Without seeing the leaves it is hard it is hard to make an educated guess. Pictures help but many times I have to see the plant/plants in person to have an idea. Does the stems go limp or do they just turn yellow and die? How high up is this going. Does the rest of the plant look healthy. Also not knowing how tall and bushy these plants are makes it more difficult. I have already trimmed off several lower yellow leaves/stems off of some of my plants. They have been mainly on the purchased plants but they are fine. As long as they are on the lower leaves/stems and not the whole plant. Normally unless I see something that tells me otherwise I don't get overly concerned about the yellowing leaves/stems. I just trim them off and go on. Yellowing can be caused by anything from aging to poor air flow. And the yellow color from poor nutrients usually affect more of the plant and are a different yellow. I imagine Dawn will comment and maybe she will have some better info for you. Jay

Sharon, I agree with everything Jay said. He and I have a very similar philosophy on yellow leaves on tomato plants.

Because much of OK has had almost non-stop moisture for the last 4 to 8 weeks, or even longer in some regions, we still have high relative humidity values and high dewpoints, and lately we have had them in combination with very warm temperatures. These weather/climate conditions are conducive to the development and spread of disease.

OLD LEAVES? Remember that yellowing leaves sometimes signal a problem, but sometimes do not. As part of the natural growth process, as plants enlarge and get really wide and tall, the lower leaves, which are the oldest leaves on the plants, often yellow and fall off. This occurs for a couple of reasons, First of all, it can happen because the older leaves are low on the plant and are being shaded by upper foliage. It also can occur because the plants are sending the energy to the new growth areas where new foliage is appearing, blossoms are appearing, and fruit is setting. There is no point (from the viewpoint of the plant) in sending energy to lower, now shaded and older leaves when the upper parts of the plant need the energy to keep growing, setting and ripening fruit. When you have older leaves low on the plant yellowing in this way, I just remove them and life goes on.

POWDERY MILDEW? Often, when we have a prolonged rainy period in May or June, I will get scattered leaves on various plants that have the form of powdery mildew (Leveillula taurica) that manifests as yellowing leaves. In its early stage only part of the leaves are yellow in a splotched pattern for a few days, and then it spreads, the whole leaf yellows, and if left alone, just eventually fall off the plant. Sometimes you will see some white patches of fungi on it, but I normally remove the leaves well before that part of it can develop. When I see PM on scattered leaves, I just go ahead and remove them to help slow down or stop its spread. There are two forms of PM that can affect tomatoes. The kind I'm talking about can be seen in the link below. I don't usually see the other kind that is more powdery instead of yellow here.

FUSARIUM WILT: Of course, this is always a possibility and one that a gardener must consider when yellowing leaves become an issue.

A couple of things to consider: First of all, there is no tomato variety on this earth that is 100% immune to fusarium wilt or to anything else. We don't talk about disease immunity in the tomato world, we talk about resistance to disease or tolerance of disease. So, even when you plant "F" tomatoes, you are merely lowering the chance of having fusarium wilt, not totally removing the chance it will occur.

Also, there are three races of fusarium wilt. When you plant a tomato that has one F after its name, it is tolerant of one race, so FF means it is tolerant of two races, and FFF means it is tolerant of three races. So, Better Boy VFN is tolerant of one race of fusarium, Big Beef VFFNTA is tolerant of two races, and BHN 640 VFFF is tolerant of three races.

Usually when it is fusarium wilt, it often starts on only one branch or only one side of the plant. Then as it advances and spreads, you see brown vascular damage on the limbs where it showed up first.

This is why Jay (and I) want to know more detail about how it spreads up the plant.

STRESS: Yellowing leaves can mean almost anything, which is why it is hard to diagnose the cause of yellowing leaves even when you are standing there looking at the plants in the garden. Yellow leaves are, first and foremost, a symptom of stress, and it can be any kind of stress. Yellow leaves can be caused by plants being too wet, too dry, too hot, aging leaves, insect-damaged, infected with various bacterial, fungal or viral diseases, or even herbicide damage.

SALT DAMAGE: With plants in some parts of the state where the soil and water have a relatively high natural salt content, yellowing leaves can be a symptom of salt damage. With plants in containers that are fed excessive fertilizer fairly regularly, yellowing leaves also can signal salt damage.

