Why Do Mint Leaves Rust?

With its scent and ability to grow almost anywhere, mint can be an addition to any herb garden. Although typically a hardy plant, mints do suffer from a few diseases. One of the most common problems is mint rust, a fungal disease that can hinder your mint plant's growth and development.

Cause of Mint Rust

The fungus Puccinia menthae causes mint rust. Like other fungi, the mint rust fungus spreads by spores carried on the air. The spores then land in the soil where they can enter the mint plants through the roots. Because the fungus feeds of the mint plant, it robs the plant of nutrients.

Why Infection Occurs

The fungus is a parasite specifically on mint. Once the fungus is in the soil where mint plants grow, the fungus has a continual source of food and will infect your mint plants year after year. Once your mint plants are infected, the fungus will reproduce via spores that disperse on the wind. This means, your infected plants can infect your neighbours' mint plants. Your plants may have been infected by someone's infected plants or the fungus may have been in the soil the plant was potted in when you purchased it.

Diagnosing Mint Rust

An infected mint plant will look sickly. When shoots develop in the spring, they will be distorted and lack the bright green colour characteristic of mints. Later, rust-coloured spots appear on the leaves. These spots are the spores. As they mature, the spores turn yellow and then black. The mint's leaves eventually die.

Controlling Mint Rust

One of the most effective ways to control mint rust is to remove all your mint plants from the infected area. Destroy infected plants and put plants that aren't infected in pots and monitor them for signs of mint rust. Turn the soil and remove any remaining pieces of mint root so the plants don't regrow. Leave the area free of mint, marjoram or savoury plants for a year to break the fungal cycle. As an extra precaution, consider not putting mint back in that area.

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About the Author

Based in Portland, Ore., Tammie Painter has been writing garden, fitness, science and travel articles since 2008. Her articles have appeared in magazines such as "Herb Companion" and "Northwest Travel" and she is the author of six books. Painter earned her Bachelor of Science in biology from Portland State University.