How to plant mint...
"Mint has shallow, creeping roots which can be broken off and placed either directly in the ground or in pots containing potting compost. Because mint is such a strong-growing plant, it can easily kill nearby plants. The solution when planting outside is to sink a bottomless container, such as an old bucket, in the ground and plant the mint in this area. The roots might not be stopped 100%, but it will do the job. Another alternative is to dig a hole to a depth of 30cm (1ft) and line it with black plastic (pierced with small holes for drainage). The soil is then replaced in the black plastic and the mint planted in that area."
The one disease that kills mint...
"Mint suffers from only one disease and that is rust - allow it to get a hold and it will kill all your mint plants. There are chemical sprays for rust, but they can often be only a part solution. The best method is to examine each plant carefully for signs of rust (orange blobs generally on the underside of the leaves) and remove any leaves affected - spray as well with a chemical if you want. If this does not remove the infection chop the plant down to ground level and burn it all - drastic, but the only cure in the late stages of rust infection. "
http://www.gardenaction.co.uk/fruit_veg_diary/fruit_veg_mini_project_january_3_mint.asp