Homemade Tomato Fungicide: Baking Soda vs. Powder, Application Methods

Clarifying the Baking Soda Fungicide Recipe

The classic Cornell formula uses **baking soda (sodium bicarbonate)**, not baking powder. Baking powder contains cream of tartar and starch—unnecessary and potentially problematic.

Correct Recipe (Cornell University)

• 1 tablespoon baking soda (sodium bicarbonate)
• 1 tablespoon horticultural oil OR 1 teaspoon mild liquid soap (castile)
• 1 gallon water

Do not use vegetable oil—it can clog stomata and turn rancid. Horticultural oil (neem, jojoba) or soap acts as surfactant/spreader-sticker.

Application Protocol

1. Mix thoroughly—baking soda must fully dissolve
2. Spray early morning or cloudy day (prevents leaf burn)
3. Cover all surfaces, especially leaf undersides
4. Apply weekly preventively; every 3-4 days if disease present
5. Reapply after rain

Target Diseases

Effective against powdery mildew, early blight suppression, Septoria. Less effective on late blight, bacterial spots.

Root vs. Foliar Application

This is a **foliar spray only**. Soil drenching with baking soda raises pH unpredictably and harms beneficial microbes. For root diseases, use Trichoderma drench or copper soil application per label.

Limitations

• Preventive, not curative
• High pH water reduces efficacy
• Can cause sodium buildup in containers—monitor
• Not a substitute for resistant varieties, rotation, spacing

Use as part of integrated program. Commercial copper fungicides (Soap Shield) offer more reliable, longer-lasting protection.