Neem Oil

    Neem can also be used to manage some fungal diseases by preventing the germination of fungal spores and impeding their penetration into leaf tissues. It will not cure already infected plant tissues but helps limit the spread to healthy tissues. In this capacity, it's most commonly used against powdery mildew.

(These are the three main types of neem oil formulations: clarified hydrophobic extract of neem oil, azadirachtin, and cold pressed neem oil. It's important to note that the most widely available formulations, those containing hydrophobic extract of neem oil as the active ingredient, do not contain azadirachtin. Without the insecticidal properties of azadirachtin, these products work by suffocating insects and coating foliage with oils to combat foliar disease. But they don't disrupt insect growth and development. However, some of these are mixed with other pesticides, such as pyrethrin, to provide greater control.)

How To Apply Neem Oil

    Foliar sprays should be applied by misting the entire plant, covering both the undersides and topsides of leaves as well as stems. A second application is usually required 7 to 10 days after the first. Neem oil isn't recommended for recent transplants, certain thin-leaved herbs or leafy greens, and plants exhibiting stress, since they can be particularly sensitive.

    Another way to apply neem oil is in a soil drench, which involves pouring the diluted solution directly on the soil. This is a good method for treating fungus gnat larvae, nematodes, and other soil-borne pests. When applied as a soil drench, the compound azadirachtin is also taken up by plants through roots and works as a mild systemic pesticide against certain insects (such as aphids) that feed on the foliage by sucking plant juices.  

How To Dilute Neem Oil for Application

    Cold pressed neem oil is highly concentrated and needs to be diluted for use. You will need either a one-quart spray bottle for small applications, or a one-gallon pump sprayer for larger applications. Always follow specific mixing instructions listed on the product label. In addition to water, a small amount of horticultural soap or plant-safe liquid soap should be added to the suspension as an emulsifier to help the water and oil mix.

    Begin by mixing the water and soap. For a gallon of water use one teaspoon of soap, and for a quart spray bottle, reduce the amount of soap to ¼ teaspoon. After the water and soap are mixed, add a small amount of neem oil, one to two tablespoons per gallon of water, or one to two teaspoons per quart of water. Apply the mixture to a small test area on plants before treating the entire plant.

    The solution for soil drenches should be prepared in the same manner as for foliar sprays, however, you will likely need a larger volume depending on the intended use. Remember that neem oil breaks down rapidly. Do not mix more than you intend to use in a single application and mix a fresh batch for repeat treatments.

    While you may see some initial knock down of pests due to the suffocating effects of oil itself, the impacts of azadirachtin on insect feeding and development are not immediately evident. So, don't get discouraged if the issue doesn't go away overnight. Instead, watch for a decrease in pest numbers over time and reapply neem oil weekly until you notice an improvement.