move through open air passages, fumigant molecules dissolve into these
forth from the air to the water phase as the fumigant diffuses through the
soil mass. The portion of the fumigant dissolved in the soil water establishes
the concentrations responsible for the kill of most soil-borne organisms.
In practice, fumigants are commonly injected through a series of uniformly
spaced shanks into soil. As the liquid volatilizes, gases begin moving
Because diffusion is greater in air above the soil surface, upward mass
of the gas may escape the soil and enter the atmosphere. As the fumigant
front moves through soil, gaseous molecules are adsorbed to soil particle
soil particles. Maximum fumigant concentration decreases, as do the sums
of CT products, with distance from the point of injection. Eventually, with
time and distance, concentrations fall below an immediate killing level. The
number of pests or disease propagules killed by fumigant treatment within
these areas depend on the number of CT units which develop within the
fumigant treated zone. The relationship between fumigant application rate
but chemical dispersion as well. If dispersal is good, increases in fumigant
application rates will result in higher CT values and provide control to a
greater soil volume. If dispersal is poor, increases in application rates will
not provide control to a larger soil volume. Reducing volatilization losses
from soil is an effective way in which to increase CT values and improve
impermeable plastic mulches (VIF, TIF) are used to increase fumigant
diffusion resistance near the soil surface. This serves to promote both
downward diffusion of the fumigant and to enhance residence time and gas
phase persistence in soil.
PEST CONTROL EFFICACY
Since their introduction to Florida agriculture, many different soil
research trials, particularly those to evaluate methyl bromide alternatives,
have provided the basis for overall generalization of pesticidal activity for
the various fumigant chemicals (see Table 2). In general, this research has
repeatedly demonstrated methyl bromide to be very effective against a
wide range of soilborne pests including nematodes, diseases, and weeds.
Chloropicrin has proved very effective against diseases but seldom nematodes
or weeds. Although with some suppression, bacterial pathogens
have not been satisfactorily controlled by any of the soil fumigants. Historically,
most of the research conducted to evaluate 1,3-dichloropropene
(Telone) has repeatedly demonstrated consistently effective nematode control
with little or no control provided for soilborne disease or weeds. Metam
sodium, metam potassium, and Dazomet are all non-selective pre-plant
soil fumigants which provide measures of fungicidal, herbicidal, insecti-
metam potassium have proved inconsistent, performing good to excellent
in some trials, while poor in others. Field research continues to evaluate
resolve problems of fumigant inconsistency with these compounds. Metam
sodium and metam potassium can provide good control of weeds when
rate, placement, and improved application technology have not resolved
demonstrated good to excellent control of nematodes, disease, and weeds
when coapplied with chloropicrin. Due to issues of off-site odors, DMDS
must now only be used in conjunction with a TIF plastic mulch. AITC, the
newest entry to registered fumigants in Florida has shown promise in
plant pathogens.
are formulated together or co-applied to increase their overall level and
spectrum of pest control. For example, chloropicrin is used both as a
stand-alone soil fumigant, as a chemical warning agent in formulation with
methyl bromide, and in formulation with 1,3-D to increase its disease control
effectiveness. Examples of co-formulated or sequential application include
1,3-dichloropropene plus chloropicrin formulated together as Telone C17
or Telone C35, or sequential applications involving a broadcast or prebed
application of 1,3-dichloropropene, followed later by chloropicrin and/or
metam sodium in separate soil applications. Given the general lack of herbicidal
activity associated with many of the fumigants, separate application of
one or more herbicides may be required to achieve effective weed control.
-
ations of chloropicrin co-applied with additional fumigants. In this co-application
approach, chloropicrin has clearly been shown to be an integral,
foundation component of any alternative chemical approach to providing
broad-spectrum pest control activity. Of the chloropicrin combinations,
Telone C-35, a combination of 1,3-dichloropropene and 35% chloropicrin,
DMDS in combination with chloropicrin (21%) has also been extensively
Table 4.2. Generalized summary of maximum use rate and relative effectiveness of various soil fumigant alternatives to methyl bromide for nematode, soilborne disease, and
weed control in Florida.
Fumigant chemical1 Maximum use rate
Relative pesticidal activity
Nematode Disease Weed
1) Methyl bromide 50/50 350 lb Good to excellent Excellent Fair to excellent
2) Chloropicrin 300 lb None to poor Excellent Poor
3) Metam Sodium (Vapam) 75 gal Erratic Erratic Erratic
4) Telone II 18 gal Good to excellent None to poor Poor
5) Telone C17 26 gal Good to excellent Good Poor
6) Telone C35 35 gal Good to excellent Good to excellent Poor to fair
7) Pic-Clor 60 300 lb Good to excellent Good to excellent Fair to good
8) Potassium N methyldithiocarbamate (Kpam) 60 gal. Poor to good Poor to good Poor to good
60 gal. Good to excellent Good to excellent Poor to excellent
10) Allyl isothiocyanate (Dominus) 40 gal. Still in assessment Still in assessment Still in assessment
1 All currently within EPA Fumigant Reregistration review with potential for new restrictions and applicator requirements.
2 Broad spectrum pest control achieved when co-applied with chloropicrin (21% wt./wt.). Provides excellent control of nutsedge but poor to fair control of annual grasses and
28 2019 Vegetable Production Handbook of Florida