Neem oil best against a wide variety of insects, including aphids, mealybugs, whiteflies, Japanese beetles, leafhoppers, fungus gnats, and other garden pests like spider mites and nematodes. Neem oil can also kill fungal diseases like powdery mildew, black spot, scab, anthracnose, and leaf spot. Dilute 1 ml oil in 100 ml of water and spray.