Fungicide resistance generates economic losses on a regional scale

Plant disease epidemics cause significant losses to global agriculture. Producers resort to intensive use of fungicides to contain these outbreaks. However, chemical control produces barely visible side effects, including the selection and spread of resistant pathogen strains. Recent research quantified the economic cost of fungicide resistance considering entire agricultural landscapes—not just isolated fields.

The team of scientists combined an epidemiological mathematical model with a regional-scale economic analysis. The work shows that as fungicides lose effectiveness, producers face reduced economic returns—even if they maintain or increase the volume of applications.

Contrary to expectations, the economic returns from fungicide application on various farms do not follow a diminishing returns curve. The study identified patterns of accelerated or decelerated returns, with implications for public policy and management strategies.

A new paradigm for estimating invisible losses

The analysis proposes a concept called the economic cost of resistance evolution. This is the difference between the ideal economic return in a landscape without resistance and that obtained when resistance is already widespread.

Estimating this cost depends on four main variables:

• fungicide price

• degree of pathogen resistance

• basic reproduction number of the disease (R0)

• relative yield loss in diseased fields.

The researchers observed a counterintuitive behavior: the cost of resistance tends to decrease as fungicide prices increase. This occurs because expensive fungicides discourage extensive treatment and reduce selective pressure on pathogens.

Furthermore, the cost of resistance does not increase continuously with the disease's ability to spread. Instead, the economic impact of resistance is greatest for pathogens with intermediate invasiveness.

The dilemma of large-scale applications

One of the main results of the study indicates that, in agricultural regions composed of many fields cultivated with the same crop, optimal decisions at the property level can become detrimental on a regional scale.

Extensive fungicide use in isolated fields favors the selection of resistant strains, which spread to other areas. Thus, even producers who use little or no fungicides suffer the consequences of regionalized resistance.

This phenomenon falls under the definition of a negative externality: the use of fungicides in certain areas imposes indirect costs on the entire agricultural community. These costs include loss of product effectiveness, increased infestations, the need for new products, and more expensive practices.

Three economic response regimes

Based on the models used, the study identified three main response regimes to fungicide use:

• Cheap and effective fungicides: applying them to all fields can be economically advantageous. Resistance, if it emerges, strongly impacts economic returns.

• Expensive fungicides: Even without resistance, extensive application may not be cost-effective. In this scenario, resistance has little additional effect.

• Mid-priced fungicides: There is a critical point of ideal coverage. Above this point, the cost of resistance increases rapidly; below this point, the return decreases due to lack of control.

The model indicates that, in the absence of resistance, the net return from fungicide application increases with the area treated. However, the introduction of resistance reverses this relationship in many cases.

Based on this, the authors suggest that agricultural policies should consider subsidies, taxes, or incentives for crop rotation to slow down the selection of resistance.

A generalizable model

The mathematical framework developed considers homogeneous crop fields and regional dispersion through natural vectors or human movement. Although simplified, the model serves as a basis for more specific analyses. It can be adjusted to consider cultivar variability, climate patterns, and differentiated management systems.

As an example, the authors cite foliar diseases of cereals and legumes that are widely spread by spores, such as soybean target spot in Mato Grosso or wheat rust in the US grain belt.

Perception versus reality: the Australian case

To contextualize the theoretical data, the authors cite a survey of 137 farmers in the Wheatbelt region of Australia. In the 2019/2020 crop year, these farmers spent an average of AU$42 per hectare on barley fungicides. In contrast, they were willing to pay AU$18 per hectare to delay or mitigate resistance. This willingness suggests a partial perception of the true cost of resistance, which can be much higher depending on the variables involved.

On a larger scale, the economic impact of pesticide resistance in the United States has been estimated at $2,5 billion per year, adjusting for inflation. Herbicide resistance, such as that identified in Alopecurus myosuroides in the UK, it can double weed management costs.

Implications for public policy

By providing a robust method for estimating the cost of resistance, the study enables the creation of more realistic, evidence-based policies. Possible proposals include:

• Variable taxation of fungicides, proportional to the risk of resistance;

• Subsidies for sustainable practices, such as crop rotation or integrated use of biological control;

• Genetic monitoring of pathogens to detect resistant variants early.

The sustainability of chemical control depends on cooperation between producers, technicians, and policymakers. Research shows that ignoring the cost of resistance compromises the economic viability of the entire production system in the medium term.

Resistance cost is not constant

Another relevant finding of the study concerns the behavior of the cost of resistance over time and price variations. When the fungicide is cheap, the cost of resistance tends to remain stable, even with price increases.

However, there's a tipping point. With mid-priced fungicides, resistance causes a sharp drop in returns. When the product becomes too expensive, resistance no longer impacts the bottom line, as growers simply abandon application.

This pattern suggests that preventive measures are more effective and viable when fungicide use is still economically advantageous. Once fungicides become ineffective, control alternatives become more expensive and less predictable.

Further information at doi.org/10.1371/journal.pstr.0000178