To the tree care professional, the words “fungi” and “bacteria” are generally associated with disease and the subsequent problems they may cause to trees.
But the reality is that microorganisms have been waging war against each other more or less since the dawn of life itself.
The use of “plant-friendly microorganisms,” such as mycorrhizae or plant growth promoting bacteria, has received some attention in arboriculture, mainly to enhance tree survival rates after planting. But agricultural research has shown that microorganisms can play important roles in enhancing plant defense responses and/or be manipulated as a direct or indirect means of biological control.
Cases have been found where susceptible hosts show little or no disease development despite a virulent pathogen being present in the soil. In others, damage by a soil pathogen may be severe at first but then decreases over time, although the pathogen still persists. These soils are defined as “suppressive.” The most important element of the soil environment that contributes to “suppressiveness” is biological, that is, the microorganisms that inhabit the soil. For the purpose of this article, such microorganisms are referred to as “biologicals.”
Biologicals operate in pathogen suppression via a number of diverse mechanisms, including competition for nutrients, antibiosis and induction of host resistance. With respect to agricultural systems where study of soil suppressive biologicals has occurred, it has been shown that nonpathogenic Fusarium and fluorescent Pseudomonas play a critical role in naturally occurring soils that are suppressive to Fusarium wilt.
In an evolutionary context, humans, with our reliance on synthetic plant protection products, are only recently recognizing the benefits of exploiting the microorganism versus microorganism fight for biological control. It’s now commonly known that while synthetic fungicides and insecticides can have impressive and rapid effects, the use of biologicals utilizes a more complex, subtle and long-term strategy. In essence, the use of biologicals allows us to fight fire with fire. Knowledge of biologicals and the potential they offer is important because of the potential for plant pathogens to overcome or develop resistance/insensitivity to synthetic plant protection products. In addition, increased legislative restrictions regarding the use and application of synthetic products means there’s a constant requirement to improve the effectiveness of existing disease control strategies and evaluate new pest and pathogen management strategies.
Over the past few years, a number of biologicals have been commercially registered to aid in horticultural crop production. These registered biologicals tend to be naturally occurring fungi and bacteria that are widespread in soil. Although mainly utilized in horticulture, these organisms offer great potential to combat tree pests and diseases of concern worldwide.
Below, we’ll discuss the potential of three commercially available bio-control organisms of benefit to arborists—with respect to pest and disease control strategies—as well as highlight ongoing research at the Bartlett Tree Research Laboratory in the United Kingdom.
Bacillus subtilis (Trade name “Serenade”)
Many species and strains of Bacillus have demonstrated bio-control activity. For example, Bacillus subtilis has a long history of study as a bio-control organism and is commonly used by the food industry. B. subtilis is registered in the U.K. for the control of Botrytis cinerea, Phytophthora and Pythium on several horticultural crop species.
B. subtilis has been shown to possess a number of human health benefits, including stimulating the human immune system when consumed, and consequently forms the basis of several medicines and probiotics. B. subtilis spores can also survive harsh conditions, such as high temperatures and, in NASA trials, the freezing vacuum of space (six years, if protected from UV). These traits make it highly suitable for long-term storage within commercial products, something that has proven a problem for many other biologicals used in arboriculture.
As a direct interaction with pathogens, biologicals generally produce a range of antibiotic and antifungal compounds that can inhibit cellular division, disrupt membranes and/or essential bacterial/fungal enzyme or protein synthesis pathways, which in turn impede growth or directly kill. In addition to direct effects on pathogens, B. subtilis is also a plant-growth promoting rhizobacteria (PGPR). Such bacteria are generally applied to the soil for long-term establishment. Once a PGPR is established in the root zone, plant growth is promoted via increased nutrient availability and uptake and/or hormonal interactions that influence growth ratios, increase photosynthetic capacity and root growth. Recent research also indicates application of B. subtilis invokes induced resistance (IR), a whole plant immune response. That is, a plant’s immune system is primed by B. subtilis to respond against future pathogen attacks.
Other species of bacteria, such as several Pseudomonas, also cause an IR response in plants. However, Bacillus are more suitable for use in commercial arboriculture due to ease of storage and greater association with increased plant growth. PGPR applied to root systems have been shown to have an effect on foliar pathogens, in addition to soil-borne species. On crop plants, impacts have been shown against chewing insects by stimulating formation of chemical defenses, as well as attractants for their natural enemies, which predate and parasitize them.
Bacillus thuringiensis (Bt) is a soil-dwelling bacterium that has been used to control insect pests since the 1920s, mainly applied as liquid sprays. During sporulation, many Bt strains produce crystal proteins (proteinaceous inclusions); these toxins have a direct insecticidal action against caterpillar defoliators by causing perforation of the gut. Bt is widely used as a larvicide against mosquito larvae and is considered an environmentally friendly method of mosquito control.
