Spray the milk solution on the plants, liberally, to kill fungus. Milk has a positive effect on the immune system of plants, giving them the ability to fight the growth of fungus. Apply a baking soda solution to kill plant fungus. Pour 1 quart of water into a spray bottle with 1 teaspoon of baking soda. Add half a teaspoon of canola oil and a few drops of dish soap. Give the spray bottle a shake and spray the solution on the plants to rid them of fungus.