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Cow’s milk is used to suppress viral and Fungal diseases in cucumber, tomato, pepper and other crops. It does this by increasing the pH of leaf surface (6.2-6.8), thus establishing a protective barrier and the crop develops systemic resistance to diseases. Milk protein (casein) inactivates the virus protein (capsid protein) in plants.
 
This technique is being used by many organic vegetable growers.
 
How to Apply: Mix 1 part cow milk with 8 part water or 15g dry milk per liter water. Spray on your plants at 7 days interval. You may also spray once in 15 days before disease appearance and at 7 days interval for reducing the spread of virus. Spray both sides of the leaves (top and bottom) and spray when the sun is not shining (morning or evening) - NO! A source said it works better in the presence of sunlight! . Note that application at higher concentration MAY not be economical and you should practice this only if cow milk is readily available and cheap. You can also use whey if you have access to it and it is cheaper.
 
This spray has been reported to be effective in controlling powdery mildew - a fungal disease prevalent during dry seasons.
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Source:
https://www.sciencenews.org/blog/food-th...ildew-woes

For those with excess milk. Or is it cost effective when compared with chemicals?

Of course it's for those with excess milk. Or those who practice pure organic farming.

Can I use goat milk? And what about commercial/processed milk?

Yes goat milk can be used. And since the main active ingredient is the milk protein, commercial or processed milk can be used.