Mint Rust? Or Spider Mites?

The mint transplants that I got for Dr. Kong are growing well, but I’ve noticed the leaves are getting these white and black spots. What is it?

I had put Golden Gate Gardening by Pam Pierce on hold at the library a few months ago, and I, just recently, finally got my hands on it. It’s a very informative book, and I definitely recommend it for any Bay Area gardeners. So, as soon as I noticed the spots on my peppermint, I read her section on Mint. Under Pests and Diseases, the only pest or disease she discusses is rust. She says, “Peppermint is subject to rust, which does not change the flavor and rarely kills the plant, but disfigures the leaves with rust-colored spots.” Could my problem be mint rust?

Mint rust is a fungus that causes orange, yellow and black spots on the leaves of mint. When I do a google image search, I get pictures that look more like this:

Image: gardening.which.co.uk

Hmmmm…. that doesn’t seem quite right. Darn. Golden Gate Gardening doesn’t seem to have the answers in this case, but I still stand by my recommendation.

Further googling, led me to this:

Image from: entomology.ca.uky.edu

This isn’t a mint plant, but the damage sure looks like what I’ve got. It’s spider mites. They live on the underside of the plant and suck the sap from the leaves leaving pale yellow spots. Spider mites are incredibly small – 1/50th of an inch – and yellow/orange in color. You usually need a magnifying glass to see them properly, although some sources say you should be able to see their webs with the naked eye.

I tried looking for the spider mites with a magnifying glass by shaking the leaves onto a white sheet of paper. There was one yellow-ish bug (I think) that fell onto the paper, but it flew off before I could get a good look at it with the magnifying glass.

The treatment for spider mites is insecticidal soap or neem oil. I’m a little uncomfortable spraying one of those things on my mint leaves, even though they’re “organic” or “natural.” I eat the mint leaves without washing them and the thought of eating insecticidal soap is kind of gross. An alternative treatment is predatory mites. These are beneficial insects that will eat the spider mites and then die off. I am so intrigued by this.

If you agree or disagree with my mint diagnosis, let me know. Also, if you’ve used predatory mites (or other predatory insects to control and insect pest problem) let me know.