No chemicals of any kind. Maybe I'll lay off the water a bit as the most affected plant is the one that gets the most water.

Thanks!

Another consideration is the type of water. Water with a lot of dissolved mineral/salts will do something like that to tomatoes and squash. I use well water for a lot of things, but the tomatoes get only collected rainwater. And never water the leaves of a tomato plant!

Hi Mark - can't tell you for sure, but it looks like a very mild case of the tomato blight that my plants were suffering. Mine progressed (rapidly) to fully yellow leaves on the lowest tier, with black spots, yellowing progressing slightly up the branches and then to the next-higher tier of leaves.

There were two products recommended to me, and I went with sulfur plant fungicide - mixed the powder with water and sprayed on the affected leaves...took a couple applications, but it's slowed and apparently controlled what had been a pretty prolific outbreak of the stuff - I had to prune back about two tiers of branches to the stem of the plants, and there's still some yellowing on some of the lowest leaves, but it seems to have stopped spreading, and I've gotten about a half dozen or so tomatoes to ripen, and the rest seem to be progressing fine.

Also, as Susan had suggested earlier in this thread, hay mulch can help - this fungus is apparently airborne, lands all over, then is spread by rain splashing it up to the lowest leaves - then it spreads higher and higher up the plant.

Hope this helps

Dave