Diagnosis and Plant Health Care
A repeatable process
Diagnose in order: identify the plant, define what is abnormal, take a site and management history, then map the pattern of damage. Distribution matters - one tree, one species, one side, one soil type, or one irrigation zone all point to different causes. Only then examine signs and symptoms.
Signs versus symptoms
A sign is the organism itself (fruiting bodies, mycelium, insect stages, frass, honeydew); a symptom is the plant's response (chlorosis, necrosis, wilt, galls, cankers, dieback). Confusing the two is a classic exam trap.
Abiotic first
Most landscape problems are abiotic: drought, flooding, compaction, planted too deep, girdling roots, frost, sunscald, salt, herbicide drift, and mechanical injury. Insects and diseases often follow stress as secondary organisms, so treating the visible pest may not address the cause. Think in terms of the decline spiral: predisposing, inciting, and contributing factors.
Plant health care
PHC is proactive and integrated: monitoring, action thresholds, cultural and mechanical controls first, biological control where appropriate, and pesticides only when warranted and labeled. Understand modes of action and rotate among them to limit resistance.
Worked example
Three different species along one street decline together while nearby trees on another street are fine. That pattern is unlikely to be a host-specific pathogen; suspect a site factor such as deicing salt, changed drainage, or root disturbance. Rule of thumb: when unrelated species fail together, investigate the site before the pest.