FUNGAL & BACTERIAL DISEASE: These would be high on my list of possibilities for causing yellow leaves simply because y'all have had oodles of moisture and high dewpoints and RH values. Sometimes you see yellowing first, well before you see any blotches that might indicate (based on their size, shape, appearance, etc.) disease.

If you only see yellow,as I mentioned earlier, then I think one of the two forms of powdery mildew that affect tomato plants might be in the early stages. (see the link below)

If you start seeing further development of brown or black specks, spots or blotches, then you must consider bacterial speck, bacterial spot, Septoria leaf spot, early blight or alternaria as possibilities.

I don't usually see bacterial speck or spot here, but early blight commonly appears about this time of the year when we have had prolonged wet weather and high humidity. I don't get too excited about it, and just remove the damaged leaves. The only way to completely prevent early blight is to spray the plants regularly with chlorathalonil, which I don't do and won't do. For chlorathalonil to be most effective, it must be sprayed on the plants virtually from the first day the plants go into the ground, and must be sprayed at regular intervals as the label states. Even then, it does not confer 100% immunity to early blight, it merely slows it down and prevents it from becoming an issue as early as it would be on plants that haven't been sprayed.

FERTILITY ISSUE: If your plants are putting out great new growth, it is doubtful there is a fertility issue. There might have been a temporary issue where the roots were so clogged with water that they temporarily were unable to take up adequate nutrients from the soil. If this is going on with your plants, feeding them might help and likely won't hurt. Often, though, this is a temporary issue caused by excessive rainfall and the plants resume normal uptake of nutrients once the soil dries out some.

Finally, I cannot remember if you have had fusarium wilt there before or not. If you have, and if the tomato plants are growing in the same place/same soil, then we have to look harder at the possibility it is Fusarium Wilt. I almost never see it here, so feel like I am not good at diagnosing it.

CROP ROTATION, PLANT SPACING: Crop rotation and proper plant spacing is very important in maintaining tomato health. My own garden is a perfect example of that right now. In the rows where I spaced tomato plants 4' apart, the foliage remains healthy despite ridiculous amounts of rainfall the last 5 weeks and constantly high dewpoints and RH values. In the rows where I spaced the tomato plants 3' apart, I am seeing some powdery mildew. In the beds where I spaced paste tomatoes 2' apart, I see a little early blight. In the new garden out back where tomatoes have not been grown before and the plants are well-spaced, there are no issues at all. My observations remind me how important both crop rotation and proper spacing are. (Not that I ever will space all my tomato plants 4' apart, probably, but at least when I put them closer together, I know exactly why they develop foliar diseases....and I know it is all my fault.)

I think you likely are dealing with a disease issue, but that's just a guess based on the weather we've had.

Dawn

Here is a link that might be useful: Powdery Mildew

Thanks, Dawn, I cut off quite a few yellow leaves myself yesterday, all at the very bottom, all below the first fruit. I figured it was weather stress as the rest of the plants seem happy.

The ones that were the worst, & that I will keep an eye on, are both Bonnie Better Bush.

Betty, You're welcome. Better Bush is one of my favorites for early and heavy production. I hope yours are okay. If y'all could just dry out a little bit up there and if the rain would stay away a few days, I feel like the heat would dry up the soil and maybe the diseases wouldn't spread so easily.

I much prefer hot and dry weather to hot and wet weather when it comes to raising tomatoes. While dry weather is not especially fun and, in general, not welcome at all, the tomato plants have almost no disease issues when the weather is dry. With rain and high dewpoints and RH values, diseases can take off and grow like mad.

It has been so dry the last couple of years that we've hardly seen the F-word (fusarium) mentioned on this forum at all until now.

Dawn

After reading your responses and checking out multiple Google images, I'm pretty sure I have some sort of fungal thing going on - I have started seeing some spots :(

Next year (why am I ALWAYS saying that?!? lol)...I'm going to have to move location for my tomatoes, solanize my soil, and not plant quite as many tomatoes in the hopes of getting away from this nonsense. I've been good about mulch and spacing (the enormous rains didn't help much), but it's probably time to do other means of prevention.
As always...thanks to Jay and Dawn specifically to tell me the things I probably know (but don't want to hear) and mentions of things to think about and/or eliminate.

Sharon