Diagnosis and Plant Disorders
Core concepts
Diagnosis is systematic: identify the plant, separate signs (the actual pest or pathogen, e.g., fruiting bodies, insects, exudate) from symptoms (what the plant looks like), establish the pattern in time and space, examine the whole plant including roots and trunk, review site history and weather, then form and test hypotheses. Abiotic disorders - drought, flooding, compaction, deicing salt, herbicide drift, frost, sunscald, nutrient deficiency, lightning - are non-infectious and usually uniform across species sharing a site. Biotic problems tend to be host-specific and scattered or spreading. Koch's postulates underpin pathogen confirmation; lab samples must be fresh, representative, and properly packaged.
Traps
- Treating dieback at the top of the crown as a crown problem when the cause is below ground.
- Herbicide injury mistaken for disease: distortion, cupping, and epinasty without signs.
- Nutrient deficiency versus high pH - interveinal chlorosis from unavailable iron at pH above about 7.
- Assuming every insect is a pest; many are incidental or beneficial.
- Diagnosing from a single leaf photo with no site history.
Rule of thumb
Default to abiotic first. Apply a pattern test: the same symptom on several species in one bed points to environment; one species scattered across the site points to a pest or pathogen.