alternaria
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downy mildew if its downy it will have a gray fuzz on the back side of the leaf spot.
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OK, I'm not seeing anything on the backside of the leaves, like grey or black dots. So, alternaria looks like a good candidate.

A fungicide then?

a fungicide can be sprayed on the plant. This can help reduce the damage from the plant, but frequently this will only lessen, not eliminate the problem.

I highly recommend spraying both the top and undersides of the leaves with a mild fungicide like Exel LG, or Actinovate. Depending on your region, you also may encounter Cucumber Beetles, which also spread leaf mold, and other diseases.

I'm growing County Fair, (left EarthTainer), Sweet Success (center) and Diva (right).

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The Sweet Success are 3-times as productive compared to the County Fair and Diva:

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Typically running in the 14 to 16 inch range:

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Cukes grow quite quickly in EarthTainers. Here is a photo taken July 9 (Diva on the left, Sweet Success on the right):

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And here they are on August 11, shooting through the rafters:

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Raybo

From what I've read, I need to prune all of the yellowing leaves and then spray the rest with a fungicide. Where I live there's a farm attached to the subdivision, and every week the entire subdivision gets our kitchen vegetable waste and grass clippings picked up. I'm not sure if a compost pile gets "hot" enough to kill off the fungus, or am I going to cause a problem for the farmer next year...