Heirloom Tomato Lower Leaf Yellowing: Septoria vs. Bacterial Spot in Organic System

Experienced CT organic gardener with 35 heirloom tomato varieties showing bottom-up leaf curl/yellowing after torrential rains—likely Septoria leaf spot or bacterial spot, managed culturally in no-spray system.

Differential Diagnosis

Feature	Septoria Leaf Spot	Bacterial Spot
Cause	Fungus (Septoria lycopersici)	Bacteria (Xanthomonas spp.)
Symptoms	Small dark spots with gray centers	Water-soaked → brown spots, yellow halo
Source	Wind-blown spores	Contaminated seed
Organic Control	Copper/sulfur sprays	Copper sprays, hot water seed treatment

Cultural Management (No Chemicals)

1. Remove affected leaves: Before they fall — trash, not compost (pathogens survive)
2. Improve air flow: Stake/prune, wide spacing
3. Water timing: Morning only — foliage dry by night
4. Mulch: Comfrey leaves (already doing) — reduces splash
5. Rotation: 2-3 years out of Solanaceae (tomato, pepper, eggplant)
6. Variety selection: Note susceptible varieties; avoid or isolate

Seed Treatment for Future

Hot water treatment (122°F/50°C for 25 min) reduces seed-borne bacteria. Research exact protocol per variety.

Organic Spray Option (If Needed)

Copper or sulfur sprays (OMRI-listed) effective for both. Your system may not need them with cultural rigor.

Top growth strong = plants will fruit. Lower leaf loss is manageable.