To be able to suggest what might be wrong with your plant, it would be helpful to know more about the environment it is in. So, here are the "usual" questions:

How large is the container? Is it self-watering or do you manually water it? Is the container in full sun, part sun or shade?

For both container plants and inground plants, it would be helpful to know these things as well: How often do you water? Have you checked the moisture of the growing medium or soil 3-5" below the surface? Is it dry, just right, or soggy? Are you feeding the plant? How often? What are the NPK values of any fertilizers you are using to feed it? What has the weather been like in your area?

Can you post pictures?

The more information you can give us, the better the chances that someone can give you an accurate diagnosis.

Betsy

You did the right thing by removing the affected leaves, which were no doubt holding fungal spores of some ilk. I spent about an hour yesterday trimming the lower leaves off of most of my tomatoes, because it sets back the inevitable onset of early blight, septoria leaf spot, other fungal leafspot diseases. Healthy tomatoes produce very well in spite of up to 30 percent leaf loss.

It is scary, though, especially since different varieties respond to disease challenges differently. Some spot and turn only slightly yellow (Roma, Stupice), others turn yellow just as the spots form (Sun Gold), and some shed affected branches fast, going from green to brown with no yellowing (German Green, Black Krim). They're all different, keeps things interesting.

I am no tomato expert, but I noticed that some of my container plants were starting to get rolled, yellow leaves at the bottom of the plant so I posted a question similar to yours with pictures on a separate blog.

The bottom line is that your problem is not necessarily a fungus, but it could also be nutrient imbalance/deficiency, particularly since you are growing in containers. I think that magnesium and potassium deficiency in particular can cause yellowing in plant leaves.

Here is one site that shows what various mineral deficiencies look like. The pictures are rather small, so you can search for magnesium or potassium deficiency to get many more pictures of what it looks like as well.

http://www.hbci.com/~wenonah/min-def/tomatoes.htm

Nate

I was looking for the same info and found this on yahoo - Hope it is helpful

Yellowing leaves on tomato plants can indicate many things.
Here's the list to run through:

1) Plants are too dry.
2) Plants are too wet.
3) Foliar leaf damage from a bacteria or virus, especially if the yellowing of the leaves is accompanied by small dark spots (probably bacterial speck) or larger brown spots with concentric circles (probably early blight).
4) A more serious disease like verticillium wilt or fusarium wilt.
5) Nutritional deficiency, with the likely culprit being a lack of one of the following: nitrogen, iron, zinc, potassium, or calcium.
6) Aphids or spider mites.
7) Root-knot nematodes. Only a issue if you are growing them in sandy soil though.
8) Tobacco mosaic virus.
9) Being too close to--and especially if grown directly underneath--a black walnut tree or trees.

So, let's take them one by one.

For (1) and (2) above, check the soil moisture. Tomato plants like to be watered consistently, but don't like having wild swings between being "too wet" and "too dry".

For (5) above, if you have fed them a couple of times this season, it is hard to imagine they have a nutritional deficiency. And, if you want to feed them the Sam's Miracle Grow, I doubt it would hurt them. I think it is a complete and balanced plant food, so plants that have been fed with it shouldn't be showing a defiency. That said, I don't feed my plants at all--I feed the soil with compost, Texas greensand, blood meal, bone meal, etc. etc. and let the soil feed the plants.

For item (6) insect damage can leave the plants yellowing. Look at your leaves. Do you see any small oval bugs that might be aphids? Turn over the leaves and look at the underside of them, especially the leaves down near the bottom of the plant. Red spider mites are tiny and barely visable to the human eye. You can tap the leaves with your finger over a sheet of paper. If tiny specks fall off the leaves and onto the paper and start crawling around, you have spider mites. For aphids, release lady bugs and they will gobble them up within a few weeks. For spider mites, spray the entire plant, and esp. the leaf undersides, with a liquid seaweed solution. The spider mite damage is likely to show up as mottled discoloration of the leaf. Some tiny webbing similar to spider webs may be visable on individual leaves.

For (7) above, root knot nematodes are a horrible problem in sandy soils. The nematodes infest the roots of the plants causing a swelling, or "knot", to appear sporadically within the root system. If you have them, there is nothing you can do now. Eradicating them is almost impossible, although there are a couple of organic solutions that help control them--like crop rotation, planting nematode-resistant varieties and planting a winter crop of cereal rye (not rye grass) and then tilling it into the soil. I hope that nematodes are not the problem.

