August 8, 2003 - 08:19

What are they, how do they work and are they beneficial?

The biological control process sparks many questions. First,
what is biological control of plant disease? It is the involvement of the use of
beneficial microorganisms, such as specialized fungi and bacteria, to attack
and control plant pathogens and the diseases they cause. So what are these
"specialized fungi and bacteria" that can attack and control plant
pathogens? They are microorganisms that are part of the normal microbiological
environment of most "healthy" soils. They are not genetically
engineered. In their native habitat, these beneficial microorganisms compete
with other microorganisms for space and food. In some cases, they are parasitic
on other microorganisms and/or they produce toxic substances that kill other
soil-inhabiting microorganisms such as Pythium sp., Phytophthora sp.,
Rhizoctonia sp. and other plant pathogens. Scientists are well aware of these
beneficial microorganisms and have studied them for many years. They have shown
that these microorganisms play a vital role in the makeup of the soil
environment and are part of the normal checks and balances that make up a
"healthy" soil.

Many beneficial fungi and bacteria have been isolated from
the soil and tested in private and university-based laboratories as to their
ability to control plant pathogens. Recently, some of the more promising of
these beneficial fungi and bacteria have been further developed and marketed to
ornamental plant growers as an alternative to traditional chemical-based

Below is a better understanding of the current crop of
beneficial organisms (biofungicides) -- how they work and, most importantly,
their advantages and disadvantages when compared to traditional chemical

How They Work

There are four different mechanisms by which beneficial or
biocontrol agents interact with other microorganisms. Most biocontrol agents
apply only one of these four mechanisms; however, some may employ more than
one. Also for the purpose of this article, I will refer to the plant pathogen
as the target organism.

Direct competition.
In this case, the biocontrol agent out-competes the target organisms for
nutrients and space. This is typically a fungus or bacteria that grows very
fast and overwhelms the target organism with sheer numbers. The target organism
is suppressed due to lack of food and space. The target organism may not die
out completely, but its population becomes so low it is no longer a legitimate
threat to the host plant. In order for this type of biocontrol agent to be most
effective, the environmental conditions must favor the growth and reproduction
of the biocontrol agent.

Antibiosis. With
antibiosis, the biocontrol agent produces a chemical compound such as an
antibiotic or some type of toxin that kills or has some sort of detrimental
effect on the target organism. Many microorganisms produce antibiotics and
toxins. Some of the more common antibiotics humans use to warrant-off infections
came originally from common soil-inhabiting fungi and bacteria. In some cases,
antibiosis can be accompanied by other detrimental mechanisms. Antibiosis is
one of the most effective methods of controlling microorganisms.

Predation or parasitism. This is the mechanism that most of us envision when we think of
biocontrol agents. In this case, the biocontrol agent attacks and feeds
directly on the target organism, or the agent produces some sort of toxin that
kills the target organism and then feeds on the dead target. Like direct
competition, the environment must favor growth and development of the predator
or parasite since populations need to be high enough to overwhelm the target

Induced resistance of the host plant. style='font-weight:normal'> Scientists have known for decades that once a plant
is infected with a pathogenic microorganism, infection triggers some sort of
biochemical reaction in the infected host plant that helps keep it from being
infected with further pathogens (super infection). The infected plant becomes
more "resistant" to other infections. Plants do not have immune
systems to protect them from infection as we do; however, they do have
physiological and biochemical systems that help inhibit infection and spread of
pathogens within tissues of the affected plant. Some biocontrol agents are
known to trigger these mechanisms, and in the case of induced resistance, host
plants are purposely inoculated with this agent in an effort to trigger the
resistant response. The microorganism that triggers the response is usually not
a severe pathogen of the host. If it were, it would defeat the whole purpose.
Induced resistance is not highly understood and is currently a very exciting
area of research throughout the scientific community.

Advantages and Disadvantages

Even though it appears as if these biocontrol agents are the
cure-all, there are distinct advantages and disadvantages to using them, when
compared to traditional chemical controls.


* If used properly, they help reduce the use of chemical-based
fungicides. This is good for the environment and is one of the most important
reasons to consider their use.

