Diatomaceous Earth (DE) is a naturally occurring, fine powder utilized in organic pest management. Derived from the fossilized remains of ancient aquatic organisms, this substance is often sought out for garden treatment. People frequently wonder if this effective insecticide can also solve common plant disease problems, specifically fungal infections. This article clarifies why DE is highly effective against certain pests but generally provides no direct control over common fungal pathogens.
How Diatomaceous Earth Works
Diatomaceous earth is composed of the microscopic, silica-based cell walls of fossilized diatoms, a type of hard-shelled algae. When mined and crushed, this material forms a powder that feels soft to human touch, yet is abrasive at a cellular level. The mechanism of action is purely physical, not chemical.
The tiny, razor-sharp edges of the silica particles cut into the protective waxy outer layer (cuticle) of an insect’s exoskeleton. Once this physical barrier is breached, the highly porous nature of DE allows it to absorb the protective lipids and moisture from the insect’s body. This dual action of abrasion and absorption leads to fatal desiccation, causing the insect to die from dehydration. For DE to be effective, it must remain dry, as moisture neutralizes its abrasive and absorbent qualities.
Direct Efficacy Against Fungal Pathogens
Despite its powerful mechanical action against insects, diatomaceous earth is not considered a true fungicide and is generally ineffective against most fungal pathogens. Fungi, such as those that cause powdery mildew, rust, or blight, are fundamentally different biological organisms than insects. They do not possess the hard, waxy exoskeleton or cuticle necessary for the DE particles to abrade and penetrate.
The mechanical process that kills an insect cannot be replicated against the cellular structure of a fungus. Fungal spores and mycelium lack the external lipid layer that DE targets for desiccation. Therefore, dusting a plant with DE for an active infection will not halt the disease’s progression.
Why Confusion Exists: Indirect Effects and Associated Pests
The misconception that diatomaceous earth kills fungus often arises from its effectiveness against certain soil-dwelling pests. DE is highly capable of killing small insects like fungus gnats, which are common carriers of fungal spores, especially those causing damping off in seedlings. By eliminating the vector of the disease, DE can indirectly prevent the spread of the infection, giving the impression that it killed the fungus itself.
Applying a layer of DE to the soil surface effectively kills adult fungus gnats as they emerge or attempt to lay eggs. Additionally, DE acts as a strong desiccant, creating a drier environment on the soil surface. This drier condition can be hostile to certain surface molds and soil-borne fungal growth, which are favored by high humidity. This secondary effect against mold is not the same as treating a systemic fungal disease.
Safe Handling and True Fungus Alternatives
When using diatomaceous earth for pest control, always select food-grade DE, as industrial-grade forms contain high levels of crystalline silica that can be hazardous. The fine powder should never be inhaled, so wearing a dust mask during application protects lung health. For best results, DE should be applied as a light, dry dust to areas where crawling pests are active.
Since DE is not a direct solution for fungal issues, gardeners should turn to established alternatives for plant diseases. Cultural controls like improving air circulation around plants and pruning infected leaves are important first steps. For active infections, proven treatments include sulfur, copper-based fungicides, or biological fungicides, which are specifically formulated to target and disrupt fungal life cycles.