For item (9) above, black walnut trees release a chemical called juglone. It can make it impossible to grow many plants beneath the tree. If this is the problem, the only solution is to plant the tomato plants somewhere else next time.

You'll notice I left items (3), (4) and (8) for last. These are 3 very likely culprits and can be hard to diagnose without seeing the plants.

If (8) tobacco mosaic virus is the culprit, you'll see a mosaic type pattern on the leaves, and maybe also on the fruit. If the problem is item (4) above, one of the wilts, like fusarium wilt (more likely in our climate) or verticillium wilt (usually found in cooler climates than ours), then I don't know of a good "fix". However, if you feed the plants and water them consistently, sometimes a plant can grow enough to "outrun" these 2 diseases and continue producing fruit, although the plants will not look good at all.

The most likely culprit, I think, is going to be item (3) above. At least, that is what the problem is in my large tomato garden 95% of the time when leaves begin yellowing and/or curling.

If you have bacterical speck, you will notice tiny specks of brown to black to almost purple on the leave surfaces.

If it is early blight, and I think it very well might be, you will notice the yellowing begins at the base of the plant and works its way up, day after day after day. You should see brown spots, usually with concentric rings, on the yellowed areas of the leaves.

To fight early blight:

a) Keep water off the leaves as much as possible. Use a soaker hose or drip irrigation.

b) Mulch under your plants to keep water from hitting the dirt and splashing back up onto the plants thereby spreading disease.

c) Remove all the damaged foliage from the plants. Usually new leaves will sprout to replace the damaged foliage. You may not be able to remove ALL the damaged foliage at once if doing so will leave your green tomatoes exposed to too much sun which can cause sunscald and ruin the fruit.

d) Spray your plants with a solution of baking soda, oil and water. To one gallon of water, add 1 tablespoon of baking soda, 1 tablespoon of a lightweight horticultural oil and 1 or 2 drops of dishwashing liquid. If you don't have horticultural oil, you can substitute vegetable oil, but I don't think it works as well. Shake this mixture well to blend it. Apply it to the plants thoroughly with a pump-up sprayer. It is best to apply it early in the morning or in the late evening. Do not apply during the heat of the day or further leaf damage can occur, especially in our hot climate. You can also try spraying with compost tea or Garret Juice (recipe can be found at the free side of the Dirt Doctor website (www.dirtdoctor.com).

Another organic solution is Serenade, a bacteria product that combats the diseases. It is new this year (as far as being labeled for use in the vegetable garden) and I am trying it for the first time. It seems to have some effectiveness. A 20-oz. bottle will cost under $10.00 at Wal-Mart. Just follow the label directions.

With regard to the leaf curl, leaf curl is like yellowing leaves and can indicate many different types of problems. I don't generally even worry about leaf curl if it shows up by itself. But, leaf curl accompanied by yellowing leaves almost always indicates that a tomato disease is present and is bacterial/viral/fungal related.

In my garden, early blight can show up anywhere between the 3rd week of May and the 3rd week of June. It is one of the most common tomato problems. If you want a heavy-duty treatement, you can try the more organic approach of spraying with a copper-based fungicide like Kocide. If you want a non-organic approach, Daconil is the solution. I don't use either of them in my garden as I believe both are more toxic than I care to be exposed to.

Post those pictures as soon as you can and you may get more accurate responses.

It is not unusual for older tomato plants to have the oldest leaves die off, especially if they are heavily shaded by the rest of the plant. Unless it is a big problem I would not worry about it other than to remove the yellowed leaves.

Betsy

try this identifying link

Here is a link that might be useful: Disease ID

Sorry I am late in replying- Work and Life got busy for a few weeks. Here are a few pictures that I took this morning:

each planter has a tomato, two basil, a pepper and either cilantro or parsley... I know its over crowded, but it is all the space I had. It has finally stopped raining everyday so now I am watering whenever it is dry when I stick my finger in (almost every day) and I water until it runs out the bottom. In the picture I am facing ESE (the tomatoes get morning and evening sun) and they probably get a little hot with the end of the day sun.
Husky Cherry red:

this leaf is starting to get yellow

I pulled this off yesterday

and Sunsugar

Yellow leaves on the plant

pulled this off yesterday

another picture of the yellowing leaves.

I am not too concerned anymore- I have had three delicious tomatoes off sunsugar and the first is almost ripe on the husky cherry red. There are more than 40 little green tomatoes on the sunsugar and twice that many flowers, and about 2 dozen little green guys on husky and again about twice that many flowers. I am just pulling off the leaves when they get more yellow than green. Does anyone have a diagnosis with these pictures?