8.2 Biotic Signs, Symptoms, and Host Patterns

Reading Living-Agent Evidence

Biotic disorders are caused by living organisms. In arboriculture, common groups include insects, mites, fungi, bacteria, viruses, phytoplasmas, nematodes, and animals. The correct diagnosis depends on the host tree, the plant part affected, the timing, the pattern of symptoms, and any signs of the organism. A leaf spot on one host may not be managed the same way as a canker on another host.

Insect damage is often grouped by feeding style. Chewing insects remove leaf tissue and may leave holes, skeletonized leaves, or missing margins. Sucking insects remove sap and may cause stippling, chlorosis, curling, honeydew, or sooty mold. Borers feed under bark or in wood and may leave exit holes, boring dust, galleries, sap flow, or branch dieback. Gall makers create abnormal plant growth, and defoliators can remove large leaf areas.

Pathogens also vary by plant part. Foliar diseases may cause leaf spots, blotches, powdery growth, or premature leaf drop. Canker diseases affect branches or stems and can cause sunken lesions, cracked bark, callus ridges, or branch dieback. Root diseases may produce thinning crowns, poor growth, mushrooms, resin, or structural concerns near the base. Vascular wilts can cause rapid wilting, flagging, discoloration in sapwood, or sectoral crown decline.

Host pattern is a major clue. If only one species or genus is affected across a mixed planting, suspect a host-specific pest or disease. If multiple unrelated species show the same symptoms in the same area, abiotic factors may be more likely. If only one side of one tree is affected, look for localized injury, sun exposure, salt spray, trenching, canker, or a branch-specific pest.

Life cycle timing matters because treatment windows depend on vulnerable stages. Some insects are controlled most effectively when crawlers or young larvae are present. Some foliar diseases are best managed through sanitation, resistant species selection, airflow, and preventive timing rather than late curative action. The exam is unlikely to require product labels, but it can ask whether monitoring should target the life stage that is actually susceptible.

Biotic Diagnosis Checklist

• Identify the host tree accurately.
• Determine whether damage is on leaves, twigs, branches, trunk, roots, or whole crown.
• Separate symptoms from signs of the organism.
• Look for life stage evidence such as eggs, larvae, adults, spores, or fruiting structures.
• Compare affected and unaffected plants nearby.
• Consider whether the tree was stressed before the organism appeared.
• Select treatment that targets the organism and timing, not just the symptom.

Many pests are secondary. Borers, canker pathogens, and decay fungi may colonize trees already weakened by drought, root injury, planting depth, or compaction. Treating the organism without addressing site stress may give poor results. Conversely, not every insect found on a tree is causing significant damage. Some are incidental, some are beneficial, and some are present below a level that warrants intervention.

For exam purposes, choose answers that improve diagnostic certainty. Inspect the host and plant part, collect evidence, monitor the susceptible stage, and use targeted controls. Avoid answers that apply a broad treatment because a leaf is yellow or a branch is dead. Biotic diagnosis is specific work.