As you probably know, tomatoes and I have a love/hate relationship. They can be a bit hateful, spiteful really...I mean, all I am asking is for that my plant produce the magnificent, beautiful fruit that is it's purpose in life....if things go well, this will happen with abandon. But when things go wrong...they go really, really wrong. It's not as if I am trying to grow a blue poppy here...but I digress...
When my friend Matt (@mmwine on Twitter) from South Florida, asked for some advice about his tomatoes, I went on high alert. He started by describing yellow leaves. My heart dropped. I asked for additional information: temperature, amount of water, pictures. When I saw the pictures, I realized there were a couple of things going on...
Let's start with this picture: The lower yellow leaves might just be a result of aging. It is difficult to tell for sure, but it doesn't appear to be the result of a nitrogen deficiency. If that's the case, try an organic tomato fertilizer to boost nutrients in the soil.
I do notice you have bark mulch around the plant. There are some types of bark mulch that, as it decomposes, can become toxic to tomatoes. I know you are in Florida and that you all often use mulch made from Mellaluca trees. These trees can be very toxic when burned. I would suspect it can also be toxic as it decomposes. For more info on this subject, here's a post on decomposing mulch and tomato toxicity.
Tomato leaves age and die from the bottom of the plant upwards... so this may or may not be a problem. Remove the leaves that are yellow and watch the other leaves for signs of yellowing.
In this next picture, we see yellowing on a different part of the plant...and this actually worries me more than the previous shot.
Here we see yellowing towards the top of the plant...and grey/brown circles. This could be a sign of early blight. Early blight is a very common fungus that affects tomatoes. It is spread by wind, watering methods, gardener's hands, and plant debris. One way to help prevent it from occurring in the future is to practice good crop rotation methods.
Organic treatment of early blight can occur in a number of ways. If it is only affecting one or two plants, you can pull those out and clear any plant litter on the ground. But you will still need to treat remaining plants in order to prevent spreading and arrest the growth of any fungus that is present.
One of the newer methods of treating this kind of fungal problem is the use of a strain of bacteria (Bacillus subtilis) found in a product called Serenade (which you can purchase from Gardener's Supply.) Serenade is OMRI certified (which means it is considered OK for use in organic gardens) and has very promising outcomes.
The good news is that you have some tomato plants that are healthy...this guy looks great, though your temps of 70-85 degrees may result in longer ripening times.
The one thing I would mention about this guy is that it looks like he is under a tree. This brings forth a few questions for me about potential tomato drama...1) is it getting enough light? Need 6-8 hours of full sun, 2) is it getting enough nutrients? The tree roots might be robbing any fertilizers you are using and 3) is it getting enough water? Tree roots can suck up water like there's no tomorrow. While it is important not to over water, it is also important to provide enough water.
Bottom line is if the tomato has to compete with the tree, the tree will kick it's butt every time.
Matt, I hope this helps...would love to know how it turns out. Whole books are written about this finicky fruit, and I'll bet my readers have several other diagnosis and/or suggestions...please do leave a comment and help a fellow gardener out!