A couple of days ago I noticed a few tan irregularly shaped spots on my cucumber leaves. The next day the spots had gotten bigger and spread to other plants. I figured it was some sort of fungus so I sprayed a fungicide on it (containing mostly just sulfur). Yet it continues to spread. There are not wet spots on the leaves. It's just dry and crackly and continues to spread. Some of the leaves have had their tips dry up. I can't make out any spores on the leaves. What is this? Help please!