Yellow Leaves with Black Spots on Container Mint

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PatrickInLaw Sacramento, CA Mar 31, 2009

Hi All,

I'm a brand new gardener (my first season doing this), doing a bit of patio container herb gardening in Sacramento, California (Sunset zones 8 & 14, USDA Zone 9b).

I have a container on our south-facing patio that has peppermint, chives, and greek oregano all potted in Miracle-Gro Organic Choice potting mix, with a good layer of glass beads on the bottom for drainage. All three plants went into the pot at the same time, about four or five weeks ago, spending their days out in the sun and their evenings indoors (until recently, when the end of frost concerns allowed me to leave the plants outside full time).

Everything is going well, with one exception. The peppermint has been getting yellow leaves with black spots on them. The leaves start by going pale, then a day or two later are full yellow with black spots, they hang around for another few days and then fall off dead. Occasionally, I find a little bit of powdery mildew on the leaves and stalks, although that has gone down a bit in the last week or so.

Only leaves that are low on the stalk turn yellow, although the yellowing is slowly spreading up the plant as the bottom-most leaves turn yellow and die off. I initially started plucking any yellow leaf, thinking that this would slow the spread of whatever vile disease is eating my peppermint alive, but it only seemed to accelarate the problem. When I don't pluck the leaves but leave them to die on their own, it seems that the yellowing doesn't spread quite as fast.

Can anyone please help me figure out what is going on here? I took some pictures of the latest damage today and posted them at http://www.patrickandvallerie.com/gardening/yellowandblack.html if you'd like to see exactly what this looks like.

For what it's worth, I've noticed a little bit of yellowing on the edges of the oregano leaves, but it seems to be just a tiny bit on the tips and isn't confined to the bottom most leaves (the oregano leaves also don't wilt and fall, they're growing rather happily). Also, I have a bit of spearmint in another pot on the patio, and that is showing an occasional bit of yellowing, but again without the black spots and falling.

Many thanks in advance,

-Patrick

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lbrabec (Lynn) Omaha, NE(Zone 5a) Mar 31, 2009

It could be a number of things.Overwatering,cooler night temperatures,lack of nourishment.Peppermint can also get spotted wilt,which involves yellowing leaves,black spots and signs that can be mistaken for powdery mildew. I will try and find a picture of this condition.

Post #6345081
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lbrabec (Lynn) Omaha, NE(Zone 5a) Mar 31, 2009

Here's a site with lots of info.
http://mint.ippc.orst.edu/mintdiseases.htm

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