Soil fumigants and tomato production, Part I
What are they, how do they work, and what does recent Ontario-based research tell us?

What are soil fumigants?

Soil fumigants act as soil disinfestants that can kill plant pests such as nematodes and fungal plant pathogens in the soil before they infect plants. In plant pathology language: the role of soil fumigants in pest management is to reduce the amount of primary inoculum present at a site in order to reduce the amount of disease in the crop. In other words, reducing the number of plant pests at a site before the crop is grown reduces the number of plants affected by the problem, or the degree to which they are affected.

In Canada, both chloropicrin and metam sodium (Vapam HL) are registered for field use. Both products are labelled for control of nematodes and plant pathogens. Metam sodium is the more popular of the two fumigants in tomato production. In Ontario tomato production, it is typically applied as a banded application through two or more shanks, concentrating on the row area. Application depths are typically 6-8 inches, although in other growing areas and in other crops like apples much deeper applications or applications at multiple depths are common.

How do soil fumigants work?

Metam sodium, which is a liquid fumigant, breaks down into gaseous methyl isothiocyanates (MITCs) after application to the soil, and then eventually into sulphur and nitrogen. It is the MITCs which are the active ingredient against soilborne pests. MITCs have two important properties: they have low vapour pressure and a strong affinity for water. Because of these characteristics, MITCs only travel a few inches from the site where they were produced (ie. the site where the metam sodium was deposited).

Recent Ontario-based fumigation research

After the emergence of tomato vine decline in 2009, our group was asked by industry to investigate whether metam sodium could help alleviate root rot and foliar symptoms associated with this problem. We completed a series of greenhouse, outdoor micro-plot, and field trials to evaluate the effect of metam sodium application on tomato growth, disease symptoms, and most importantly, tomato yield. Our objective was to evaluate the potential of metam sodium under ideal fumigation conditions (greenhouse and micro-plot trials) and typical industry practices (small-plot and strip trials).

The results of our trials are summarized in Table 1 (greenhouse and micro-plots) and Table 2 (field trials). We saw some positive effects of metam sodium fumigation on plant growth, disease incidence, and tomato yield in our controlled studies, but we didn't see any increase in tomato yield as a result of fumigation in field trials.Effect of metam sodium vs. nontreated control (P ± 0.05)

Table 1. Summary of trial results from greenhouse and outdoor micro-plot trials conducted to evaluate the influence of metam sodium fumigation on tomato growth and yield, and root rot symptoms, 2010-2011
Trial ID Soil Source Location Effect of metam sodium vs. nontreated control (P ± 0.05)
Significant decrease in root rot? Significant increase in biomass? Significant increase in yield?
Greenhouse #1 One infested field
Yes
Yes
Not measured
Greenhouse #2 Three infested fields (incl. 2010A)
No
No
Not measured
Micro-plot 2010 One infested field (2010A)
No
No
Yes
(high rate)
Micro-plot 2011 Two infested fields (2010A, 2011A, 2011D)
No
No
No
Table 2. Summary of results from small-plot and strip trials conducted to evaluated the influence of metam sodium fumigation
on tomato yield (t/acrea), 2010-2012.
Treatment Label Rate # Shanks Site A (2010) Site D (2011) Site D (2012)b Site E (2012)
Non-fumigated control -
-
30.2 a c
35.3 a
59.6
45.3 a
Metam sodium Med
3
33.1 a
-
-
-
Metam sodium High
2
-
37.0 a
-
-
Metam sodium High
3
33.2 a
41.6 a
54.9
52.5 a
Metam sodium High
4
-
-
62.1
51.8 a

a Site A was a small plot field trial, Site D and Site E were strip trials. All trials were fumigated in the spring.
b Statistical analysis was not possible due to missing plots.
c Numbers followed by the same letter in a column are not significantly different at P ± 0.05, Tukey's adjustment.

You may wonder why there is no difference among the treatments at some of these sites, when the means in the fumigated plots seem higher than the control plots. The statistical test used to compare the fumigation treatments tests to see if the numerical differences among treatments are because of the treatment effect or if it is due to something else like chance or experimental error. The data is too variable to say that the numbers (yields) are different because of fumigation. It is possible that similar differences would have occurred if no fumigation treatments had been applied to the site. Another way to look at it is by examining the data from each fumigated strip. In Figure 1, we have plotted the raw data from each strip at Site E. You should be able to see that sometimes one treatment appears to have much higher yield than another but at other times the yields among treatments are very similar.

Figure 1. Tomato yield in individual plots (clear circles) and treatment means (dark circles) at Site E

Figure 1. Tomato yield in individual plots (clear circles) and treatment means (dark circles) at Site E fumigated with metam sodium for management of tomato vine decline, 2012. Statistical analysis revealed no significant difference among treatment means (Tukey's adjustment, P ± 0.05). (text version)

Our conclusions from this research are that metam sodium is not an effective tool for improving yield associated with root rot fungi and vine decline under field conditions. So are our results similar to what others have found? To find out, check out Part II and III of this series which will appear in future issues of Hort Matters. You can contact Cheryl Trueman (ctrueman@uoguelph.ca, 519-674-1500 x63646) to obtain full reports of the research summarized in this article.

Acknowledgements:

The authors gratefully acknowledge the contributions of cooperating growers and industry members involved in this research including our grower cooperators, Chris Renwick (UAP), Irwin Schmidt (AMVAC), and Agris for donation of metam sodium to research trials. Financial support for the research results presented in this article was provided by the OMAFRA/U of G Partnership and the Ontario Tomato Research Institute.

References:

Blecker, L.A., and Thomas, J.M. (2012) Soil fumigation manual: a national pesticide applicator certification study guide. National Association of State Departments of Agriculture Research Foundation.

Duniway, J.M. 2002. Status of chemical alternatives to methyl bromide for pre-plant fumigation of soil. Phytopathology. 92:1337-1343.

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