Hi...I'm in Oakland, California, and planted a large patch of Corsican Mint ground cover last December. It was really doing beautifully -- lush, green, filling in -- however, around July I noticed some brown/black patches and it just went off a cliff (pics below). It's possible a heat wave inflicted this, but the uniform blackness made me wonder if it's a disease of some sort? I water 2x/week, but it's simply never recovered at all.

Any thoughts or advice? I'm tempted to let it continue to see if it recovers, and also considering planting new plants in the bad areas, but if there's truly a disease here, I don't want another round of plants wiped out.

Thanks in advance for your help.