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Can Soil Amendments Reduce Disease Severity in Trees
(Part 1)
From Roots to Canopy: Boosting Tree Defenses Against Pests and Diseases
Dr. Glynn Percival
Senior Arboricultural Researcher
Bartlett Tree Research Laboratory, Charlotte, NC
Introduction
Over the past decade, professionals in urban tree management have faced a significant increase in pest and disease outbreaks, including sudden oak death, emerald ash borer, beech leaf disease, and bacterial leaf scorch. Unmanaged, these pests and diseases can cause severe damage potentially resulting in the loss of major tree species.
When tree removal and destruction are not viable options, managing these outbreaks primarily involves the use of synthetic plant protection products that exhibit direct toxicity to the targeted pests or diseases. However, the overuse of these products has raised several concerns, including the impact on non-target organisms due to spray drift, groundwater contamination, accidental exposure to the public, and the development of pesticide resistance in target populations. Consequently, there is now a need to develop more environmentally benign solutions.
A Different Approach
Western medicine emphasizes that the prevention of infectious diseases such as typhoid, diphtheria, measles, hepatitis, and smallpox is primarily achieved through vaccination. This process involves injecting the human body with a weakened strain of a pathogen, which stimulates the production of antibodies specific to that disease, thereby conferring immunity.
This raises the question: can we apply the same “vaccination” principles to protect trees by enhancing their inherent immune systems? The answer is yes. The concept of vaccinating plants against pests and diseases, known as induced resistance, is not new. It was recognized in the early 20th century as a method to boost trees’ natural immune responses to plant diseases.
Induced resistance leads to the accumulation of antimicrobial proteins, fungi-toxic enzymes, phenolics, and terpenoids within leaves, stems, and roots. This results in thicker, more lignified leaves that are more resistant to degradation by enzymes released by foliar diseases. In conifers, this process enhances resin production, phenolic compounds, and the formation of a wound periderm. Because multiple defense mechanisms are activated, it is highly unlikely that pests and diseases can develop resistance to this approach.
Moreover, induced resistance has been shown to provide protection against a variety of biologically different pathogens, including fire blight (bacterial), Phytophthora root rot (oomycete), powdery mildew (fungal), Ceratocystis spruce wilt (vascular wilt fungus), and horse chestnut leaf miner (insect).
Induced resistance and soil amendments
Research conducted at the Bartlett Tree Research Laboratory has demonstrated that the application of specific soil amendments can trigger an induced resistance response in trees (Photographs 1 and 2). Utilizing soil-applied products presents numerous opportunities for managing tree pests and diseases without the need for spraying. Tree resistance can be enhanced by exposing trees to natural and/or synthetic soil amendments, either at the time of planting or around the base of established trees, using techniques such as air-spade technology. Specific soil amendments known to induce resistance include:
Chitin
Chitin, a naturally occurring component of fungal cell walls, can also be derived from waste crustacean shells, including those of crabs, lobsters, crayfish, and shrimp. In soils, chitin serves as a nutrient source, stimulating soil microorganisms to produce chitinolytic enzymes that degrade the chitin molecule. Elevated levels of these enzymes in the soil contribute to the suppression of pathogenic fungi such as Rhizobium and Fusarium root rots. Additionally, the proliferation of chitinolytic bacteria, such as Bacillus licheniformis, Stenotrophomonas maltophilia, and B. thuringiensis, aids in controlling Oomycetes like Phytophthora cactorum. Recently, chitosan, a derivative of chitin, has shown promise as an insecticide. It effectively controls various aphid species and lepidopteran pests through ingestion of treated foliage, with chitosan either applied to the leaf surface or translocated within the plant’s vascular system.
Biochar
Numerous studies have demonstrated that the addition of biochar enhances soil fertility and quality (Photograph 3). Beyond modifying the physical and chemical properties of the soil in the rhizosphere, research indicates that soil-applied biochar also induces resistance to fungal diseases such as Botrytis cinerea (grey mould) and Leveillula taurica (powdery mildew), as well as the insect mite pest Polyphagotarsonemus latus. Zwart and Kim (2012) found that a 5% biochar application (by soil volume) significantly increased stem biomass in Acer rubrum compared to plants inoculated with Phytophthora cinnamomi. This suggests that biochar amendments have the potential to mitigate disease progression and physiological stress caused by Phytophthora canker.
Phosphites
Inorganic phosphite salts when applied to plants as a soil drench, operate through two mechanisms: directly targeting the disease and indirectly stimulating plant defense responses. These responses include the accumulation of phytoalexins, hypersensitive cell death, cell wall lignification, and the production of lytic enzymes that inhibit pathogen growth. Research has demonstrated that potassium phosphite salts are effective in controlling Oomycetes, such as Phytophthora root rot and canker pathogens, Venturia inaequalis (apple scab), and pathogenic bacteria including Erwinia amylovora (apple fire blight) and Pseudomonas syringae pv. aesculi (bacterial bleeding canker).
Look for Part 2 in the February 2025 TREE Press.
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