Plant Disease vs. Injury: When Visual Diagnosis Fails

Gardener asks how to distinguish fungal/bacterial attacks from sun/fertilizer burn—symptoms often overlap.

The Hard Truth

Visual diagnosis alone is often insufficient. Symptoms overlap because:

• Pathogens cause tissue death → looks like burn
• Burn creates entry points for pathogens
• Both cause chlorosis, necrosis, wilting

Definitive Diagnosis Requires

1. Laboratory culture: Plate damaged tissue on media, identify pathogen
2. Microscopy: Spores, hyphae, bacterial streaming
3. Molecular tests: PCR for specific pathogens
4. Pattern recognition: Distribution, progression, host specificity

Field Clues (Not Proof)

Feature	Pathogen	Injury
Spread pattern	Expands from focus	Uniform, matches exposure
Margins	Often distinct, halos	Diffuse, matches contact
Progression	Worsens over time	Static after event
Signs	Spores, mycelium, ooze	None
Host range	Specific	All species affected

Practical Approach

• Treat cultural issues first (water, light, nutrients)
• If symptoms spread → suspect pathogen
• Submit sample to diagnostic lab for valuable crops
• Use broad-spectrum only as last resort

When in doubt, culture it out.