Calamintha (calamint or catnip) looks like it is dying (growing, insect, plants)

[SDBorn]

I planted 10 calamintha grandiflora plants and 9 of them are growing beautifully, but all of a sudden one looks like it is dead, almost overnight! What could have happened? They are all planted in the same mulched bed, they all get the same amount of water etc. Could some underground insect kill the roots or something? I'm at a loss.

[J&Em]

They are usually pretty trouble free but on occasion higher humidity, heat and just normal changes at this time of the year will cause some to decline. If you haven't uprooted it try cutting it back (sterilize the pruner immediately afterwards with bleach just in case it is an infected plant) and throw out all parts and pieces that have been cut off or fallen off (you should do this with any plant that suddenly dies off as a precaution to preven the spread of whatever caused the problem). It may surprise you with new growth in a week or two. If it doesn't have any signs of green pull it out and inspect the roots. If they are nonexistent it is possible something like a vole chewed on it. If they appear very dark or black it can be the sign of infection, often from being too wet in the case of calamint, which likes of moisture but does not do well in very wet conditions over longer periods.

[SDBorn]

Thanks much, appreciate it!