The biological control process sparks many questions. First,what is biological control of plant disease? It is the involvement of the use ofbeneficial microorganisms, such as specialized fungi and bacteria, to attackand control plant pathogens and the diseases they cause. So what are these”specialized fungi and bacteria” that can attack and control plantpathogens? They are microorganisms that are part of the normal microbiologicalenvironment of most “healthy” soils. They are not geneticallyengineered. In their native habitat, these beneficial microorganisms competewith other microorganisms for space and food. In some cases, they are parasiticon other microorganisms and/or they produce toxic substances that kill othersoil-inhabiting microorganisms such as Pythium sp., Phytophthora sp.,Rhizoctonia sp. and other plant pathogens. Scientists are well aware of thesebeneficial microorganisms and have studied them for many years. They have shownthat these microorganisms play a vital role in the makeup of the soilenvironment and are part of the normal checks and balances that make up a”healthy” soil.
Many beneficial fungi and bacteria have been isolated fromthe soil and tested in private and university-based laboratories as to theirability to control plant pathogens. Recently, some of the more promising ofthese beneficial fungi and bacteria have been further developed and marketed toornamental plant growers as an alternative to traditional chemical-basedfungicides.
Below is a better understanding of the current crop ofbeneficial organisms (biofungicides) — how they work and, most importantly,their advantages and disadvantages when compared to traditional chemicalfungicides.
How They Work
There are four different mechanisms by which beneficial orbiocontrol agents interact with other microorganisms. Most biocontrol agentsapply only one of these four mechanisms; however, some may employ more thanone. Also for the purpose of this article, I will refer to the plant pathogenas the target organism.
Direct competition.In this case, the biocontrol agent out-competes the target organisms fornutrients and space. This is typically a fungus or bacteria that grows veryfast and overwhelms the target organism with sheer numbers. The target organismis suppressed due to lack of food and space. The target organism may not dieout completely, but its population becomes so low it is no longer a legitimatethreat to the host plant. In order for this type of biocontrol agent to be mosteffective, the environmental conditions must favor the growth and reproductionof the biocontrol agent.
Antibiosis. Withantibiosis, the biocontrol agent produces a chemical compound such as anantibiotic or some type of toxin that kills or has some sort of detrimentaleffect on the target organism. Many microorganisms produce antibiotics andtoxins. Some of the more common antibiotics humans use to warrant-off infectionscame originally from common soil-inhabiting fungi and bacteria. In some cases,antibiosis can be accompanied by other detrimental mechanisms. Antibiosis isone of the most effective methods of controlling microorganisms.
Predation or parasitism. This is the mechanism that most of us envision when we think ofbiocontrol agents. In this case, the biocontrol agent attacks and feedsdirectly on the target organism, or the agent produces some sort of toxin thatkills the target organism and then feeds on the dead target. Like directcompetition, the environment must favor growth and development of the predatoror parasite since populations need to be high enough to overwhelm the targetorganism.
Induced resistance of the host plant. Scientists have known for decades that once a plantis infected with a pathogenic microorganism, infection triggers some sort ofbiochemical reaction in the infected host plant that helps keep it from beinginfected with further pathogens (super infection). The infected plant becomesmore “resistant” to other infections. Plants do not have immunesystems to protect them from infection as we do; however, they do havephysiological and biochemical systems that help inhibit infection and spread ofpathogens within tissues of the affected plant. Some biocontrol agents areknown to trigger these mechanisms, and in the case of induced resistance, hostplants are purposely inoculated with this agent in an effort to trigger theresistant response. The microorganism that triggers the response is usually nota 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 excitingarea of research throughout the scientific community.
Advantages and Disadvantages
Even though it appears as if these biocontrol agents are thecure-all, there are distinct advantages and disadvantages to using them, whencompared to traditional chemical controls.
* If used properly, they help reduce the use of chemical-basedfungicides. This is good for the environment and is one of the most importantreasons to consider their use.
* They help reduce the risk of developing pathogenresistance to traditional chemicals. Due to the overuse of certain chemicalfungicides, some common plant pathogens such as Pythium sp. and Botrytis sp.have become resistant to these fungicides. This is less likely to happen withbiocontrol agents because the beneficial organism co-evolves along with thetarget organism and adapts to the changes. Something a chemical cannot do.
* In most cases, they are safer to use. Most biocontrolagents have very low or no toxicity to humans and other mammals. This is atremendous benefit in this day and age.
* They tend to be more stable than chemical pesticides ifstored properly. These are living organisms and must be stored as such. If theyspoil, 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 enterthe 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 hostplant, especially if mistakes are made and rates are miscalculated.
* Biocontrol agents tend to be more difficult to implementwhen compared to chemicals. Since most of these products have to be implementedprior to the onset of disease, greater preparation by the user is necessary.Biologicals work best in greenhouses that routinely scout for diseases andinsects and detect problems early.
* In most cases, they have a narrower target range. Most arenot broad-spectrum products. Identification of the correct target organism isimperative.
* They may not work as quickly as chemicals. Since theirpopulations need to take time to build up they can take more time to beeffective. That is why it is necessary to apply them prior to the onset ofsevere disease outbreak.
* These products do not eradicate the pathogen or rescue thehost from infection. They have to be administered prior to the onset ofdisease, 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 extremetemperatures.
* In most cases, biocontrol products are more expensive touse. This includes both time and money. They may be a bit more expensive topurchase initially, and they take more time to initiate, if used properly.
* They may not be compatible with the use of other chemicalfungicides and bactericides. The product label should be checked to see withwhat chemicals the product is compatible. Many of these beneficials are fungi,and some of the more common greenhouse fungicides have the potential to killthese beneficial microorganisms.
Currently there are close to 40 commercial products that aremarketed as biological controls worldwide. Not all of these are available inthe United States. For greenhouse floriculture and perennial production, thereare about a half dozen products that are currently popular (See Figure 1, page43). Of these, PlantShield appears to be the most widely used. Plantshield isthe T-22 strain of the soil inhabiting fungus Trichoderma harzianum (TH). TH’smode of action against the target organism is multifaceted. It uses bothantibiosis and predation against many common soil-inhabiting fungi that causeroot and crown rots such as Pythium, Rhizoctonia, Fusarium and Sclerotinia. Itappears to be one of the most popular biofungicides in the greenhouse industryand 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 workfor you, two simple rules must be followed. First, all of these products mustbe used in conjunction with standard disease cultural controls. Culturalcontrols include : growing plants in a well-drained media; not over watering;keeping the greenhouse relative humidity below 85 percent; practicing strictsanitation; and making sure that the nutrient and pH conditions of the hostplant are within the ideal range for proper growth and development. This willhelp assure that the environment is favorable for the growth and development ofthe beneficial organism.
Second, all of these biocontrol products must be applied atpreplant or prior to the onset of disease. In most cases, they will not rescueplants that are already infected. If you abide by these two criticalconditions, the likelihood of you having success with a biocontrol agent isgood. If you don’t, they won’t work.
Manufactures who have traditionally been the source ofchemical fungicides will be producing and marketing biofungicides. Growers needto be aware of what products are available, the way they work and theirlimitations. It will be a while before we see a biofungicide that controlsPythium sp. as good as Subdue. However, under the proper growing conditions,biofungicides can be a viable alternative to chemicals.