* They help reduce the risk of developing pathogen
resistance to traditional chemicals. Due to the overuse of certain chemical
fungicides, some common plant pathogens such as Pythium sp. and Botrytis sp.
have become resistant to these fungicides. This is less likely to happen with
biocontrol agents because the beneficial organism co-evolves along with the
target organism and adapts to the changes. Something a chemical cannot do.

* In most cases, they are safer to use. Most biocontrol
agents have very low or no toxicity to humans and other mammals. This is a
tremendous benefit in this day and age.

* They tend to be more stable than chemical pesticides if
stored properly. These are living organisms and must be stored as such. If they
spoil, they are no longer affective.

* In most cases, they have lower re-entry interval (R.E.I.)
times. This is a significant factor especially when it is necessary to enter
the production facility immediately following application.

* In most cases, they are less phytotoxic. Because they are
"natural" they are less likely to cause toxic effects on the host
plant, especially if mistakes are made and rates are miscalculated.


* Biocontrol agents tend to be more difficult to implement
when compared to chemicals. Since most of these products have to be implemented
prior to the onset of disease, greater preparation by the user is necessary.
Biologicals work best in greenhouses that routinely scout for diseases and
insects and detect problems early.

* In most cases, they have a narrower target range. Most are
not broad-spectrum products. Identification of the correct target organism is

* They may not work as quickly as chemicals. Since their
populations need to take time to build up they can take more time to be
effective. That is why it is necessary to apply them prior to the onset of
severe disease outbreak.

* These products do not eradicate the pathogen or rescue the
host from infection. They have to be administered prior to the onset of
disease, in most cases at preplant.

* They may have a shorter shelf life if not stored properly.
Remember, these are living organisms that don't take well to extreme

* In most cases, biocontrol products are more expensive to
use. This includes both time and money. They may be a bit more expensive to
purchase initially, and they take more time to initiate, if used properly.

* They may not be compatible with the use of other chemical
fungicides and bactericides. The product label should be checked to see with
what chemicals the product is compatible. Many of these beneficials are fungi,
and some of the more common greenhouse fungicides have the potential to kill
these beneficial microorganisms.

The Products

Currently there are close to 40 commercial products that are
marketed as biological controls worldwide. Not all of these are available in
the United States. For greenhouse floriculture and perennial production, there
are about a half dozen products that are currently popular (See Figure 1, page
43). Of these, PlantShield appears to be the most widely used. Plantshield is
the T-22 strain of the soil inhabiting fungus Trichoderma harzianum (TH). TH's
mode of action against the target organism is multifaceted. It uses both
antibiosis and predation against many common soil-inhabiting fungi that cause
root and crown rots such as Pythium, Rhizoctonia, Fusarium and Sclerotinia. It
appears to be one of the most popular biofungicides in the greenhouse industry
and can be an asset to a disease management program if used properly.

Keys to Success

In order for any of these biological control agents to work
for you, two simple rules must be followed. First, all of these products must
be used in conjunction with standard disease cultural controls. Cultural
controls include: growing plants in a well-drained media; not over watering;
keeping the greenhouse relative humidity below 85 percent; practicing strict
sanitation; and making sure that the nutrient and pH conditions of the host
plant are within the ideal range for proper growth and development. This will
help assure that the environment is favorable for the growth and development of
the beneficial organism.

Second, all of these biocontrol products must be applied at
preplant or prior to the onset of disease. In most cases, they will not rescue
plants that are already infected. If you abide by these two critical
conditions, the likelihood of you having success with a biocontrol agent is
good. If you don't, they won't work.

Manufactures who have traditionally been the source of
chemical fungicides will be producing and marketing biofungicides. Growers need
to be aware of what products are available, the way they work and their
limitations. It will be a while before we see a biofungicide that controls
Pythium sp. as good as Subdue. However, under the proper growing conditions,
biofungicides can be a viable alternative to chemicals. style="mso-spacerun: yes">

About The Author

Steve Nameth is professor and associate chairperson in the Department of Plant Pathology at The Ohio State University, Columbus, Ohio. He may be reached by phone at (614) 292-8038 or E-mail at

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