibition of feeding during critical growth periods allows the plant to establish a strong root system which can better tolerate nematode populations if they increase later in the season. In many cases, the inhibition of nematode feeding and/or movement delays reproduction and population growth so that populations at the end of the season are still considerably less than in nontreated soils |
