White spots on mint leaves signal that the herb is under stress from its environment. Although mint is resilient, this discoloration requires immediate diagnosis to protect the plant’s health and flavor. Identifying the specific nature of the white spots will guide the appropriate response. The two most frequent culprits are tiny pests feeding on the leaves or a prevalent fungal infection.
Pinpointing the Cause: Pest Damage
If the white spots appear as tiny, scattered specks, the damage is likely caused by minuscule pests, most often spider mites. These arachnids feed by piercing plant cells and sucking out the chlorophyll, leaving behind a characteristic pattern called stippling. This damage manifests as fine, pale dots across the leaf surface that do not rub off easily because the discoloration is internal to the leaf tissue.
Spider mites thrive in warm, dry environments and are frequently found on the undersides of leaves. A more advanced infestation often includes fine, silky webbing stretched between the leaves and stems, which the mites use to travel and protect their colonies. Other pests, such as thrips or whiteflies, can cause similar stippling damage, but webbing points directly to spider mites.
Pinpointing the Cause: Fungal Disease (Powdery Mildew)
If the white material looks like a coating of flour or talcum powder, the cause is almost certainly powdery mildew, a widespread fungal disease. Unlike the internal spots from pests, this is a growth of fungal mycelium and spores on the surface of the plant tissue. The white coating can be easily rubbed off the leaf, which differentiates it from pest damage.
This fungus is favored by specific environmental conditions, particularly warm days, cool nights, and high humidity within the plant canopy. Poor air circulation, often seen in dense groupings of mint or in indoor environments, creates the moist air that allows fungal spores to germinate and spread rapidly. The disease can appear on the leaves, stems, and leaf petioles, covering the plant in a fuzzy, white layer.
Treatment and Recovery Strategies
To begin recovery, immediately prune and dispose of all heavily affected leaves and stems, regardless of whether the issue is pest or fungus. This physical removal reduces the total number of fungal spores or pests, slowing the spread. Discard the cuttings in the trash rather than composting them to prevent re-infestation.
For pest issues like spider mites, a strong spray of water can dislodge many arachnids, especially from the undersides of the leaves where they congregate and lay eggs. Follow this by applying an organic insecticidal soap or neem oil, which suffocates the mites and acts as an anti-feedant. These treatments should be repeated every seven to ten days to target newly hatched mites.
For powdery mildew, the goal is to alter the leaf surface environment to inhibit fungal growth. A simple solution of one teaspoon of baking soda mixed with one quart of water and a few drops of mild liquid soap can be sprayed directly onto affected plant surfaces. Neem oil is also effective against fungal spores, but apply it when temperatures are below 90°F to prevent leaf burn.
Prevention is critical for long-term health, and improving air circulation is paramount for both pests and fungus. Ensure the mint is not overcrowded by surrounding plants and consider using a small fan if grown indoors. Adjust watering habits by watering the soil directly and avoiding overhead watering, as wet foliage encourages fungal development.