This project funded by Horticulture Australia Limited compiled and reviewed information on the implications of enhanced metalaxyl breakdown and to recommend management practices to minimise the risks associated with the fungicide’s use in Australian horticulture.
This report provides an overall perspective on enhanced degradation of metalaxyl, as well as other factors that affect metalaxyl persistence in agricultural systems.
Metalaxyl is highly effective against serious diseases caused by Oomycetes fungi, such as potato late blight, downy mildew and damping-off on vegetables, or cavity spot on carrots.
As metalaxyl is a site-specific fungicide that is also very selective in its activity against target fungal pathogens, it is susceptible to both fungicide resistance and rapid degradation.
The development and impact of metalaxyl resistance is well known. However, the phenomenon of rapid degradation of fungicides such as metalaxyl is relatively new, and there have been very few studies on its impact on Oomycetes disease control in horticultural crops.
It should be noted that although pesticide biodegradation in soil may adversely affect the control of soil pests, this process is also an important mechanism for degrading, detoxifying, or assimilating pesticides.
This helps to prevent a build-up of pesticide residues, and soil and groundwater contamination.
Growers and agricultural advisers must be aware of the consequences of excessive metalaxyl use.
A better understanding of the fungicide’s properties, as well as effects of soils, irrigation, and cultural practices, will help to maintain metalaxyl products availability for use against major, economically significant soil borne diseases.
Enhanced biodegradation by soil microorganisms was found to be the most important factor in reducing metalaxyl persistence in soils.
Enhanced biodegradation can be defined as the accelerated degradation of a pesticide after repeated applications to soils.
Enhanced degradation of metalaxyl has been reported in sites that have a history of consecutive years of metalaxyl soil applications.
In soils with an enhanced degradation problem, metalaxyl breaks down so rapidly that it does not provide appropriate disease control.
For example, in a sandy soil that had no prior history of metalaxyl application, its half-life was 82 days.
This was reduced to as little as 4 to 10 days in paddocks with enhanced degradation.
In laboratory studies, a single exposure of different soils with no history of metalaxyl treatment was sufficient to increase their subsequent capacity to degrade the fungicide.
This may be due to the wide range of microorganisms (fungi, bacteria and actinomycetes) capable of degrading it.
In comparison to the soil system, most plant canopies do not support high microbial activities. Hence, enhanced degradation is unlikely to occur on plant canopies following metalaxyl spray applications.
High microbial activity in soil is usually associated with high organic matter levels. Therefore, metalaxyl tends to degrade faster in soils that are high in organic matter.
- The rate of metalaxyl degradation in soils can also vary with soil depth.
Organic matter levels are lower with increasing soil depth and hence conditions are believed to be less favourable for its degradation.
- Cultural practices may influence the persistence of metalaxyl in soil.
Enhanced degradation was not detected in intermittently cropped red ferrosol soils in Tasmania.
These soils had two or four years of pasture in between crops and metalaxyl soil applications.
- In contrast, rapid degradation was found in intensively cropped soils, where the fungicide had been applied to soil in carrot and potato crops in consecutive years.
- Further studies are required in order to better understand the effects of cropping practices, metalaxyl use, and types of crops, on metalaxyl persistence.
- It is not known whether affected soils can recover from enhanced metalaxyl degradation.
Further investigations are required to determine the recovery potential of enhanced degradation affected sites.
A long-term approach for a sustainable metalaxyl usage must involve the following measures for integrated disease management :
- Use suitable crop rotations with plants that are not susceptible to Oomycetes fungal pathogens, thereby reducing the frequency of metalaxyl applications.
- Do not plant one susceptible root crop soon after another. • Use crop cultivars with moderate to high resistance to Oomycetes pathogens.
- Reduce metalaxyl leaching and run-off, by reducing the slope, avoiding over irrigating, constructing drainage ditches, and improving soil management to reduce compaction.
- Encourage soil management practices that improve soil structure for better water infiltration, increased biological antagonists, disease suppression and reduced pathogen levels.
- Develop alternative chemical methods for use in alternation or in addition to metalaxyl for Oomycetes soilborne disease control.
For example, a new class of systemic fungicides that can activate plant natural defences may offer a new perspective in disease control.
- Develop and introduce the use of biological control methods. As metalaxyl is selective in its activity, it is likely to be compatible with most biological control methods.
The author would like to thank Dr. Doris Blaesing and Mary Trebiico of Serve-Ag Pty Ltd, and Dr. John Mathiessen of CSIRO Entomology, Western Australia, for their comments and proof-reading of the report.
The funding of this project by Horticulture Australia Limited, Australian Potato Industry Council, and levies from vegetable industries, is gratefully acknowledged. .
The Australian Government provides matched funding for all HAL’s R&D activities.