Neem oil for plant fungus: mixing ratios that work

What neem does and does not do

Neem oil coats the leaf, which stops fungal spores germinating and smothers soft-bodied insects such as aphids, whitefly, mite and thrips. It is preventive and early-curative: it slows powdery mildew, rust and leaf spot that have just started.

It will not cure established late blight, bacterial wilt, or any viral disease. If lesions are spreading daily, neem is the wrong tool and delaying costs you the block.

The mix

Neem does not dissolve in water; without an emulsifier it separates and lands as burning droplets. Always mix the oil with the soap first, then add water.

• Preventive spray: 5 ml cold-pressed neem oil + 2 ml mild liquid soap per 1 litre of water (0.5%).
• Active infection: 10 ml neem + 3 ml soap per 1 litre (1%) — never stronger.
• Use lukewarm water; cold water leaves the oil unmixed.
• Spray within 8 hours of mixing — azadirachtin degrades fast.
• Test on a few leaves and wait 24 hours before spraying a whole block.

Timing decides whether it works or burns

Spray at dusk or in the first hour after sunrise. Oil plus direct midday sun scorches leaves, and UV destroys azadirachtin within hours. Cover the underside of leaves — that is where mildew, mite and whitefly live and where most people never spray.

Repeat every 7 days as prevention, every 5 days during an active infection, for three applications. If nothing improves after three rounds, re-diagnose rather than increasing the dose.

Common mistakes

Doubling the concentration to compensate for poor coverage is the fastest way to defoliate a crop. Other frequent errors: using refined rather than cold-pressed neem, spraying flowering plants at midday when pollinators are active, and mixing neem with sulphur or copper sprays within two weeks — the combination burns leaves.

Frequently asked