|Soil fumigant nematicides are compounds which vaporize (fume) easily, moving
through soil air spaces, diffusing into soil water to kill nematodes and other organisms
and then degrading or escaping through the soil surface. Fumigants generally require some
form of soil sealing such as tarping, soil packing, or irrigation to prevent the gas from
escaping from the soil too quickly. All fumigants are good nematicides if they reach the
nematodes and remain in contact for sufficient time to kill them. Applications are made by
injecting the products through shanks or, with metham sodium, by applying the fumigant in
sufficient water to penetrate the soil to the desired depth. All these compounds kill
other organisms besides nematodes, including plants, and can be used only before planting.
In the past, there have been several fumigant materials that provided excellent control of
most nematode species when applied before planting. However, the use of many of these
compounds has been suspended due to health and/or environmental risks. Today, the only
fumigant products which remain available include methyl bromide, chloropicrin, methyl
bromide plus chloropicrin, metham sodium (i.e. Vapam, etc.); 1,3-dichloropropene (e.g. 1,3
D) (Telone II); and 1,3 D plus chloropicrin (Telone C-17). However, methyl bromide is too
expensive for routine use in mint and is rapidly being phased out because of the concern
that it contributes to the depletion of the ozone layer.
Few studies on the effectiveness of fumigation have been completed in mint. Jensen (unpubl.) observed that preplant fall fumigation with Telone at 15, 30 or 50 GPA reduced root populations of root-lesion nematodes at the first harvest to 8, 23 and 1%, respectively, of those in nonfumigated plots. Hay yield at harvest was 3.3, 2.2 and 3.4 times larger than that in nontreated plots for the 15, 30 and 50 GPA rates, respectively.
More recently, the effects of preplant fumigation with Telone II, at 25 or 59 GPA, and Telone C-17 at 27.5 GPA on mint production were studied for four years after treatment (Ingham, 1995). The active ingredient in Telone II (1,3-D) is an excellent nematicide but only impacts soil fungal pathogens at very high rates. Telone C-17 includes 1,3-D but is also 17% chloropicrin, an additional fumigant material that is effective against soil borne fungi. Large blocks in the fields were fumigated with the different materials, and nonfumigated strips served as check areas. Treatments were not replicated within a field, but hay and oil yield estimates were made from two-four areas within each treatment. Estimates of the cumulative gross and net value for each treatment, based on an average price of $15/lb oil, are summarized in table 1.
Response to fumigation varied considerably between sites and appeared to be related to the characteristics of the different fields. Site one had heavy wilt and lesion nematode pressure. Benefit was realized as early as the first year after fumigation with Telone C-17. In site two, there was no sign of wilt or lesion nematodes after four years. Without sufficient pathogen pressure, no benefit was realized because the nontreated plots had yields as high as in fumigated plots. Site three had little wilt but low to moderate populations of root-lesion nematodes (P. penetrans). Thus, the expense of fumigating with Telone C-17 did not pay for itself but benefit was realized with Telone II (at 25 GPA) which only controls nematodes but is less expensive than Telone C-17. Yield was not increased further with the higher rate of Telone II (59 GPA) so the extra cost did not pay for itself.
Results from the trials in these three fields suggest that growers should carefully consider field history before fumigating, or the treatment may not pay for itself. Treatment with Telone II is sufficient in fields where nematodes are the primary concern and wilt has not been prevalent. It does not appear necessary to exceed 25 GPA, and lower rates may also be effective. Telone C-17 is recommended where there is a history of nematodes and heavy wilt pressure. Condition of check plots in site oneafter one season suggested that this field may have been taken out of mint after only one year if it had not been fumigated before planting.
Table 1. Effect of Telone fumigation on cumulative gross and net crop value over four years - Willamette Valley Peppermint 1990-1994.
1Cumulative crop value over four years based on $15/lb oil.
2Gross value of treatment minus gross value of nontreated check and cost of treatment (Telone C-17 = $371/A, Telone II @ 25 GPA = $312/A, Telone II @ 59 GPA = $738/A)
Unfortunately, not enough trials have been completed to determine the density of root-lesion nematode at which soil fumigation becomes an economical control measure. In the studies described, prefumigation densities ranged from 1100 to over 6000 P. penetrans/quart soil in sites one and three, respectively. Furthermore, the extent of yield loss from a given density of nematodes also appears to be dependent on presence/absence of Verticillium. Double fumigation with Telone II followed by a metham sodium product, such as Vapam, has been very effective for controlling root-knot nematodes and Verticillium in potato . However, no published information from recent tests on fumigation of mint fields with metham sodium products appear to be available. Very early tests in fields infested with mint nematode demonstrated that rototilling in Vapam (270 GPA) was more effective at increasing yield the first year (8.5 fold increase) than injection (4 fold increase) but neither application method was as effective as D-D (a product similar to Telone) which increased yield 12 fold at 50 GPA (Jensen and Horner, 1952). No information on wilt pressure was provided, however.