Mint leaves have black spots. Is it mint rust?

11 years ago

I have taken a lot of gardening tips by reading this forum and I'm very grateful to you all. I signed up because I couldn't find a solution for my problem. Last year I received tiny spearmint and applemint plants from my friend and I put them in a container with fresh potting soil. The spearmint didn't do very well in terms of yield (may be the applemint was taking over?) but they both together never filled up the entire pot. They were very hardy and I left them outside all winter. When they came back this spring,
I pulled out the applemint completely and planted a new spearmint plant
I bought at the local nursery hoping to harvest more spearmint. But today I noticed tiny black spots on the leaves of my old plant. There are sunscald like marks on some leaves. The plant hasn't been overgrowing like most mints do, it's pretty stunted. It's in the same soil as last year
and the new plant from the nursery is doing fine. The bushier one at
the back is the new one and the rest tinier ones scattered are the old
ones that survived from last year. It probably isn't receiving as much
direct sun as it should but it is placed outside in my bright patio with
a lot of light. I try my best to move it around so it gets about 4-6
hrs of direct sunlight a day. Any advice on what's going on with the
spots and low yield? Can I rescue the old plant or should I discard it and replace the potting soil to let the new one grow without any disease? TIA!

Featured Answer

James Lim

11 years ago

I have nothing helpful to tell you about your mint plant, I am new to all this green stuff myself. But I thought I should mention that you have very nice nails. :) have a nice day.

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Comments (6)

lodewijksport
11 years ago
last modified: 11 years ago
let it grow a bit then cut them all down to groundlevel , in most cases the new shoots won't be infected. Also do not pot it shallow , fill it with more earth so there is more ventilation going on. spearmint is a real hassle when it comes to leaf rust , i stopped growing it in my climate because we have very high humidity throughout the year ( below sea level ). Some mint like banana mint and corsican mint are more resistant to leaf rust. strawberry mint and pineapple mint usually get infected as well. chocolate mint is more resistant but eventually without care it gets infected as well.

if you want mint that is more resistent to fungi or pests then get banana mint or some of the wild species like corsican mint , mentha aquatica or some asian/ japanese mint species. I have grown most of the cultivars and hybrids and they all got infected so i stopped growing them. it is also better to apply a layer of mulch like small gravel or something similar because most fungi are soil borne especially in pots.

Right now cut back the shoots close to ground level and give it more ventilation, apply a layer of mulch/gravel once new shoots have appeared.
patio_garden thanked lodewijksport
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fatamorgana2121
11 years ago
I'd say bug rather than a disease is the culprit here.

FataMorgana
patio_garden thanked fatamorgana2121
patio_garden
Original Author
11 years ago
Thanks lodewijksport for the suggestions! I will cut back the infested parts and fill the pot with more potting soil and add mulch once the little guys grow up a bit. I hope this will work. I will keep those more resistant varieties in mind too.

@fatamorgana2121 Thanks, I didnt think about it. It could possibly be bugs. It doesn't seem to be mint rust because there are no rusty orange spots or lesions. I found a couple of white flies and flying aphids just starting to build their colony in each plant. I have a tomato, pepper, basil and some flowers. I'm in the process of getting rid of them by inspecting everyday to remove new ones. I hope it's not a disease.
SW (Sydney, USDA 10b)
11 years ago
It looks like the faeces of insects, probably the aphids.
The over-wintering spores of mint rust are indeed black, but they're a bit bigger than that, they only appear on the undersides of the leaves, and they're latched on tight.
Daixy
11 years ago
Those look like bug poo to me. If you have access to neem oil, mix some up with dish soap or castille soap and that should get rid of the bugs; and in effect, the poop. Just be sure to spray early in the morning or late afternoon so the leaves don't cook.

All the best :)