Please help me figure out what is wrong with my tomato plants. This is a grape tomato plant I got as a transplant. I have 2 other cherry tomato plants in a raised-bed garden with these, and they're doing fine. I first noticed this discoloration/disease? on the plant over the weekend.

It's been very hot and humid (mid 90's) for about 5 days straight. I watered the garden once during this period, as it had rained recently. This appears on all the leaves on about the lower 30-40% of the plant. The upper leaves look fine, so far. I want to know if this is just weather related or if it's some kind of disease, in which case I'll remove the plant before it can spread to anything else.

Thanks in advance!