Understanding microbial biofungicides and how to use them effectively

By Matthew Krause

Microbial biofungicide use continues to gain acceptance and momentum in horticultural and agricultural production.

To growers new to microbial biofungicides, the learning curve for using them effectively against foliar and soilborne plant diseases can seem rather steep.  To growers who tried and had little, if any, success with biofungicides in the past, trying again and getting consistently effective results will seem challenging.

Growers in both groups need not worry.

Since the first registered biofungicides were introduced over 30 years ago, the number of available products and formulations has increased substantially. In addition, improvements in user-friendliness, quality, stability, and technical knowledge have led to significantly improved efficacy and cost-in-use with many biofungicides.

This article will help growers select, test and get the most out of microbial biofungicides in disease-management programs.

1 - What are microbial biofungicides?

Registered microbial biofungicides contain live cells of specific, unique strains of bacteria and fungi as active ingredients.

In some cases, the secondary metabolites produced by these microorganisms may also be included in formulated products.

Electron microscope photo of Clonostachys rosea fungi. M – Mycelium / SP – Spores	Electron microscope photo of Clonostachys rosea fungi. H: Hyphae

2 - How do microbial biofungicides work?

They usually work against plant pathogens via two or more of the following nontoxic modes of action (MOAs):

1. Competition for space and nutrients

Clonostachys rosea blocking the growth of Botrytis cinerea by competing for nutrients and living space.

The active strain colonizes and outcompetes pathogens for space on plant surfaces and/or in adjacent soil to prevent pathogen establishment or block pathogen access to the host plant.

The active strain consumes nutrients on or around the host plant that pathogens would otherwise use to grow, spread and actively infect the plant

2. Antagonistic metabolites released against plant pathogens

Metabolites are substances produced and released by microorganisms during their growth and activity under different conditions

Defensive agents released by the active biofungicide strain inhibit or disrupt pathogen growth, spread and establishment on or around the plant.

3. Hyperparasitism or predation to disable or kill pathogens by feeding on them

Hyphae of Clonostachys rosea J1446 coil around the plant pathogen Rhizoctonia solani and degrade its cell walls by secreting digestive enzymes (β 1,3 glucanase, chitinase)

With hyperparasitism, the biofungicide strain actively infects and feeds on plant pathogens. It weakens them and reduces their presence and activity on and around the host plant.

With predation, the biofungicide strain physically attacks, digests, and consumes plant pathogens, effectively disabling or eradicating them from the host plant environment.

4. Inducing host defense in plants against pathogen attack or spread

The biofungicide strain interacts with the host plant to locally or systemically prime or fully invoke the plant’s own defense mechanisms against pathogen infection and spread.

Induced resistance responses may vary in their strength and duration. Depending on the inducing agent and induced resistance mechanism, plant species and varieties may vary in their receptiveness and sensitivity to the inducing agent

3 - The benefits of microbial biofungicide use in agriculture

Now that you know what biofungicides are and how they work, here are 7 reasons why growers use them:

• Effectively protect against plant diseases as alternatives to or as program partners with synthetic fungicides

• Improve productivity and product shelf lives due to low restricted-entry and pre-harvest intervals

• Low health and safety risks to workers, consumers, plants, beneficial insects, and the environment

  NOTE: personal protective equipment (PPE) must be worn during their preparation, application, and restricted entry intervals (REIs) to protect workers from the remote risks of developing sensitivities to the biological components of these products
  Reduce or break pathogen resistance to synthetic fungicides

• No known resistance to any of their active ingredients’ multiple modes of action

• Improve market access and crop values due to increasing consumer demand for greener or more sustainably produced crops

• Improved plant, soil and environmental health with continued use, which can lower pest and disease pressures over time
  

4 - Microbial biofungicides in Plant Disease Management

Central to plant disease management theory is the plant disease triangle.

This triangle illustrates the interrelationship among susceptible plant host, conducive environment and virulent pathogen. When something disrupts or changes any of these elements, it results in the reduction or control of a given disease.

In true integrated disease management (IDM),
biological and synthetic fungicides are regarded as important tools
for effectively disrupting one or more of these elements.

The most successful IDM programs are those that are proactive rather than reactive.

They usually include:

• Knowledge about the historical and potential pathogens that can affect the crop, including their biological characteristics, how they spread, and the influences of plant-associated, environmental and cultural factors.
  

• Exclusion of pathogens from the crop and crop environment and/or effective reductions in pathogen levels in field soils
  

• Proper sanitation and crop hygiene to prevent spread of disease from infected plants and contaminated equipment or inputs to non-infected plants
  

• Control of environmental, physical and chemical factors where possible to limit conditions that favor pathogen development, plant stresses, and disease spread
  

• Continuous disease monitoring and accurate pathogen diagnoses to identify unexpected issues when they arise and to document them for future reference
  

• Identifying, understanding and optimizing biological and synthetic fungicide tools for cost-effectively preventing or minimizing crop diseases.
  

These components used together disrupt or eliminate environmental, cultural, and biological factors that are essential for pathogens to successfully grow, develop, and cause disease.

So, as you can see, biofungicides are most effective when combined with other disease management approaches.

5 - Integrate microbial biofungicides into your crop management program in 8 easy steps

Properly selected, applied and timed microbial biofungicide treatments directly exploit pathogens’ biological vulnerabilities. They also support crops’ physiological health, innate tolerance to plant diseases, and, ultimately, quality and productivity.

