4.3 Chemical Stress Beyond Herbicides: Growth Regulators, Adjuvants, Salts, Metals & Air Pollutants

The final Abiotic Disorders task asks the arborist to diagnose and evaluate chemical stress and pollution injury. The outline's own list is broad: herbicides, miticides, insecticides, fungicides, antibiotics, heavy metals, plant growth regulators, antidesiccants, oils, soaps, fertilizers, surfactants, spreaders and spreader-stickers, inert ingredients, and air pollution. Herbicide phytotoxicity and de-icing salt are covered elsewhere in this guide; this section covers the rest.

Plant Growth Regulators

Paclobutrazol and flurprimidol are triazole and pyrimidine gibberellin-biosynthesis inhibitors. By blocking the synthesis of gibberellic acid, they suppress internode elongation. In arboriculture they are applied as a soil drench, soil injection, or basal trunk application and are taken up by roots and translocated in the xylem.

Legitimate uses include reducing shoot elongation between utility line-clearance cycles, reducing the frequency of clearance pruning, and improving stress tolerance in some situations through the associated reduction in leaf area and increase in root-to-shoot ratio.

The concentration trap is explicitly named in the ISA outline. These active ingredients are marketed in different concentrations — an 8 percent formulation and a 22.3 percent formulation, for example. A rate that is correct for one product is an overdose or an underdose for the other. Because the material is persistent in soil and is taken up over a season or more, an overdose is not correctable within the season.

Overdose symptoms:

• Extremely short internodes producing a compressed, tufted shoot habit;
• Undersized, thickened, unusually dark green leaves;
• Persistent suppression across multiple growing seasons, because the residue remains active in the soil;
• In severe cases, distorted growth and dieback.

Diagnostic rule: uniform, whole-tree dwarfing with dark thickened foliage and no pathogen signs, in a tree under utility or municipal management, should raise a PGR question and prompt a review of the application record.

Distinguish these from ethylene-based growth regulators and from sprout inhibitors, and from antitranspirants and antidesiccants (film-forming polymer emulsions applied to foliage to reduce winter desiccation), which can themselves cause injury by trapping heat or by being applied at temperatures outside the label range.

Adjuvants, Oils, Soaps and Inert Ingredients

Adjuvants are added to spray mixtures to improve performance and are a real and under-recognized source of phytotoxicity.

Horticultural oil is the most common source of adjuvant phytotoxicity in tree care. Standard cautions: do not apply above the temperature ceiling on the label; do not apply to drought-stressed plants; do not apply when foliage will not dry promptly; observe the label's minimum interval before or after sulfur applications, because the combination is phytotoxic; and observe named sensitive species, which commonly include blue-needled conifers where oil strips the glaucous bloom, along with maples, redbud, and others listed on the label.

Insecticides, miticides, fungicides and antibiotics applied as trunk injections deserve separate mention: injection is a wounding technique, and repeated injection at the same points produces cumulative internal injury and decay columns. Prescription of an injection program should account for the wounding cost across the expected treatment horizon.

Fertilizer Salt Injury

Fertilizers are salts. Over-application, application to dry soil, or application without irrigation raises the soluble salt concentration in the root zone, lowering soil osmotic potential and producing physiological drought: the root cannot draw water from a solution more concentrated than its own tissue.

• Symptom: marginal leaf scorch progressing inward, indistinguishable in appearance from drought scorch or from de-icing salt injury.
• Diagnosis rests on application history plus measured soluble salts (electrical conductivity of a saturated paste extract), not on symptom appearance.
• Foliar burn from a granular product landing on wet foliage is a separate, immediate contact injury.
• Remediation is leaching with sufficient irrigation where drainage permits, which is only possible if the site drains.

Heavy Metals and Soil Contamination

Urban soils commonly carry legacy contaminants: lead from paint and leaded fuel near roadways and old structures, zinc and cadmium from galvanized materials and tyre wear, copper from treated timber and fungicides, arsenic from historic orchard sprays and CCA-treated wood, and petroleum hydrocarbons from fuel and lubricant spills.

The professionally important point is that most established trees tolerate levels of metals that are unacceptable for human exposure. Phytotoxicity in trees is uncommon except at gross contamination levels. What changes is the site management recommendation:

• Screening soil where site history suggests contamination, particularly before recommending a site for edible production or a children's play area;
• Managing exposure with mulch and turf cover to prevent dust and direct soil contact;
• Recognizing that trees may be part of a phytoremediation or, more commonly, a phytostabilization strategy — using cover and root systems to stabilize contaminated soil rather than to remove the contaminant.

Copper phytotoxicity in the rhizosphere and boron toxicity from reclaimed water are the two genuine tree-health metal and micronutrient toxicities most commonly encountered.

Air Pollution Injury

Two exam-relevant discriminators. Upper-surface stipple points to ozone; lower-surface glazing points to PAN. And air pollution injury characteristically appears across multiple unrelated species in a corridor or downwind of a source, following the exposure gradient rather than any taxonomic pattern — which is the single best way to separate it from a biotic disease.

The Differential Discipline

Chemical injury is a diagnosis of pattern and history, and there is no laboratory test for most of it.

1. Map the affected population. Multiple unrelated species affected together points to abiotic; one species affected points to biotic or to a species-specific sensitivity.
2. Map the geometry. A sharp line, a directional gradient, a spray boom height, a drip line under an irrigation head, a downwind plume — these are physical distributions, not biological ones.
3. Establish the timeline and obtain records. Pesticide application records, fertilizer records, PGR records, and irrigation water quality reports resolve most cases.
4. Confirm the absence of signs. Abiotic disorders produce symptoms only. Secondary organisms will colonize chemically injured tissue, so the presence of a sign does not exclude a chemical cause, but their absence strongly supports it.