Based on your description of soil composition, I would first suspect the direct relationship between too much water/not enough air in soil to be the logical suspect. After that, I would suspect high root temperatures. These are just guesses based on the scant info provided and some familiarity with the odds favorites.

Al

I am growing two kinds of Cuc's in 5 gallon buckets this year with excellent results.

I used fresh Sta-Green Container soil in all my buckets.

I have plenty of holes at the bottom/sides of the buckets and there is a piece of landscaping cloth in the bottom to keep the soil from washing out.

I fertilize with 15-30-15 MG and Epsom salts every two weeks. 1 teaspoon each of MG and Epsom to a gallon.
About 12 ounces of the mixture above gets dumped into each container.

I have about 25 x 5 gallon buckets in my garden. Several kinds each of peppers, tomatoes, cuc's suger snap peas.

I water when needed using a moisture meter to make sure I do not overwater or under water. The meter makes a big difference in containers since it is so different than ground planting watering.

Just throwing out info, maybe something will help.

el34 YOU say you are planting in 5 gal buckets , could you tell us how often you have to water, and about how much each time. One person wrote that his tomato plants get 2 gal. of water each day. I am experamenting with 5 gal. buckets and find one of my tomato plants with almost ripe fruit ia taking a little over 1 half gal. a day.

Hey dang,
Each bucket seems to have it's own requirements. I use a moisture meter and it tells me how much to water. I water if the level gets down to 1 or 2 on the meter. If the meter is in the 3 or 4 range, I don't add water.

See my link below for pic's of my meter. It was $5 at Lowe's and a great little gadjet. You can see the meter face.

If the meter shows that I need water, I just fill up the bucket which usually has about 2 or 3 inches from the top of the soil to the bucket rim. I would say every two or 3 days if there's no rain, I do a complete soaking till the water runs out the holes at the bottom of the bucket.

Here is a link that might be useful: Click here for 3 pages of garden pics

I forgot to say that it hasn't seemed to matter much if I over water a bucket since I have composted container soil that breathes like a sponge and I have so many holes in the bottom of the buckets that the water just flows out if I over water.
I read that this also helps to flush salts from fertilizers and the Epsom salts I use. It's seems to be good to flush the whole bucket with fresh water now and then.

el34 I saw your post and pix. very nice and all your plants look so fresh and healty, I am just getting well into container gardening, and am now finding a few 5 gal buckets and hope to have eniugh for next spring garden.

A friend of mine owns a small bakery here. He gives me the 5 gallon buckets free. He gets honey and molasses in the buckets for the bakery.

The dang buckets are $5 apiece at a local home center. He gave me about 40 buckets this year.

It's possible you can get freebies at a bakery?