How to tell which problem it is
Check the undersides of a few affected leaves, with a hand lens if you have one. Mint rust shows as yellow spots on the upper surface with small cinnamon-brown pustules beneath them, mostly on the lower leaves, and the pustules leave a rusty powder on your finger. Spider mites leave fine pale or bronze speckling, sometimes with webbing, and the tiny mites sit along the veins underneath. Round brown dead spots with no pustules can come from four-lined plant bugs, yellow-green bugs about 1/4 inch long with four black stripes that hide under leaves. Yellowing with no spots at all points to the roots or the soil.
Why it happens
Rust is the most common disease behind spotted mint. University of Connecticut notes that mint rust infects spearmint and peppermint and completes its whole life cycle on mint, so it needs no second host plant. Cool temperatures and leaves that stay wet favor it, and Iowa State University Extension notes that rust and leaf spot show up most where plants are crowded or air circulation is poor. Badly infected leaves yellow and drop. UConn also advises against taking cuttings from infected plants, since the disease can travel with them into a new pot or bed.
When leaves yellow evenly with no spots, look at water and food. Mint likes consistently moist soil, but soil that stays soggy, especially in a pot without drainage or one standing in a saucer of water, starves the roots of air. UConn notes that root and crown rots make herb leaves yellow and wilt and turn the roots dark brown or black. Pale, yellow-green older leaves on a plant that has grown in the same pot for a year or more can mean it has used up its nutrients; Iowa State notes that container mint needs occasional fertilizer. Spider mites are worst in hot, dry weather and on plants that have been allowed to dry out.
What to do now
For rust, cut off infected stems and leaves and bag them rather than them, thin crowded stems, and water the soil instead of the leaves, in the morning. If rust keeps coming back on the same plants, UConn suggests discarding them and starting over with a less susceptible variety. For spider mites, spray the leaf undersides hard with water every 2 to 3 days and keep the soil evenly moist. For yellowing in a pot, make sure water runs freely out the bottom and empty the saucer. If the plant hasn't been fed, use a balanced fertilizer at the label rate; UConn notes that heavy nitrogen produces the soft growth spider mites prefer.
Next season
Give mint room and air. Iowa State recommends dividing established patches every three to four years to keep them vigorous and uncrowded, and regular cutting keeps fresh growth coming. Replant only healthy, spot-free pieces, and don't take cuttings from plants that had rust. UConn notes that improved spearmint types such as Kentucky Colonel are less susceptible to rust than standard spearmint, and that the peppermints Murray Mitcham and Todd's Mitcham resist it under moderate disease pressure. Set pots where leaves dry quickly after rain.
Growing it again? When to plant mint for your ZIP code
- RustOrange or rust-brown powdery pustules on leaf undersides, with yellow spots on the upper surface — the spores smear orange when rubbed.
- Spider MiteStippled, bronzed leaves with fine webbing on undersides; damage intensifies in hot, dry weather.
- Root RotPlants wilting despite wet soil, with brown or slimy roots — wet-soil pathogens destroying the root system.
- Nitrogen DeficiencyUniform yellowing starting on the oldest, lowest leaves while new growth stays green — nitrogen is being pulled from old tissue to feed new growth.