Ongoing work at Bartlett Tree Research Laboratory is assessing the use of B. subtilis, among other biologicals, as a treatment against infection of tree wounds, such as those following the excision of bacterial bleeding cankers caused by Pseudomonas syringae pv. aesculi and fungal infections following pruning of fruiting apple trees. Wounds compromise the natural barriers of the tree and are potential portals of entry for several wound pathogens and decay fungi. This research is evaluating how biologicals protect wounds from these pathogens by direct colonization of the wounded areas, as well as any secondary influence on stimulating callus growth.
Trichoderma harzianum (Trade name “Trianum”)
Trichoderma is one of the more common soil fungi on Earth. Similar to any organism, it demonstrates robustness for living and interacting with other organisms. Many Trichoderma species act as direct biological controls of several root and foliar pathogens. When Trichoderma is applied to a soil, it rapidly colonizes roots, increasing nutrient uptake, photosynthetic rate, yield and plant growth, as well as tolerance to drought stress.
Trichoderma out-competes pathogens for nutrient resources and simultaneously produces antibiotic compounds, inhibiting pathogen growth. In addition, Trichoderma can directly parasitize a wide range of economically significant pathogenic fungi and fungi-like oomycetes such as Armillaria, Pythium, Phytophthora, Rhizoctonia, Botrytis and Fusarium.
Initially, Trichoderma grows toward the hyphae of the pathogen, attracted by the release of their exudates (naturally secreted compounds). Trichoderma then attaches itself to the pathogen, coils around its hyphae and mechanically penetrates the pathogen’s cell walls using appressoria (a specialized cell that is used to infect host organisms). Following this process, cell wall-degrading enzymes and antibiotic compounds are secreted to aid parasitism, culminating with the death of the host hyphae.
On Armillaria fungi, Trichoderma penetrates the resilient outer layers of rhizomorphs to parasitize the dense hyphae inside; rhizomorphs are vital to the spread of Armillaria, especially between tree root systems.
Trichoderma is also highly resistant to many of the antibiotic compounds released by pathogenic fungi and bacteria, as well as a number of commercially available synthetic fungicides.
This unique property makes Trichoderma especially compatible with integrated pest management programs, because it can be used alongside or in rotation with synthetic plant protection compounds to increase their efficiency and/or reduce their usage.
Work at the Bartlett Tree Research Laboratory has shown Trichoderma to be beneficial in suppressing Armillaria infestations, with a single application providing increased protection for up to 18 months.
Beauveria bassiana strain (Trade name “Naturalis”)
Unlike Trichoderma– and Bacillus-based biologicals that focus on disease control and promoting plant health, Beauveria bassiana is a species of fungi, many strains of which are entomopathogenic, i.e., an organism that can act as a parasite of insects, either killing or seriously disabling its host. If host death occurs, entomopathogenic fungi multiply within the insect body, releasing spores that can re-infect further insects.
The commercially available B. bassiana strain targets a broad spectrum of invertebrate pests including spider mites, whitefly, aphids, thrips and European cherry fruit fly. (Other entomopathogenic strains of fungi tend to be specialized to one species or group of insects.) B. bassiana is effective at all the stages of the insect pest life cycle, although direct contact with the target organism is required.
Research has recently shown that in some cases, B. bassiana can inhabit woody plant species without any detrimental effects on their biology and increase disease resistance against insect pests. B. bassiana can also be mixed with physical insecticides, such as spray oil or horticultural soaps, to further improve the effectiveness of treatment.
Pros and cons
Biologicals are composed primarily of living organisms, generally making them less robust than chemical products. A fundamental problem often relates to maintaining these organisms in a live state after packaging – they tend to have a relatively short shelf life, often requiring refrigeration to ensure their viability.
Care also has to be taken with the application of biologicals as effectiveness can be influenced by prevailing weather conditions and soil temperature. For example, soil temperatures less than 10 degrees Celsius tend to inhibit biological activity. These issues, plus limited government regulation and knowledge regarding their use and application, have resulted, in general, in limited confidence in the use of biologicals for pest and disease management.
The application of biologicals attempts to modify soil, leaf and/or whole plant micro-ecosystems by enhancing or balancing specific fungal and/or bacterial populations. It’s important to emphasize that when a tree is heavily infested or infected, the natural ecosystem is considered to be “malfunctioning,” which has allowed the pathogen(s) to establish, attack and eventually overwhelm the tree.
As with all ecological associations and interactions, a combination of external variables can influence the effectiveness and outcome of a biological treatment. For example, other microorganisms present on the plant or within the soil matrix may either stimulate or inhibit any artificially applied biological and/or the release of exudates and chemicals from tree roots differing between species. Also, the age of a tree also influences the quantity and form of exudates released.
Soil nutrient availability may also influence the effectiveness of treatment, as some biologicals control soil-borne pathogens by outcompeting them for specific nutrient resources necessary for survival.
Due to the factors above, the results of biological treatments have often been variable, with effectiveness lower than desired for use by the arboriculture industry. But in recent years, a number of highly targeted biologicals have become available that show high efficiency rivaling chemical products. Also, there are a number of commercial products available with potential uses against multiple tree pests and diseases. As research continues, more will be developed which, combined with a growing application expertise within the arboriculture industry, means it’s likely a greater use of biologicals to promote tree health and manage pests and diseases will occur.