Milk spray for powdery mildew

The claim

Diluted milk sprayed on leaves controls powdery mildew.

What the evidence actually shows

It does, and the data are better than most gardeners citing it realise.

Work published in Crop Protection tested fresh cow's milk against powdery mildew (Sphaerotheca fuliginea) on zucchini squash across five greenhouse experiments. Milk above 10% in water, applied twice weekly, produced 10% disease severity. The fenarimol or benomyl fungicide comparison gave 9%. The water control gave 56%.

That is a folk remedy performing at essentially the same level as a conventional fungicide in a controlled trial, which is why this entry exists - a registry that only debunks is not credible, and honest reporting means saying when the kitchen remedy wins.

Two practical caveats from the same work. Once-weekly application needed 20 to 50% milk for equivalent control, so frequency and concentration trade off against each other. And concentrations above 30% produced harmless saprophytic mould growth on upper leaf surfaces - cosmetically unappealing but not damaging.

Why it works

Not fully settled. Proposed mechanisms include milk proteins generating reactive oxygen species in sunlight that are hostile to the fungus, direct antifungal activity of some milk proteins, and a physical or pH effect on the leaf surface altering spore germination. The mechanism being incompletely explained does not weaken the outcome data.

What to do instead

Nothing · use it, at 10% or above, twice weekly, on early infection. It sits sensibly alongside potassium bicarbonate in a low-input mildew programme.