Root Zone Disease Management—Go Biological or Not?

Read More in The Sun Gro’er Issue 1/2 (2003)
In recent months there has been a lot of hype in industry journals and trade shows about biological fungicides (BFs). The intention of this article is to provide some background information on these types of products and also to provide some guidance in determining if biological fungicides are right for you or your customers.
A biological fungicide (BF) is a living organism that is used to suppress or control a plant disease typically caused by a pathogenic (disease-causing) fungus. Biological fungicides utilize microorganisms from one of two groups – fungi or bacteria. The most common root diseases found in greenhouse production are Pythium, Phytophthora, Rhizoctonia, Thielaviopsis, and Fusarium.

So a BF is a fungus or bacteria purported to provide some sort of control of one or more of these diseases. But the word “control” can be misleading. BF’s do not provide control in the sense that they can clean up an existing disease problem like some chemical fungicides are believed to do. A BF’s main form of control is prevention through disease suppression, so it’s essential they are used before disease appears. Ideally, BFs should be applied before or during planting so that their suppressive benefits can become established in the root zone (rhizoplane) and on or around plant roots (rhizosphere).

Product labels perform the primary means of communication about a product and proper use of that product to the user. So it is important to note that when a company develops a biological organism that shows the ability to ‘suppress’ a disease or induce a plant to perform better in the presence of a disease, they can choose two different routes in developing a label:

  1. The product can be registered as a microbial inoculant. This usually means that the product does something to promote healthier or stronger plants. In some cases, this may be may be a result of disease suppression. But, if labeled this way, a supplier typically cannot make disease control (fungicidal) claims.
  2. The other route, is to label the product as a biofungicide or biological fungicide. This requires the supplier to register the product with the USEPA and state regulatory agencies. This is a much more demanding process and involves significant time and money to perform the testing to prove it is safe to people and the environment as well as support disease control claims.

Placing restrictions on the use of a product is a way to limit the cost and increase the speed the registration process. Limiting a product to enclosed structures (i.e. greenhouses) is one strategy that some suppliers have chosen to use which is why some of the products listed below have that restriction on their label. Other strategies may include limiting target crops, limiting re-application or application rates, requiring certain PPE and the like. The point is that the user has to comply with restrictions on the label.
Interestingly enough, some companies may choose to label a product both ways. It that case, the same active ingredient is labeled as a microbial inoculant and a biofungicide. Nevertheless, as mentioned above, all of the products mentioned in this article work only to prevent disease through disease suppression and will not “control” or “eliminate” a disease after it has infested a crop. But for the sake of keeping things simple, we will use the very general definition of the term ‘biological fungicide’.
You might ask, “How do organisms suppress plant disease organisms?” BF’s work mainly by one of several methods, they may include:

  1. Competition- BF’s compete with disease organisms for food supplies. Plants excrete sugars and/or other nutrients into the soil as they grow. Fungi and bacteria in the soil use these materials as food sources. BF’s are usually more aggressive in seeking out these food sources and therefore prevent the growth of disease organisms by significantly reducing or eliminating the food supply.
  2. Antibiosis- This is direct control through the release of substances that can kill other organisms and form a “barrier zone” around plant roots into which other organisms will not grow. Many bacteria use this mechanism including Streptomyces, which produces the chemical streptomycin. Incidentally, this is the active ingredient in Agrimycin or Agri-Strep.
  3. Predation or Parasitism- Some BF’s actually feed on other organisms in the soil, by attacking them. This works well when disease populations are low, but if there is a large infestation, the BF most likely will not be able to keep up. This is a mechanism that Trichoderma and Gliocladium use in their control of other organisms.
  4. SAR (Systemic Acquired Resistance)- This is something that is just beginning to be understood. SAR involves the symbiotic relationship between the BF organism and the host plant. The organism releases chemicals that the plant interprets as disease causing. This triggers the plant to engage its own defense mechanisms, which may include such events as thickening of cell walls, enhanced rooting and increased reproductive traits. The last point is what has created the most interest, since it can increase the yield of fruits and vegetables.

