Lately there is a lot of talk about the development and use of bio-controls for the management of diseases in greenhouse bedding plant production. Each season, bio-control products are introduced that are labeled for a wide variety of plant pathogenic organisms. Some of these products have had labels for years. Others are new.

Biocontrol products for the control of greenhouse plant diseases are here to stay and the more educated you are on how they do and do not work, the wiser your choices will be. Here are some current biofungicides, how they work and their advantages and disadvantages compared to traditional chemical-based fungicides.

Natural controls

USDA defines biological control of plant disease as " 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." These specialized fungi and bacteria are microorganisms that normally inhabit most soils. They are not genetically engineered.

In their natural habitat, these fungi and bacteria compete with other microorganisms for space and food. Some biocontrol agents produce toxic substances that parasitize and/or kill other soil-inhabiting microorganisms such as Pythium, Phytophthora, Rhizoctonia and other plant pathogens.

Scientists have known about these parasitic microorganisms and 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.

For years, these parasitic 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 biocontrol fungi and bacteria have been further developed and marketed to ornamental plant growers as an alternative to traditional chemical-based fungicides.

How they work

There are four mechanisms by which biocontrol agents control other microorganisms. Most biocontrol agents apply one or more of these mechanisms in the process of interacting with other mircoorganisms.

Direct competition. The bio- control agent out-competes target organisms for nutrients and space. This is typically a fungus or bacterium that grows very fast and overwhelms the target organism. The target organism is suppressed due to lack of food and space. The target organism may not die completely; however, the population becomes so low it is no longer a problem.

Antibiosis. The biocontrol agent produces a chemical compound such as an antibiotic or some type of toxin that kills or has a detrimental effect on the target organism. Many microorganisms produce antibiotics and toxins. Some of the more common antibiotics that humans use to ward off infections came originally from fungi and bacteria.

Predation or parasitism. This is the mechanism most often associated with biocontrols. The biocontrol agents attack and feed directly on target organisms.

Induced resistance in the host plant. It has been known for decades that when a plant is infected with a pathogen, that infection triggers some sort of reaction in the host that helps keep it from being infected with other pathogens. 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. Some biocontrol agents trigger these mechanisms and in the case of induced resistance, host plants are purposely inoculated with this agent in an effort to trigger this resistant response.

Advantages of biocontrols
· Biocontrols help reduce the use of chemical-based fungicides.
· They help reduce risk of developing pathogen resistance to traditional chemicals.
· In most cases, they are safer to use.
· They tend to be more stable than chemical pesticides if stored properly.
· In most cases, they have lower restricted-entry interval (REI) times.
· In most cases, they are less phytotoxic.

Disadvantages of biocontrols
· Biocontrols tend to be more difficult to implement compared to chemicals.
· In most cases, they have a narrower target range.
· They may not work as quickly as chemicals.
· These products do not eradicate the pathogen or rescue the host from infection.
· They may have a shorter shelflife than chemical controls if they are not stored properly
· In most cases, biocontrols are more expensive.
· They may not be compatible with other chemical fungicides and bactericides.

What's available?

To date, more than 40 commercial products are marketed as biological controls. Not all of these are available in the United States. In greenhouse floriculture and perennial production, only a few growers are using biologic& regularly (see chart, Page 65).

PlantShield appears to be the most widely used. PlantShield is the T-22 strain of the soil-inhabiting fungus Trichoderma harzianum (TH). TH uses both antibiosis and predation against many common soil-borne pathogens that cause root and crown rots such as Pythiurn, Rhizoctonia, Fusarium and Sclerotinia. It also has activity toward many foliar pathogens.

Keys to success

For any biological control agent to work, two simple rules must be followed:
1. All of these products must be used in conjunction with standard disease cultural controls.
2. All of these products must be applied before disease onset. They will not rescue plants that are already infected.

If you abide by these two critical guidelines, the likelihood of being successful with biocontrols is good. If you don't, they won't work.

Growers must be aware of what biocontrols are available, the way they work and their limitations. It may be a while before we see a biofungicide that controls Pythium as well as Subdue does. However, under proper growing conditions biofungicides can be a viable alternative to chemicals.

For more information on bio-controls, go to the USDA Web site, www.barc.usda.gov/psi/bpdl/bpdl.html.