It’s either early blight or one of the leaf diseases – there are two for tomatoes, Septoria (tends to be smaller spots) and bacterial spot. If the leaves start yellowing from the bottom of the plant up, that argues for early blight. Without culturing in a lab it’s hard to say which it is but the treatment for the home gardener is the same:
1. Pull off the leaves that are spotted and yellow so the plant can be better monitored and some of the spores are removed.
2. Never get the foliage wet when watering. (It’s amazing how many people routinely spray the leaves of plants every evening because they think that they are somehow “refreshing” the plant. This is a prescription for every leaf disease in the book.) Water deeply less often.
3. Start spraying immediately with a copper fungicide – this is an organic treatment that is a bit stronger than other organic fungicides. If the garden is right next to a pond, lake or stream, however, do not use copper as it is toxic to aquatic life. (In such cases I would use Actinovate, a natural bacteria.) Spray under the leaves and the stems too. Although the fungicide won’t “cure” the problem it can protect the newer growth long enough so that tomatoes can be harvested. Next season start spraying early first using something like Actinovate or Seranade, and switching to the copper once the plant has a problem. (Do not alternate these – the copper will kill the bacteria in the other two products.)
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