Brown Patches In Corsican Mint - Knowledgebase Question

Mercer Island, WA
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Question by gunbykranz9
September 8, 2003
My corsican mint plants have suddenly developed brown patches in the middle which are spreading out; at first sight they appear to have dark soil in the middle of the plants, but the leaves have actually turned brown and matted. They were planted last fall in well prepared soil and were, I thought, becoming nicely established and very green and healthy looking. I have been watering them every other day during the summer due to little rainfall here, but they do not appear too wet or soft, nor do the plants appear to be dried out. They are planted in a part sun/part shade area between and to the side of flagstone pavers. Companion plants blue star creeper, bunchberry, salal and ferns all look fine, and no sign of pests other than a few slugs, which I don't believe like the taste of mint. Any ideas? I hate to lose these beautiful plants. Thank you

Answer from NGA
September 8, 2003
Corsican Mint is susceptible to verticillium wilt, mint rust and mint anthracnose. Without actually seeing the symptoms, it's difficult to diagnose. I can only provide information on the three most common diseases:

Verticillium wilt affects the mint plant gradually. Symptoms of verticillium wilt include stunted, erect plants as a result of shortened internodes. Leaves become yellow and eventually reddish, and opposite leaves bend towards each other into a crescent shape because of asymmetric growth. Yellowed leaves tend to fall off.

The symptoms include light-yellow, blister-like lesions on young shoots in the spring, and brownish-red spots surrounded by a yellow halo on the leaves later in the season. Rusted leaves fall off and defoliation can be severe. When rust infects young shoots, the shoots are usually twisted and break off at the infection point. Later in the season the leaf spots become dark brown and the overwintering spores are produced.

Anthracnose, also called leopard spot, is another common disease. Symptoms appear as small, sunken, brown spots on the lower leaves, stems, and stolons. These spots enlarge to form oval lesions with light-gray centers and reddish-brown borders. If lesions are numerous enough, they unite and cause defoliation, or cause large cankers which can split the stem. Long periods of wetness exacerbate anthracnose infections. Anthracnose overwinters on plant debris so clean the garden up in the fall.

Hope the above information helps you determine the cause of the spots on your Corsican Mint leaves!

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