Managing Fungicide Resistance

Plant pathogens can develop resistance to fungicides in much the same way that insects develop resistance to insecticides. Most commonly, the gene for resistance is already present in the population at very low levels and through repeated exposure, resistant strains of a pathogen survive and increase as susceptible ones are killed. Eventually, a pathogen population comes to be dominated by the resistant strains, and control breaks down. However, there are some key differences between the development of fungicide resistance and insecticide resistance.

With insecticides, almost all products are at risk for resistance development if overused. For fungicides however, there is a broad spectrum of potential risk, based on the chemistry of the product and its mode of action (i.e. how and where in the fungus, it works). Fungicide resistance was unknown until the early 1970s when a new class of chemistry was introduced. These products, known as the benzimidazoles, had the advantage over previous fungicides of being systemic; they were taken up and translocated throughout the plant providing long term protection, but also eradication of existing diseases. Unfortunately, they differed in one other way from other fungicides. They controlled fungi by interfering with cell division at one very specific site in the fungus.

It is this single site mode of action that makes the benzimidazoles so susceptible to the development of resistance. A single mutation that alters the target site, could result in a fungal strain that is not affected by the fungicide; i.e. it becomes resistant. And in reality, this is what happened, with resistance to this group of fungicides showing up within only a few years of their introduction. In the greenhouse, we know some of these products as Benlate (benomyl), which was registered until the early 1990s, and more recently, the closely related product Senator (thiophanate-methyl).

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And why was fungicide resistance not a concern until the advent of the benzimidazoles? Earlier fungicides (many of which are still being commonly used very effectively today) had a very different mode of action, often acting on many different sites within the fungus. The chances of genetic mutation occurring at multiple sites and inhibiting the action of these products, is almost negligible. Collectively, these types of products are described as having 'multi-site' mode of action, and the chances of resistance developing to them are very low. Products registered for greenhouse use in Canada, that fall into this category include Daconil (chlorothalonil), Captan and copper. There are other products where the resistance risk is also considered low, although the mode of action is not well understood. These include products such as Aliette (fosetyl-Al) and Milstop (potassium bicarbonate).

Since the development of the benzimidazoles, there have been many other chemical classes of fungicides developed. Many of these newer products are also classed as having 'single site' mode of action and are therefore at risk of resistance development. In fact resistance to many of these products has been documented. They include:

• the phenylamides - e.g. Subdue Maxx (metalaxyl)
  the dicarboximides - e.g. Rovral (iprodione)
  
• Demethylation inhibitors (also known as the Sterol inhibitors) - e.g Nova (myclobutanil)
  
• QoIs (also known as the strobilurons) - e.g. Compass (trifloxystrobin)

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Being aware of the problem is one thing; doing something about it can be quite different. A number of strategies have been identified as being useful in managing fungicide resistance. Some of these have limited usefulness in Canadian greenhouses, but it is still worthwhile understanding the concepts behind the various approaches.

1. High risk single site fungicides should not be used on their own, but used as a tank mix or alternated with other products. The use of two different modes of action will reduce the survival of resistant fungal strains. The problem with this approach is that we don't have a large selection of registered fungicides to choose from, and in some instances, only one product may be registered against a particular disease. It is interesting that pesticide manufacturers are now starting to introduce new fungicides to the market that are a premixed formulation of two different products, with the aim of reducing the likelihood of resistance development. There are now several products in the process of being registered in Canada that use this approach.
   
2. Limiting the number of applications of high risk products. It has become fairly standard for new fungicides to carry a label statement restricting how frequently the product can be used. However as with the previous strategy, the success of this relies on having a variety of fungicides registered with different modes of action. When only one effective product is available, the potential for overuse is very high.
   
3. Where possible, high risk fungicides should be used as protectants, before diseases become well established. Considering many of the newer single-site fungicides work very well as eradicants, this would seem to be counter intuitive. However, the larger the disease population, the greater is the likelihood of selecting a resistant strain. This is the opposite approach taken for managing insecticide resistance, where preventative spraying is discouraged. For disease management though, it is a more effective approach, but only if used when conditions are conducive to disease development.
   
4. Integrated Disease Management is one of the most important tools growers have available to them to control the development of fungicide resistance. Many approaches are encompassed under this term including cultural controls, biological controls, use of resistant varieties, good sanitation etc. However, greenhouse growers, more than any other farmers have an additional tool available to them to assist with disease control, and by extension, fungicide resistance management.
   In the greenhouse, the environment can be controlled in ways that other growers can only dream about. And the environment is critical in the development of disease outbreaks. A good understanding of disease biology and its environmental requirements should be the first step in a disease management program. Above ground conditions such as humidity, temperature, air movement, irrigation, plant spacing, light levels can affect the development of foliar diseases such as botrytis, powdery mildew, leaf spots, downy mildew and rusts. The environment in the root zone such as pH, salt levels, temperature, water retention and oxygen impacts on the development of root and crown diseases.
   Many of the conditions that encourage disease development are also outside the preferred growing conditions for the crop and as a result put stress on the plants. Stressed plants are more susceptible to disease than those grown under optimal environmental conditions.

Fungicide resistance is something that all growers need to be aware of and to implement strategies to limit its impact. Although many newer products are at high risk for resistance, we still have a number of older products for which resistance has never been documented, or which are at low risk. These should be used to take the selection pressure off the high risk fungicides. Integrated disease management strategies and in particular, environmental management should also play a key role in any resistant management strategy.

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