I don't understand what is happening to my Colocasia Black Beauty. It has been growing beautifully the last few months. However, about 3 days ago I noticed yellow spots on a few of the largest, healthiest leaves. Within 24-48 hours, the leaves turn a mustard yellow with rust looking blotches and have died. This morning, I noticed my Colocasia Pink China is now starting to get the same yellow spots. So far, it is only on one leaf. At first, I thought it could be spider mites, but see no obvious webbing. Can spider mites take down large, healthy leaves in 2 days without any visible signs they are even there? I am in Zone 8b Coastal South Carolina and we had good rain last week, but this week we've had no rain and it has been very humid - temps in the 80's with the heat index being in the mid to high 90's. My Colocasia are planted in the ground and the soil is moist but not soaking wet. I am hoping someone here can give me an idea of what might be happening to them so I can fix it before they all die. Hopefully, the pictures will help identify the problem. Thanks!