For the horticulture market, it seems to produce a slightly larger and healthier plant and reduces crop time. This is one of the mechanisms used by some Bacillus species. Before we address some of the benefits of using BF’s over chemicals, let us look at some of the products currently available to the horticulturist.
Bacillus subtilis MBI 600: Formulations containing this organism are branded as Subtilex and manufactured by MicroBio Ltd., who is a subsidiary of Becker Underwood, Inc. Subtilex is labeled as a seed treatment for suppression of Rhizoctonia, Fusarium or Pythium on field crops such as Cotton, Peanuts, Soybeans, Alfalfa
Wheat, Barley and Corn. The mode of action, according to the Microbio Group’s website is to exclude other organisms and exude an anti-fungal metabolite that will suppress or kill pathogens. The bacteria, since it is a spore former, can persist in the soil for some time supposedly making the shelf life close to 2 years. There are no data available to our knowledge to support this although it may be possible. Other products containing Subtilex include growing media products manufactured by Premier. Premier also claims control of Pythium, Rhizoctonia and Fusarium and general germination and growth stimulation.
While there is data primarily geared toward Ag crops, there is limited University trials evaluating MBI 600 compared to commonly used biological fungicides. Tests used by Premier show treated mix compared to untreated competitive mix. Comparisons were not made to a chemical control or a similar mix not containing the biological agent, meaning the data is “confounded” – apples to oranges. General anti-microbials like ZeroTol will kill B. subtilis. Therefore, if a grower uses ZeroTol or other materials that kill bacteria then Subtilex is not for them. Premier claims no re-application of Subtilex is necessary. There are no known Subtilex products that a grower can use for incorporation or liquid re-application. Currently it is only to be used in enclosed structures-greenhouses and glasshouses with PPE such as respirators and gloves.
Bacillus subtilis GB03: Gustafson LLC manufactures this organism and formulations containing this organism for greenhouse and nursery use are manufactured and marketed by Growth Products, Ltd. under the brand name CompanionÒ. There are two formulations labeled for greenhouse use – Companion Dry Super Concentrate and Companion Liquid. This is one of the species that has both microbial inoculant and biological fungicide labels. The Dry Super Concentrate and one of the liquid formulations are actually labeled as microbial inoculants. These products have broad labels and can be used outdoors and in shadehouses. The other formulation is Companion Biological Fungicide. It is a liquid and is only labeled for enclosed structure application and with the use of PPE such as gloves and respirators. Companion liquid biological fungicide is labeled for suppression and control of Rhizoctonia, Pythium, Fusarium and Phytophthora. Gustafson LLC markets a water-applied formulation for the Ag seed market, called KodiakÒ. It is labeled to “suppress” Pythium, Phytophthora, Rhizoctonia, Fusarium, Sclerotinia and Anthracnose– a wider label than products using B. subtilis MBI600. The Companion label recommends re -application every 3-4 weeks if needed. It is unknown why products containing B. subtilis ‘MBI600’ does not require reapplication but B. subtilis ‘GB03’ does. Growers can use Companion Dry Super Concentrate in their mix or apply the liquid biological fungicide product as a root drench with any re applications using Companion liquid biological fungicide. Although claims have been made on relative efficacy of the MBI600 and GB03 strains, again, no known research or practical trial data are readily available to support such claims.
Streptomyces lydicus WYEC 108: This organism is used in formulations called Actino Iron or Actinovate Granular and manufactured and marketed by Natural Industries, Inc. Actino-Iron is formulated on humic /fulvic acid for ornamentals and Actinovate is formulated as a soluble powder or on zeolite for turf. They are marketed as microbial inoculants. No claims are made on disease control but the products are reportedly in the registration process for that reason. If the organism is similar in range of control to Mycostop, it could be labeled to have a broad range of control, which may include Pythium, Phytophthora and Fusarium. Mode of action is competition and antibiosis. No reapplication is reported to be necessary for annuals and perennials. Actino-Iron also has the capability of adding iron safely and would be considered a benefit for situations where iron deficiency is a problem. The company claims that the organism makes nutrients more available as well. There is limited data to support these claims. There are apparently no significant storage considerations with these products before or after mixing. The same anti-microbial materials that affect Bacillus will affect Streptomyces although there may be exceptions. Growers can use the granular for preplant-incorporation and a liquid concentrate for re-application.
This product has a well proven track record in the greenhouse industry. Streptomyces griseoviridis K61. This organism is used in a formulation called Mycostop and produced and marketed by AgBio Inc. As with S. lydicus, this microbe populates the plant roots and exudes substances that suppress or kill pathogenic organisms.
Labeled for control of Pythium, Fusarium, Alternaria, and Phomopsis spp. As with Actino-Iron, it is claimed to be a growth stimulant. One study at the Vineland Research Station in Ontario (HortTechnology Vol. 13(1), pgs. 149-153) shows excellent results with this product on plant growth although not as efficacious against Pythium as Subdue. The formulation is temperature sensitive and needs to be refrigerated before use. One of the formulations can be used as a pre-incorporation material but at very low rates (4 grams per yard!). These two factors make it very unlikely that it will be adopted for pre-incorporation.
