Could be a variety of different things. Could be heat, salt damage, fungal disease damage. Not knowing if you have been fertilizing lately, had rain, heat, etc., makes it difficult to diagnose. Also, some varieties just do this. Considering where you live, you have had rain, heat, etc., so it could be any of the above.
Here is info from University of Arizona on salt burn.
"Salt Burn Symptoms include the edges of the leaves turning brown. This common problem is caused by a buildup of salts in the soil and subsequently in the plant tissue. Salts can accumulate from inefficient watering practices or if an excessive amount of fertilizer is used. Symptoms typically are the browning of leaf margins of browning of the entire leaf (Fig. 7). Avoid this problem by periodically watering slowly and deeply to leach salts past the root zone. Also, carefully follow the instructions on fertilizer containers and do not over apply. ("If a little bit is good, then a lot must be better", does not hold true for fertilizer!) By applying the fertilizer in a 'split application', i.e. applying half the recommended amount in two applications timed about 2 weeks apart will minimize the potential for fertilizer 'burn'. Water thoroughly before and after fertilizing to help prevent burn. "