February 2013 Culture, Plant Health

Soil Biofungicides: Biological Warfare at its Finest By Chris Hayes

You would like your plant's root system to be in balance with the soil, your growing media. Balance in water, balance in food, balance in microbes and balance in organic/inorganic matter all group together to maintain the health of the roots. Wackiness within any part of this balance can put the roots at risk allowing plant pathogens lingering nearby to attack. Not good, so most growers try to defend their roots frompathogens. Growers can defend them with soil biofungicides — biological warfare at its finest!

Within the soil are living microbes that naturally prevent pathogens from damaging a plant's root system. Some soil microbes lightly suppress pathogens; others rise to the occasion and sustainably prevent and control root rotting pathogens. The best of these have been commercialized and undergone a rigorous review to become fully registered soil biofungicides in the United States, Canada and other countries. Registration indicates that the product has been subjected to many tests that demonstrate safety to workers, the environment and the end-user. Registration provides evidence that the products have been subjected to many efficacy tests and shown to work in the field, the subterranean field of the roots.

For example, federal U.S. EPA registration, as well as registration within states, requires many answers to the on-target and non-target effects of soil biofungicides. The Feds and the state of California require data showing efficacy on plants for different root rotting pathogens. No intel on on-target effects, no registration.

Effective and Registered Soil Biofungicides

Registered soil biofungicides (Table 1) give rates and frequencies shown to be effective against root rotting pathogens on plants listed on their label. Toxicity effects on non-targets are tested, as well as run-off effects on water and the environment. Compounds that may contaminate our water, food or air risking the health of humans, animals, plants and the environment are of concern. Registered soil biofungicides carry "Caution" on their label and have low or zero-hour Restricted Entry Interval (REI); some are organically certified by OMRI (Organic Material Review Institute) or NOP (National Organic Program, UDSA). The effectiveness and safety of registered products is something to consider; performance issues can leave your roots open to attack.

Preventing damage from root rots begins with a good defense, a.k.a. good biological warfare, to protect the roots from pathogen attack. Soil biofungicides are fungal or bacterial active ingredients that best prevent and should be used on clean plant material, e.g. young plants or healthy plants; or after a chemical curative treatment if plants are suspected to be tainted with a disease or pathogen.

Programs using conventional, IPM, sustainable and organic methods of growing should all use soil biofungicides early to prevent issues and not rely on them to cure an internal disease. Select the right product that controls the pathogen on the types of plants you grow. Read the label for rates and frequencies.

Methods of Application and Selection

Soil biofungicides can be applied by incorporating them into the growing media either onsite or many soil mixing companies are willing to incorporate, just ask them. Some can be mixed in water and applied as a normal part of your irrigation program. Table 2 gives a general overview of different methods to apply soil biofungicides to plants.

Soil biofungicides are living organisms that need to survive to defend the homeland of the roots. They need to be able to inhabit the environment of the growing media to protect the roots against pathogen attack. Soil biofungicides can be fungal based (more advanced types of microbes called Eukaryotes) or bacterial based (more ancient types of microbes called Prokaryotes). Both major groups of microbes have their strengths and weaknesses to environmental effects, e.g. soil pH, temperatures and moisture, compatibility to other inputs, modes of action and shelf life. A general description is given in Table 3.

Environmental conditions are controlled by the grower to favor healthy plant development. A healthy plant is the first step to making money; selling it and getting paid are the ultimate goals. Selecting the proper soil biofungicide that can grow in this environment can be critical to help you produce a healthy plant.

In general, fungi tend to have more environmental diversity than bacteria and tolerate greater changes in soil pH, temperatures and moisture; and higher levels of salts and metal ions in your growing media. Fungi tend to grow at both acidic and alkaline pH and some can grow at cool to cold soils up to warm to hot soil temperatures near the roots. On the other hand, bacteria tend to have longer shelf life. Product labels and technical support documents should provide information on conditions that allow a soil biofungicide product to survive in your growing media. Store the soil biofungicide as suggested on the label; use rates and frequencies as suggested on the label.

Other crop inputs can also influence a soil biofungicides' survival and activity in the growing media. Not only is environmental compatibility important but so is chemical compatibility. Can you use bactericides when using bacterial-based products? Can you use fungicides when using fungal based products? Will other chemical inputs like insecticides, herbicides, growth regulators, fertilizers, disinfectants, surfactants, as well as greener inputs like organic nutrients help, hurt or have no effect on the soil biofungicide? Make sure you read the label and contact the manufacturer to be sure that you have answers to these questions. Most manufacturers have developed information that will help you put a program in place that maximizes the effectiveness of their product in your hands.

