Is Over-Reliance on Single-Action Fungicides Putting Your Crop at Risk?

Fungicides have long played a critical role in modern agriculture, offering growers a powerful line of defense against crop diseases that threaten yield and quality. However, as resistance continues to develop and spread across key regions, a growing concern looms: Are we relying too heavily on single-action fungicides to protect our crops?

Fungicide resistance isn't just a theoretical problem—it’s happening now, across continents, and impacting crops from wheat and grapes to soybeans and vegetables. Growers who lean too hard on a narrow range of fungicide modes of action may be unknowingly accelerating this resistance. This over-reliance can reduce the long-term efficacy of these products and leave entire harvests vulnerable.

The issue at hand is not just about chemistry. It’s about stewardship, strategy, and long-term sustainability.

What Are Single-Action Fungicides?

Fungicides classified as single-action, or site-specific, target a single location within the fungal metabolic pathway. They are therefore very effective—until they aren't. That one point of attack can be quickly neutralised or circumvented by pathogens, making the fungicide useless.

Among the most popular single-site fungicides are succinate dehydrogenase inhibitors (SDHIs), triazoles (DMIs), and strobilurins (QoIs). Although these chemistries have revolutionised the field, their high specificity carries a cost: a greater chance of resistance emergence.

• A single-site fungicide can lose efficacy in just 3 to 5 seasons under heavy use.
  
  

• Over 60 fungal species have shown resistance to one or more single-action fungicides globally.
  
  

This is particularly alarming when considering the limited fungicide pipeline. Developing a new molecule can take over a decade and cost upwards of $300 million.

The Science Behind Fungicide Resistance

Resistance occurs when a fungal population undergoes genetic shifts. When a fungicide is applied, susceptible strains are killed off, but resistant individuals survive and reproduce. With repeated exposure, the resistant population grows until the fungicide becomes ineffective.

The problem isn’t the fungicide itself—it’s how it’s used. Applying the same mode of action season after season, often without rotation or tank mixing, sets the stage for resistance.

Some of the major contributors to resistance include:

• Continuous monoculture cropping without break crops
  
  

• Poor timing or sub-lethal application rates
  
  

• Relying on one product because of cost or familiarity
  
  

Fungicide resistance is especially insidious because it builds slowly, often unnoticed, until control failures become apparent—and by then, it’s usually too late.

Real-World Impacts on Crop Health

The effects of resistance aren’t theoretical—they’re already being felt across the world. In the U.S., resistant strains of Cercospora beticola have become a massive problem for sugar beet growers. In Europe, Zymoseptoria tritici, the cause of Septoria leaf blotch in wheat, has developed resistance to multiple classes of fungicides, significantly narrowing control options.

In response to resistance, growers often employ more fungicides, higher dosages, or more frequent sprays, all of which increase expenses and have an adverse environmental impact without addressing the underlying problem.

Rotating or combining fungicides with various mechanisms of action is one method of diversifying control tactics. Multi-site protectants can offer wider protection and postpone resistance when incorporated into a spray programme.

For this reason, many integrated management strategies suggest treatments like fungicides based on propineb. Antracol propineb 70 wp, for example, is recommended for the management of blight and leaf spot diseases because it provides multi-site protection and lessens the selection pressure on single-action fungicides.

Why Multi-Site Fungicides Are Making a Comeback

Unlike single-action fungicides, multi-site products attack fungi at several points simultaneously. This makes it harder for pathogens to develop resistance. Multi-site fungicides, like chlorothalonil, mancozeb, and propineb, are often used in rotation or tank mixes to complement more targeted fungicides.

They’ve been around for decades and are sometimes viewed as “old tech,” but their value is being re-evaluated in light of rising resistance. These fungicides:

• Lower the risk of resistance due to their broad-spectrum action
  
  

• Provide insurance for more fragile, single-site chemistries
  
  

• Are cost-effective in many cropping systems
  
  

This resurgence is crucial for high-risk crops like grapes, tomatoes, and potatoes, which face relentless pressure from fungal pathogens.

“Overuse of a good thing is still overuse.”

This insight—simple but sharp—captures the dilemma many farmers face. A product that works well can become a crutch. But even the best tool fails when used in isolation without strategy.

Better Practices: Integrated Fungicide Management

The key to keeping fungicides effective lies in how they are used, not just in the choice of which ones we select. Integrated fungicide management focuses on smart usage through diversification and precision.

Key principles include:

1. Rotate Modes of Action (MoAs):
Switching between fungicides with different modes of action (MoAs) reduces selection pressure on any single target site. Look for FRAC (Fungicide Resistance Action Committee) codes on labels and plan rotations accordingly.

2. Use Tank Mixes Wisely:
Mixing fungicides with different MoAs can enhance control and reduce resistance risk, if both components are practical. Avoid “cosmetic” mixes where one partner isn’t truly active.

3. Apply at Optimal Timing:
Too early and you waste it. Too late and disease sets in. Using weather data and disease forecasting models can help nail the timing.

4. Combine with Non-Chemical Measures:
Cultural practices like crop rotation, resistant varieties, proper irrigation, and residue management can reduce pathogen pressure.

For a deeper dive into these strategies, the Crop Protection Network offers data-driven guides tailored to specific regions and crops.

When Regulations Restrict Options

Many single-action fungicides are under regulatory review due to concerns about their environmental impact, especially in the EU and parts of Asia. In some regions, actives like chlorothalonil have already been banned.

This tightening landscape underscores the need for resilient strategies. Growers must adapt, not just comply. Shifting away from dependence on a few favored products isn’t just about avoiding penalties—it’s about securing future harvests.

If you're managing fungicide programs in increasingly restricted markets, consider referencing this EFSA pesticide database to stay updated on product approvals and restrictions.

FAQs

1. What is the most significant danger of using only one type of fungicide?
   The significant risk is the rapid development of fungicide resistance. This can render the product useless and leave your crop exposed to aggressive fungal infections.

2. Can I still use single-site fungicides?
   Yes—but use them strategically. Rotate with other MoAs and integrate them with multi-site fungicides. Avoid repeated use in the same season.

3. How do I know if resistance is building on my farm?
   Look for declining performance even with proper application. If disease persists despite treatment, consult with an agronomist or test samples to confirm resistance.

4. Are multi-site fungicides safe for all crops?
   Most are broad-spectrum but always check the label. Some crops or stages of growth may be sensitive to certain active ingredients.

Thinking Beyond the Sprayer

When illness strikes, it's easy to lose sight. Seize the sprayer, strike forcefully, and hope for the best. However, crop protection necessitates planning ahead. Although resistance doesn't appear right away, your options might be limited by the time it does.

Creating a robust, diverse illness management programme is insurance, not just a best practice. It's how you safeguard your soil, your investment, and your next harvests. Avoid making one method of action the weakest link in your chain. Rather, create a toolbox that is flexible enough to react to evolving threats.

Smart farming is about how, when, and why you use it, not just what you use.