Resistance management – more important than ever

As with any crop, insects, diseases, and weeds found in horticulture production can and do develop resistance to their controls. This is evident by a reduction or complete failure of control using an effective crop protection product when appropriately applied under label conditions. The nature of specific resistances can be complex but most importantly resistant individuals pass on these characteristics to their progeny thus making resistant populations.


The resistance challenge is not new or unique to growers of horticulture crops.However, the diverse nature of the crops and target pests and the reduced availability of active ingredients makes resistance management particularly challenging for this sector.Few field crops have the duration and intensity of pest pressures faced by many horticulture crops. The marketability of horticulture crops is so impacted by quality that even lighter pressure that may not limit biological yield may be intolerable. Both of these factors mean more applications per season are usually required for horticulture crops, promoting selection pressure and leading to greater resistance potential. 


Contributing to the resistance management issue in horticulture is (delete also) often reduced availability of active ingredients. The size of the crop protection market for minor use crops in Canada limits direct registration activity. Though the Minor Use Program does contribute, it cannot fully compensate for all gaps. A further complication is the cancellation of existing active ingredients as a result of the Pest Management Regulatory Agency re-evaluation program. Cancellations can be quite justified as a result of human health or environmental concerns. Updated data that might mitigate risks however simply may not be developed in re-evaluations for minor use crops due to the small potential return. 


In the big picture, the number of tools is simply getting more limited, especially factoring in current resistance across many pests. For fungal disease control, multi-site fungicides such as captan, chlorothalonil, and mancozeb have long been part of the control strategy. They are valued not only for their disease control spectrum but the very low risk of resistance developing. Numerous highly effective single-site fungicides were introduced along the way with resistance reported in many products over time. Resistance management efforts have suggested tank mixing single-site fungicides with a multi-site to delay the onset of resistance. While effective, changing regulatory requirements have now severely limited or eliminated this strategy.


The introduction of the neonicotinoids which brought systemic and long-lasting insect control along with low mammalian toxicity were great innovations in many production systems. Re-evaluation decisions have begun to restrict or eliminate uses of neonicotinoids due to environmental concerns. Their predecessors in organophosphates and pyrethroids – which have already had many instances of resistance – are also facing increasing regulatory pressure. Other insecticidal groups will be forced to take on more of the load in the near future, notably the diamides and spinosyns.


Resistance to herbicides in weeds has been the more prominent issue among field crops certainly. However, horticulture crops have had their share of difficulties such as linuron resistance in pigweed as one example. The appearance and spread of weeds with resistance to multiple herbicide groups is also becoming of greater concern. Adding to the challenge, horticulture crops lack the herbicide resistance traits such as glyphosate, glufosinate, or dicamba found in some field crops, limiting their use. Perennial crops have the further problem of limited tillage and rotation options. 


We know resistance management is a particular challenge in horticulture. What can we do about it? The classic recommendations of course still apply. Use scouting and diagnostic services to confirm pest presence and pressure where appropriate. Rotate groups of crop protection products. Rotate crops, if possible, to avoid repeated exposure of the same active ingredients over time. Use alternative controls such as tillage for weeds and netting for insects. These methods have been proven and remain important.


But they probably won’t be enough. Resistance continues to develop, following the processes of evolution. Dynamic scouting methods that provide feedback on existing resistance will certainly help, such as the quick tests for herbicide-resistant weeds being developed in Ontario and Quebec. These tests would allow a grower to test small weeds for herbicide resistance in season with a quick turnaround. An alternative mode of action can then be used on the same weeds to prevent further spread of resistance by pollen and seed.


For longer-term sustainability, it’s clear that more tools are necessary. More biological crop protection products are being used than ever in multiple crops. Having followed several trials, I can say that efficacy in field production can be highly variable. I have seen complete failures and also excellent control. Efficacy is typically more consistent in rotation with conventional products. They offer a different mode of action and take some selection pressure off the remaining effective conventional groups. The U.S. IR-4 program – equivalent of Minor Use – has recently created a new category of research called integrated solutions to explore conventional and biological combinations. Managing resistance is a key focus of this program. Cucumber beetle was one of the first integrated priorities selected in the program debut in 2018. With few solutions, especially without neonicotinoid seed treatments, this a perfect fit for exploring integrated control.


Integrated control measures are critical to maintaining effective crop protection with fewer tools and growing resistance. New effective methods must be welcomed. Our future strategies must have resistance management as a principal factor or we face a tough road ahead.


Publish date: 
Friday, August 2, 2019

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