Trichoderma harzanium Rifai strain KRL-AG2: This organism is formulated into products labeled as Root- Shield Granules and Plantshield HC, which is manufactured and marketed by BioWorks, Inc. Trichoderma harzanium Rifai strain KRL-AG2 (often called T-22) is a genetically engineered and patented Trichoderma
(US patent No 5260213). RootShield Granules are labeled to have efficacy in control of Pythium, Rhizoctonia, Fusarium, Cylindrocladium and Thielaviopsis. The labeled rate is 1.0 to 1.5 pounds per CY. Re-application is needed 10-12 weeks after initial application. RootShield is compatible with many insecticides and fungicides although it is not compatible with general anti-microbials like Zero -Tol. Since RootShield is a fungus instead of being a spore forming bacteria like some of the above-mentioned products;
it does have some storage considerations. It is currently recommended that once incorporated in a media, that it be used in less than six months. In extremely hot climates or during the summer months this time frame is reduced. Despite some of these handling/ environmental issues, RootShield has a significant positive track record of use in the greenhouse and nursery industry.
Gliocladium virens GL-21: This organism is used in a formulation labeled as SoilGard 12G and manufactured and marketed by Certis, Inc. Mode of action is very similar to Trichoderma based products since Gliocladium is similar to Trichoderma. Labeled to be antagonistic to Pythium and Rhizoctonia. Application rate is 1 to 1.5 lbs per CY. Storage precautions are similar to Trichoderma products although the SoilGard 12G label indicates that temperatures of 75° to 100° F may accelerate loss of bioactivity
with above 100° F being detrimental. This product is not well known in the marketplace but the organism has been used in the industry for quite some time marketed formerly under the brand name GlioGard. .
One might ask the question; “What makes biofungicides better than the chemical controls that I have been using for years?” The advantages of biocontrols over chemical controls are often seen more as indirect benefits. Biological fungicides often have growth stimulatory effects that are somewhat separate from the effect on suppression of disease organisms. Although these growth stimulatory effects are reported to be real, they are not consistent, meaning that customers will not always observe a positive effect. Handling and safety requirements are more “relaxed” when using BF’s. Biological fungicide products usually have zero re-entry intervals whereas most chemical fungicides have a 12 REI. In most cases, BF’s do not require any special protective clothing although there are exceptions as noted above. This provides a safer and more productive work environment for employees.
Ease of use can be enhanced in some cases depending on the perspective of the user. While chemical controls can be used as a preventative measure, biological products must be used as a preventative measure if to be successful. Therefore, if BF’s are incorporated into the mix before planting, they will begin colonizing the root zone, providing protection from the time of transplanting. Chemical applications used as a preventative are often viewed as an indiscriminant use. So, chemical applications are not made until later or when growers see a problem, which means the chemical has to ‘catch up’ and growth has often been checked.
Additionally, with chemicals there is always the possibility, especially with repeated applications, for disease organisms to build up resistance to the chemical, thus reducing its effectiveness. Resistance is not apparently an issue with BF’s since their control mechanisms are so different from chemical fungicides. Then of course, is the environmental stewardship aspect of biologicals. Since most of the organisms occur in nature, there is not the concern attached to using them as there is with using chemicals that are man-made and may be perceived to linger in and harm the environment.
Cost can also be a consideration. If a grower is in the habit of using preventative chemical treatments and then goes back and hits hot spots with additional treatments, they could save money in the long run with BF’s. However, growers following IPM practices and spot application of chemical fungicides to specific crops for specific problems will not be as likely to save money by switching to biological fungicides. On the other hand, growers who are cavalier about utilizing proper IPM techniques thinking that biological fungicides will correct cultural problems will often be disappointed. In the end, biological fungicides are not a magic bullet that will solve all disease problems for a grower. For growers that are concerned about employee and environmental safety, biological control organisms serve a valuable purpose. But any grower that is considering switching their main disease control to biologicals must be aware that none of the products claim to cure an existing problem. Therefore, all the past advice and recommendations related to IPM techniques such as sanitation, buying clean plant material, well-thought out fertility programs and monitoring the physical environment (watering, air movement, temperatures) are even more critical when using biological fungicides in a disease management program.
Sun Gro Horticulture can provide custom blends with several of the above products and has considerable experience with their use. The Technical Specialist Team is available to assist growers in determining what products would work best in their conditions.
~Mark Thomas and Rick Vetanovetz