Question: My sister sent me some “Neem oil” to use for powdery mildew, but I haven’t a clue how to use it. Could you tell me?
Answer: “Neem oil” is processed from seeds of the neem tree, native to eastern India and Burma, and is a wonderful product that can be effective as an insecticide, miticide, and fungicide and is safe for home use. It biodegrades quickly, and exposure to neem oil poses no threat to humans or other higher animals, or to “beneficial” predator insects (only to those who feed on plants). Though it doesn’t directly kill plant-feeding insects, it does act as an irritant, discouraging them from feeding on the plant. For this reason, it’s best used regularly as a preventative, or to attack pest problems in their early stages.
We’ve found its best use is as a treatment for powdery mildew, the very fine, white powder that appears on leaves and, sometimes, blossom stems. Though powdery mildew rarely will kill a plant, it will scar the surface of leaves and lead to short-lived blossoms. It tends to be a particular problem when there is cool, stagnant, humid air. Spraying with neem is an easy, immediate, non-toxic solution for dealing with powdery mildew on a large number of plants. There is minimal damage to open blossoms (a few varieties appear more sensitive), and leaves will look shiny and clean.
We spray with 100% pure neem oil, at a rate of 1 tsp. per quart of water. To this, add 1/4 tsp. of liquid dish soap, which is need to mix the oil well in the water. If the oil is very thick, use warm water when mixing. Spray all plant surfaces generously with a fine mist. Best of all, it’s not toxic to humans–we don’t use either masks or gloves when using it. That said, don’t be so careless as to swallow it or spray it into your eyes (not lethal, but not pleasant either). As for the odor, it smells a lot like sesame oil. To store neem oil, keep at room temperatures (it thickens if chilled), and shake well before using.