The Q&A Archives: Rust Or Bugs on Mint Plants

Question: I have a orange mint that has some problem but I can't tell what it is. It originally had spider mites, but I thought I got rid of them by spraying with a miracle grow plant food mix on a regular basis to keep it moist. I haven't seen any webs or movement on the plant like I used to. But now, between the veins, there is some sort of speckled browning that causes the leaf after time to become brittle and die. There are brown speckles under the leaf, but I am not sure they are not dirt that splashed up on the bottom of the leaves from watering. This has been going on for a month now and I have it on two of my plants (both orange mints). I have been picking off the bad leaves and trying to clean the bottom of the leaves by wiping them with my fingers. I have stopped spraying the leaves in worries that it was a fungus or rust that I was helping by leaving the leaves wet. The plants sit in my window sill and are open to the sun/weather but also sit on top of a radiator (due to lack of a better place). Please help me out. Thanks.

Answer: Well, let's make sure the spider mites are gone. They are fairly tenacious about hanging around. Use a magnifying glass and check the undersides of the affected leaves for a "dirty" or "rusty" appearance. Maybe that's what the brown speckles you see are. Look for very, very fine webbing. You didn't say how frequently you sprayed with the plant food, but spider mites are best controlled by washing the plant off daily with a strong stream of water. If the problem is bad, you may need to treat them with insecticidal soap.

If you determine that the brown speckles aren't mites, it could well be a rust, which is a fungus that can be transferred via water splashing on the soil, etc. Cut off any affected leaves. Mints can typically take a fair amount of pruning, and although they may not look vigorous over the winter, they should come back next spring. Also, you didn't say if the radiator is on when the plants sit on it, but warm heat wouldn't be a good location for them. I hope this info helps!

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