I am hoping someone can help me identify what is killing my tomato plants.

The attached picture shows a typical leaf about midway through the progression from healthy, green leaf to limp, brown, dead leaf.

What is happening: The bottom leaves were affected first. It has slowly (over the course of several weeks), but steadily moved up the plant. The leaves first start to turn pale yellow. Then the spots appear, then the leaf turns brown and limp. The plants had been doing great up until this began. They are about four feet tall, have all set at least 1-2 dozen fruit and had been growing at a steady clip. I have seen no insects, and no other indication of stress until this yellowing/spotting pattern appeared.

Set-up: I have four plants in 4 self-watering, 5 gallon buckets on a breezy, covered porch. They get about 8-10 hours of direct sun daily. Two are Fox Cherry and two are Genovese Costoluto. The leaves are never wet, but always well watered from below. I grew them all from seed and have been successful with these varieties in the past. They are potted in good quality potting mix that was new for these containers. The only other plants nearby are two cucumbers in containers that are thriving and pumping out fruits daily.

I am in the US Virgin Islands (if that makes any difference).

Thanks in advance for any help!