Modes of Action

So, why use soil biofungicides when we have chemicals that work; good question. Chemicals can work and have for years but they are not silver bullets. Their Mode of Action (MOA) is conducive to pathogens developing resistance. Rotating with a living organism less likely to develop resistance can help keep chemistries, especially curatives, around longer. Environmental concerns, worker safety concerns, animal health and non-target (e.g. bees, other pollinators and beneficials) concerns as well as phyto concerns have caused growers to look at alternatives.

Most soil biofungicides can be used on edible plants as well as ornamental, woody, foliage and tropical plants and have low or zero hour REI. Soil biofungicides prevent root rotting pathogens from taking over and can work as well as chemicals under low to moderate pathogen challenges faced by most growers.

Soil biofungicides may possess multiple MOA that add to their usefulness. Understanding how these products work can help you with your program. They can survive near the roots; some can survive on the roots shielding them from damage. MOA help establish the re-application frequencies given for fungi and bacterial based products. Using different ways to help protect the roots give soil biofungicides versatility as part of a rotational program or as a stand alone under certain situations.

Modes of Action for soil biofungicides can determine their longevity as a preventative, which will influence re-application frequencies. Some are widely understood; others are still being investigated; still others have yet to be discovered.

Here's what we know today:

1. Exclusion. Soil biofungicides can grow in the media or near plant roots (termed Rhizosphere Competency) or can grow on the roots (termed Rhizoplane Competency) shielding them from pathogens. The soil biofungicide grows in such a way as to dominate the same area a pathogen wants to occupy and physically exclude it.

2. Competition. Some soil biofungicides can remove simple organic and inorganic compounds released from roots as part of their waste removal system. Beneficial microbes feeding upon root exudates remove enticements that may actually attract pathogens.

3. Parasitism. Some soil biofungicides can attack and eat root rotting pathogens feeding directly upon them for food. They release enzymes that dissolve the cell wall of these pathogens killing them for extended periods of time.

4. Antibiosis. Some soil biofungicides release secondary metabolites that have an inhibitory effect on pathogens; most importantly cell wall or cell membrane formation. Many bacterial based soil biofungicides use this MOA and require shorter application intervals.

5. Nutrient availability. Nutrients must be in a reduced form (making them water soluble) to make them available for absorption by a plant's root system. Some of the soil biofungicides release reducing agents and chelating agents that make nutrients more soluble and available to a root allowing for better utilization of the fertilizer being applied.

6. ISR (Induced Systemic Resistance). Certain soil biofungicides, especially Trichoderma and Bacillus species, have been shown to "turn on" specific compounds produced in a plant's root system that naturally defend the plant from the inside. Production of Phytoalexins and Jasmonic Acid by a plants root system from addition of these soil biofungicides allow some plants to activate their internal immune system. This Mode of Action is still not well understood or predictable but is a hot topic for researchers around the world.

7. Others. There are MOA still to be discovered and explained; good research topics for future graduate students around the world.

Summary

Soil biofungicides are living microbes that can successfully prevent many types of root rotting pathogens under diverse environmental conditions on a range of plants using multiple mechanisms of control. Many of the top growers in the United States as well as other U.S. growers and growers around the world have successfully used soil biofungicides for decades to prevent and control root rotting pathogens.

Using labeled soil biofungicides to defend the plant before disease outbreaks occur; knowing the environmental conditions where they are applied are conducive; knowing what other inputs are not harmful; having general knowledge and asking questions up front aid in the efficacy of the products. Applying the proper soil biofungicides for an indentified root rotting pathogen early before the pathogen gets a foot-hold in the plant can make the difference in performance and the need to use chemicals. Growers and their staff are in constant contact with their plants; they love them and take good care of them to make a profit. Safety for the people, safety for the plants and safety for the environment are becoming more of a concern in the edible and ornamental plant growing world. Soil biofungicides offer this safety; at times they can be used alone or in a rotational program if stress becomes dominant. Identifying problems early and preventing further spread are the backbone of a biological program. Soil biofungicides can and do work.

Soil Biofungicides: Biological Warfare at its Finest