The following section provides recommendations for developing your own effective microbial biofungicide-based disease management programs.

1. Think holistically about your crop

Be proactive first, then reactive and therapeutic.

Biofungicides work best preventively. Although they can effectively protect non-infected plants if applied before pathogens attack, they will stop the pathogens but won’t eliminate damage from plants that have already been infected by these pathogens.

True curative control of plant diseases only occurs with a few fungicides, and usually does not persist. However, most synthetic and some biological fungicides can provide therapeutic control by reducing pathogen levels or activities enough to shock, delay or reset disease progress on affected plants.

2. Knowledge is power

Know and understand the potential pathogens that can affect each of your crops, their biological characteristics and their disease cycles so you can anticipate potential disease issues before they happen. You can then select products that exploit pathogen weaknesses, and decide in advance if and when to apply these products in your program.

3. Choose the best manufacturers and distributors to add value to your operations

The way microbial biofungicides are produced and formulated can each have a big impact on their efficacy. Two products with the same active ingredient may yield wildly different results.

That is why you should consider manufacturers with in-house expertise in fermentation and production of their formulations. This helps ensure the quality of the product you are using from batch to batch.

As for the manufacturers and distributors, seek out those that offer the best technical support. This can determine the success or failure of your program.

4. Select the right products and formulations for the pathogen(s), crop, and crop environment

Consult with manufacturer or distributor technical representatives and their online resources as well as crop consultants, extension specialists and other growers with substantial biofungicide experience for the correct products and formulations for each crop-disease combination.

Here are some formulations by application type:

• Foliar spray, soil spray, seed spray, fog, drench, dip, slurry: Liquid, ES, EC, WP, WG, WDG.

• Top dress, in-furrow, pre-plant incorporation (soils and growing media): Granule or G.

• Dry seed treatment: WP, DP or Planter Box.
  

5. Choose the proper application method

Consult experts to find the most effective application methods, rates and use patterns according to the pathogen, crop and crop stage.

Keeping application equipment properly maintained and calibrated is essential for optimal efficacy and preventing product waste.

6. Carefully read labels for storage, handling, mixing, application requirements and limits

Since most microbial biofungicides contain living microorganisms and/or metabolites or extracts from their fermentation, they generally cannot tolerate temperature and physical extremes as much as most synthetic fungicides. As such, they have limited shelf lives under specific storage conditions and particular application requirements or restrictions.

The cellular active ingredients in liquid, wettable powder and wettable granule microbial biofungicides are not water soluble and must be kept suspended by regular agitation during application to ensure even distribution on the crop.

Determine whether there are mesh sizes of in-line or other filters used in sprayers, injectors and irrigation systems that are incompatible with potential biofungicides.

Observe the required PPE and REI for each product (biological and synthetic).

7. Test new products on a smaller scale before going all-in

Determine whether the new tool is worth the investment compared to your current program before fully committing. Here is how:

• Identify what the measures of success with the new product should look like before you test: effects on disease, plant growth, marketable yield, marketable quality, return on investment, etc.

• Test on a large enough scale to make a reasonable side-by-side comparison, but small enough not to put your crop at substantial financial risk if the biofungicide does not perform as expected.

• Try integrating into your current program vs. replacing major components of your program.

• Document your observations and results: the good, the bad and the ugly. Keeping and referencing this information will help in making program adjustments that optimize efficacy and cost-in-use, and making current and future decisions.
  

Your smartphone camera is a great tool for documenting side-by-side comparisons, unexpected results, and other visible observations as you encounter them.

8. Consider compatibility, complementarity and potential synergy with other inputs and practices

Tank-mix or rotational compatibility with other biological and synthetic pesticide products can help economize applications of tools that prevent or resolve different plant health issues to improve marketable yields. Be on the lookout for unexpected results – both positive and negative.

6 - Things to know before using agricultural biofungicides

It is difficult for pathogens to develop true resistance to biofungicides because they often work via two or more non-specific MOAs. This also makes them ideal rotational partners for many synthetic fungicides, as they can help prevent or delay fungicide resistance development.

When used alone, they work best against labelled crop diseases at low to moderate pathogen levels under conditions that favour typical disease development (low-moderate disease pressures). They are likely to be ineffective under high disease pressures, where pathogen levels are high and/or conditions are suddenly highly favorable for disease development. That is why they are mainly used in combination with compatible synthetic and/or other biological fungicides to manage diseases.

Like synthetic fungicides, they do not persist at effective levels indefinitely. Microbial active ingredients in many soils – or growing medium-applied products may last from 2 to 12 weeks, depending on soil, environmental and plant factors as well as cropping practices. For example, foliar biofungicides typically need to be applied frequently since they usually do not spread to new growth, are not systemic, and can be negatively affected by environmental stresses such as UV light, low relative humidity, temperature extremes, and incompatible crop inputs.

They cannot fix or overcome unhealthy cultural practices and growing conditions. Although growers can use microbial fungicides’ unique biological characteristics to manage major plant diseases, it is essential that they avoid circumstances that promote pathogen spread, growth and development as well as limit practices that make their crops more susceptible to diseases.

Microbial biofungicides are not miraculous. But using the information and recommendations in this guide should help you build, evaluate and implement your own effective biologically-based plant disease management programs using biofungicides.