My cucumber's first big leaves are getting the same old familiar lesions - and it isn't from too much water:
I've kept this pot on the dry side, watering only twice a week, and not til soggy. This self-watering pot was planted with fresh Miracle Gro potting mix, with a layer of Miracle Gro Organic Choice mix midway up the pot. Since I'm not keeping it wet, I've watered from top and bottom when watering. With tap water. There should be no nutritional deficiencies.
It is crowded, by the three bean plants. And strangely, roots are coming up out of the top of the soil, from both types of plant (I think). There are some fungus gnats, but not in the soil that I can see. No aphids.
For the most part, the beans are happy. First bean flower bud turning lavender already, at age 3.5 weeks. Been watering with tap water. One bean plant looks a bit chlorotic - one whole branch (closest to the lights) kinda yellow with green veins, sort of the opposite of the cuke leaves.
And then there's light. At first, I kept the lights above the bean plants, until the height discrepancy between beans and cucumber reached 5-6" (~14cm). Since then - maybe a week - the cuke has had a 26W CFL less than 2" from its top. A similar 6500K bulb above the beans, but very little of that light made it through the chassis of the 3500K spotlight on the cukes.
I just noticed the brown lesions along the veins today. I checked Sunday's pictures - the lesions weren't there, though perhaps in retrospect the edges and veins were getting a bit light already.
For now, I switched a 6500K bulb in over the cuke - so beans and cukes both have 6500K.
I'm thinking the brown-lesion problem is just lack of light. Maybe...