Mint Fungus

Verticillium Wilt

• When the fungus verticillium wilt infects mint plants, it is often called mint wilt; this is the most serious of mint diseases. The soil-borne fungus quickly spreads from plant to plant and through using contaminated tools. The fungus enters the plant through injured roots, and symptoms include stunted growth and twisted, deformed tops. Mint plants infected with mint show yellowing leaves, starting at the bottom of the plant and moving up, until the plant dies. Because there is no chemical control for verticillium wilt on mint, it is essential to select healthy, certified disease-free mint plants to grow. Infected plants must be removed and discarded, all tools sterilized to eliminate further contamination.

Mint Rust

• Mint rust is the other predominant fungus that infects mint plants, though not as devastating to a crop or a group of plants as mint wilt can be. That is because the fungus is an airborne fungus and does not permanently violate the soil as wilt does. The symptoms of mint rust begin as distorted new spring growth and orange to reddish-brown spots on the underside of leaves. It spreads to mint stems, the spots growing larger and eventually turning brown or black with infected brown leaves falling from the plant. Though chemicals are ineffective in controlling mint rust, early detection of the fungus and removal of infected leaves and plants can stop the spread of mint fungus. Care should be taken to any infected leaves that flal around the plants.

Anthracnose

• Anthracnose, commonly referred to as "leopard spot," is not as prevalent a fungus infection for mint plants as wilt or rust, but still can be a threat and damaging. Small brown spots that look sunken can appear on the lower leaves and stems of mint plants after long periods of rain and moisture remaining on the leaves. The spots can grow, turning gray with brownish edges. Anthracnose can cause leaf drop and stem split. Anthracnose fungus over-winters on fallen infected leaves and does not enter the soil. Good sanitary practices of cleaning all fallen foliage and crop or plant rotation are the proper controls of this fungus. Disease-resistant mint plants are recommended.

Powdery Mildew

• During periods of higher humidity and cooler nighttime temperatures, your mint plants may develop powdery mildew. Native varieties of mint are less susceptible to the fungus. Appearing as a powdery white to grayish coating in the mint leaves, the fungus can quickly spread, causing leaf drop. Overhead irrigation can eliminate the fungus, as the water causes the fungus to slip off the leaves of the plant, though within a few days of ceasing, the overhead irrigation powdery mildew may return. A fungicide application for powdery mildew will control the fungus, but don't spray near, or during, harvest time.