Neem oil is the go-to natural insecticide in most parts of the world due to its minimal effect on the environment and low to nonexistent toxicity for humans and pets.
Extracted from various parts of Azadirachta indica, an ornamental tree related to mahogany, the oil and solids contain a natural insecticidal chemical called Azadirachtin.
Whether used as a neem soil drench or foliar spray, neem oil interrupts the life cycle of harmful pests and can sometimes kill them outright, yet won’t harm pollinators or beneficial insects that don’t come into direct contact with the oil.
But merely knowing you can kill common plant pests such as aphids with neem may not be enough for some houseplant enthusiasts who discovered the honeydew has led to a fungal infection.
Fear not, because the same treatment you use to kill the aphids may also cure your plant.
Does Neem Oil Work as a Fungicide?
Neem oil on plants not only acts as an effective fungicide, but it also gets rid of pests that create honeydew, which can encourage fungal growth.
It even kills many bacterial infection forms, making it one of the best all-around remedies for your plant’s health.
What Fungi or Diseases Does Neem Oil Kill?
Neem oil can help protect against a large number of fungal infections and some diseases and virii.
Some of the conditions neem oil helps treat include:
- Black Spot
- Fire blight
- Gray mold
- Leaf spot (Bacterial and Fungal)
- Powdery Mildew – Details in our article Neem oil and Powdery Mildew
- Root Rot – Read these Tips on Neem Oil and Root Rot
- Sooty Mold
- Stem Rot
- Tip Blight
- Verticillium Wilt
It has also been successful when proactively treating seeds against phytopathogenic fungal diseases.
How Effective is Neem Oil against Fungus?
As with any product, neem oil works best when used as a preventative, and its effectiveness will decrease the longer you wait to treat an existing infection.
For example, a neem foliar spray will eradicate a new sooty mold infection in as little as a few days but may not be able to save a plant from an advanced case of rot.
Neem Oil Fungicide Recipe – Making an Antifungal Foliar Spray and Soak
Take a gallon of water and slowly blend in 1 tablespoon of pure castile soap to serve as an emulsifier (i.e., it allows the water and oil to mix).
Now add 4 teaspoons of 1% percent clarified hydrophobic neem oil to make a foliar spray or 2 tablespoons of 100% percent cold-pressed crude neem oil for a root soak.
Both of these mixes will have their target areas and benefits, so you may wish to use only one or a combination, depending on the situation.
Foliar sprays are purely topical and evaporate in an hour or less, leaving behind no residue.
This option not only targets any visible fungus but will also kill aphids, mealybugs, plant scale, and other insect pests that produce honeydew. This substance attracts sooty mold and other fungal infections.
Meanwhile, root rot and other subterranean fungal issues will require a soil soak.
Soil soaks are also absorbed by the plant, turning it into a systemic insecticide that lasts up to 22 days and can also cause an immune system reaction in the plant that helps eliminate fungal infections that have entered the plant itself.
Tips for Using Neem Oil as a Fungicide
Treating fungus with neem isn’t much different than fighting plant pests, and some of the same cautions apply.
Only use the foliar spray at dusk or dawn to be gone before pollinators, and beneficial insects become active.
Treat the planet as soon as possible, and be aware that even the best efforts may not be enough for a plant that’s already nearly dead.
Always test a neem product on a small portion of the plant 24 hours before treatment begins, as plants can have sensitivities or allergies, just like humans.
Finally, saplings and new growth are often very tender and highly susceptible to chemical burns, so avoid using neem oil on those plants/areas if possible.
Neem is also used in Powdery Mildew Control.