Hi all. So, I just got transferred to San Diego CA and am relatively new to gardening. I've got 5 potted basil plants in large terracotta planters (b/w 10-13 gallons each) that have recently started developing some problems; mainly, over the last 6-8 weeks they've started developing yellowing leaves that drop easily, some black mold looking substance on the underside of the leaves, and the leaves have been drooping.

What is bizarre is that on each of the five plants, not ALL of the plant is effected; some of the leaves, about half-ish, are that deep healthy green with firm leaves. However, sometimes even on the same branch, there will be leaves as I've described. On each plant, I'd say about 40-60% of the total leaves are effected.

I've doing some research and I've basically be able to find that the problem may be:

1) I'm watering too little.

2) I'm watering too much.

3) The plants need more fertilizer.

4) The plants are over fertilized.

5) They're getting too much sunlight.

6) They're not getting enough sunlight.

7) It could be mold.

8) It could be a parasite.

9) It could be friggin aliens.

10) The Illuminati.

I've tried accounting for all of the above (cutting back watering for a week, increasing watering, increased fertilizing, turning the plants to change the sunlight exposure, telling the aliens to leave me alone, etc...) and have seen no functional change. The plants aren't especially huge compared to their pots, approximately 18" tall, about 14-16" across, not super dense foliage, so I'm not sure if they could be rootbound.

Anyway, if anyone could offer some advice to help me save these guys I'd appreciate it. They were super healthy up until about 2 months ago, with a great harvest every 8-10 days, but since then it's just all gone to hell. Thanks for any help in advance.