Bacterial spot identification & organic treatment
Xanthomonas spp.
How to identify bacterial spot
A seed- and splash-borne bacterium that riddles tomato and pepper leaves and fruit with small, scabby dark spots. Unlike fungal spots it has no rings and no fuzzy growth, just a greasy, water-soaked look at first.
Symptoms
- Small, dark, water-soaked spots on leaves that turn brown-black and may drop out, leaving ragged holes
- Raised, scabby brown spots on fruit, sometimes with a cracked, corky surface
- Yellow halos around leaf spots in wet weather
- Heavy defoliation on susceptible varieties in warm, wet seasons
Host crops
Tomato, Pepper.
How it spreads
Carried on and in seed, spreads via splashing rain/irrigation, and moves readily on hands, tools, and stakes when foliage is wet.
Favorable conditions
Warm (75-86°F), humid or rainy weather with frequent leaf wetness; wind-driven rain moves it fast through a planting.
Progression
- Seed/transplant introduction (At planting) — Bacteria may already be present on seed or transplants with no visible symptoms.
- Leaf spotting (Warm, wet weeks) — Small water-soaked spots appear, often on younger leaves first.
- Fruit lesions (Mid-late season) — Scabby raised spots reduce marketable fruit; heavy leaf loss exposes fruit to sunscald.
Organic treatment for bacterial spot
- 1. Copper + mancozeb-alternative copper program · medium effort
Apply an OMRI-listed fixed copper spray every 5-7 days starting at first symptom; copper suppresses spread but won't cure infected tissue.
When: Onset through harvest
- 2. Avoid working wet plants · low effort
Stay out of the patch when foliage is wet; bacteria smear onto hands, tools, and clothing and start new infections.
When: All season
- 3. Remove severely spotted leaves · low effort
Prune out heavily infected foliage to reduce the bacterial load splashing onto fruit.
When: As seen
Prevention
- Buy certified disease-free or hot-water-treated seed; avoid saving seed from infected fruit.
- Rotate out of tomato/pepper ground for 2-3 years.
- Use drip irrigation, not overhead sprinklers.
- Disinfect stakes and cages between seasons.
Easy to confuse with
Bacterial speck (smaller, pinprick spots on tomato only), Early blight (target rings), Septoria leaf spot.