Answer: 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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