Exploring factors that can contribute to inconsistent results with fumigation
Cheryl Trueman, M.Sc., College Professor, Ridgetown Campus, University of Guelph; Janice LeBoeuf, Vegetable Crop Specialist, OMAFRA – Ridgetown; Anne Verhallen, Soil Management Specialist – Horticulture, OMAFRA – Ridgetown
In two previous articles on this topic, we discussed what fumigants are and how they work, and shared research results on the efficacy of metam sodium from trials in Ontario and other regions conducted in field tomato production systems. In Ontario, recent research results indicate that metam sodium is not an effective tool for improving yield associated with root rot and vine decline under field conditions. In other regions of North America, results are mixed. In this article, we’ll explore factors that can contribute to inconsistent results with fumigation.
A number of factors can contribute to the success (or lack of success) of a fumigant application with metam sodium. These factors include: the method of application, the conditions at the time of application, activities that occur at the site post-application, inherent soil properties, and the history of fumigation at the site. These factors may impact the size of the fumigated zone in the soil profile, or the length of time the target pests are exposed to the fumigant. Keep in mind the following facts about metam sodium that were discussed in Part I of this series:
- Metam sodium is a liquid fumigant that breaks down into gaseous methyl isothiocyanates (MITCs) after application to the soil. The MITCs are the active ingredient against soilborne organisms, including both pests and beneficial organisms. .
- MITCs have low vapour pressure and a strong affinity for water, therefore they only travel a few inches from the site where they were produced (ie. the site where the metam sodium was deposited).
Method of application: In field tomato production in Ontario, metam sodium is most commonly applied through shank injection, centred on the row area. This is a form of banded application that reduces the total amount of fumigant applied per acre of land (and correspondingly, the cost of treating a field) rather than a broadcast application that would treat the whole field area. In the case of metam sodium, growers need to decide both the optimum width of the band and the optimum depth of application, taking into account the rooting characteristics of the crop, the location of the pest in the soil profile, and any critical period of infection (ie. is the pest a concern if roots grow out of the treated zone several weeks after transplanting?).
Band width from a single shank injector is limited by the lateral movement of the product from the injection point. Spray blades can be used to deliver a wider band of product to a set depth. Application from one injection depth will cover only a portion of the main rooting zone in tomatoes. If this is not adequate, there are applicators that deliver product at multiple depths. However, wider bands and multiple application depths increase the cost of treatment.
Application conditions: It is important to ensure that metam sodium is applied under optimum conditions to ensure maximum movement within the soil, and to limit dissipation of MITCs before they can effectively kill target pests. Optimum conditions are fully explained on the Vapam HL product label, and include soil temperature between 4 and 32°C at a 3-inch depth, and soil moisture between 50 and 80% field capacity. Properly loosening the soil and limiting the amount of residue can also improve results. Finally, the soil surface should be smoothed and sealed immediately after application in order to prevent premature dissipation of the product.
Post-application activities: Remember that only the soil that comes in contact with the fumigant will be disinfested, therefore any activities that introduce soil from outside the fumigated zone may contaminate the fumigated area with soilborne pests. These could include bed reshaping, herbicide or fertilizer incorporation, or possibly sidedressing or V-ditching. Also keep in mind that plant roots will grow out of the fumigated zone and into infested soil during the growing season. Depending on the target pest and the size of the fumigated zone, this could impact the degree of disease reduction or yield benefit that could occur as a result of fumigation.
Inherent soil properties: We all know that soil properties can vary both within and among fields. For example, yield maps often show areas of a field with higher or lower yields. These differences can occur for a number of complex reasons, such as interactions between soil texture, soil moisture, and soil biota. Anyone who has ever sampled for nematodes knows that soil populations can vary considerably from one sampling point to the next, even over short distances. These factors all make it more difficult to determine if the application of a fumigant like metam sodium effectively reduced pest populations and increased yield, or if differences in these factors are due to natural variation.
As discussed in Part II of this series, the success of metam sodium applications among different fields can also vary considerably. Triky-Dotan et al. (2007) explored this variation further by studying the efficacy of fumigation on soil from 34 sites, using the exact same application methods and conditions. Soils were collected, inoculated with Fusarium oxysporum f. sp. radicis–lycopersici spores, and fumigated with metam sodium. The length of time it took for the MITCs to dissipate and the mortality of the F. oxysporum f. sp. radicis–lycopersici spores were then measured. They found that the longer it took for MITCs to dissipate, the greater the spore mortality. This result reinforces the importance of properly sealing in a fumigant after application. However, the group also found that spore mortality among fields ranged from 0% for 5 of 34 fields, to over 90% for 17 of 34 fields. Soil texture, organic matter concentration, and soil pH accounted for little of this variation. In other words, some sites had very high MITC dissipation rates which resulted in no or very poor spore mortality, and these differences couldn’t be accounted for because of differences basic soil properties. These results suggest that it may not wise to assume that the fumigation results obtained at one location will be the same at another location.
History of fumigation: The history of metam sodium application at a specific site may also affect the potential for an economic return on fumigation. Accelerated degradation is a phenomenon that has been identified for a number of pesticides in soil. In short, after repeated applications of a given pesticide, members of the soil microbial community that are able to rapidly decompose the pesticide evolve or increase in numbers. As a result, the pesticide is decomposed so rapidly in the environment that it may no longer be effective. Triky-Dotan et al. (2009) and Triky-Dotan et al. (2010) present follow up studies to their initial 2007 research in which they investigate the effect of metam sodium application on the soil microbial community. Accelerated degradation was documented in some of the tested soils after as few as one or two applications of metam sodium, and as a result, metam sodium failed to control the diseases in question.
Last but not least: In any situation where you are considering fumigation as part of your pest management program, ask yourself this: what is your target pest? Have you conducted the appropriate monitoring and sampling to determine if the pest is at sufficient levels to cause economic damage? This information will help you determine if fumigation is a potential solution, and which application method and rate are most appropriate. The foundation of a pest management program is correct pest identification! Remember that fumigation is a broad-spectrum management tactic that is toxic to both soilborne pests, and beneficial soil organisms. Additional information on sampling methods and thresholds is available in the post Resources for Vegetable Crop Scouts (April 2012).
Triky-Dotan, S. et al. 2007. Generation and dissipation of methyl isothiocyanate in soils following metam sodium fumigation: Impact on Verticillium control and potato yield. Plant Dis. 91:497-503.
Triky-Dotan, S. et al. 2009. Accelerated degradation of metam-sodium in soil and consequences for root-disease management. Phytopathology 99:362-368.
Triky-Dotan, S. et al. 2010. Microbial aspects of accelerated degradation of metam sodium in soil. Phytopathology 100:367-375.