Milk diluted with water helps prevent some types of fungus from endangering plants, as well as giving plants added nutrients. Diluted milk can be poured directly into the soil or sprayed onto plant leaves.
Houseplants, tomato and garden plants all benefit from a diluted milk mixture. Skim, low-fat or whole milk can be used for plant care, because it's the proteins in milk that provide the benefit. Always dilute the milk, and never pour milk directly onto plants, because too much milk fat isn't healthy for them. There are no hard and fast rules as to the dilution amounts. The highest concentrated milk mixtures contain around 40 percent milk and 60 percent water, and the lowest concentrations contain 10 percent milk to 90 percent water. Anywhere along this spectrum is fine.
To make a milk spray to use as a natural fungicide for plant leaves, dilute milk with water inside a spray bottle. When the plants are in bright light, spray the leaves thoroughly with the mixture. Repeat every 10 days for best results. Milk sprays are particularly effective in combating a plant fungus known as powdery mildew, which causes white spots on leaf surfaces. Cucumbers, squash, pumpkin, grape and zinnia plants escape this disease when sprayed before being infected.