I should add that all of these were initially planted in sub-irrigated containers, all 40 cm wide and 40 cm deep. I tried some of the new plantings in traditional terra cotta containers with new soil/substrate, but they failed to grow here also. I'm beginning to think that what I thought was powdery mildew is actually Phytophthora blight, and in that case, did using sub-irrigated planters contribute to it?

What kind of fertilizer did you use and how often was it applied?

It's a 3-5 nitrogen-potassium. In Spain. Options were very limited, and seemingly all the same/similar composition. Not a three number ratio like I see in posts from the U.S. I used it every two to three weeks, which was less than the instructions on the bottle indicated (1x week) because I was afraid of burning the plant (though I have to be honest I don't know much about that). Also, I had planted in a substrate ratio of 40% compost to 60 percent coconut coir (easy to get in bricks up to my terrace). I did not mulch because I thought it may not be necessary with thesub-irrigation. But, also, mulch is not so easy to get here, or get up to my terrace. Apologies for my lack of brevity, and thank you.

The new seedlings were never fertilized.

I'd agree with the above that the primary problem is your containers are far too small for the plants you list. Unless you are growing dwarf varieties of those plants. That causes all sorts of stress for the plants and makes it much more susceptible to disease and pest problems in addition to all the water and nutrient issues it creates.

Dave

Ok. That definitely helps to understand for the first set of plants. Thank you very much. I can get larger containers, at least to try some squash again, maybe not the climbers. I also do have a drip irrigation system in place (previous tenant), though I didn't use it because I was trying the sub-irrigated system. Will the drip irrigation improve my water management in this case, using containers with standard drainage?

However, the second set of plants never reached more than about six inches high, including squash, and including those in new soil with lots of compost. Even at 3 inches high they were already yellow (after an initial week of green following germination). They only made small leaves, and they failed to grow further than 3-6 inches. In that case I don't believe it was space (yet). Most outer leaves beginning to die within days.

Any SVB holes in stems? I would use controlled/time released fertilizer if required. sub-irrigation for direct sow may also have been your problem.