skootsi, I think that it may be a (low) pH condition which in turn is allowing the uptake of one or two elements to a toxic level. I would favor iron toxicity.
"Except for watermelons, all the cucurbits
mentioned here grow best in soils with a pH of
6.2 - 6.8. Watermelons grow well in soils with a
pH of 5.5 - 6.8
I would do such things that will raise the pH of the soil.

i would have my soil tested...

and THEN act accordingly ...

BTW ... that looks like municipal compost ... i once bought that kind of stuff.. and it burnt up large swatches of my garden... i dont think it was properly 'finished' ....

ken

Depending upon where OP lives in CA, the soil generally has a high to quite high pH already. So increasing it would make things worse.

Tell us what you did for soil prep before you planted.
What did you mix in, if anything?
Fertilize?

Also please describe your watering practices.
What method? (hand, drip, sprinkler, ooze?)
How often?
How do you decide to water again?

Looks suspiciously like salt injury to me. This is often common in arid climates. Factors like soluable salt buildup/soil composition and watering practices can contribute to it.

skootsi, Depending upon where OP lives in CA, the soil generally has a high to quite high pH already. So increasing it would make things worse.
The document I linked states that water melons do better than some other cucurbits at a lower soil pH.
You stated that your water melons are growing better than the cucumber and squash.
From that I inferred that (low) pH might be the predisposing condition that led to the leaf symptoms.
The symptom could be salt injury but I could find no evidence that water melons would react any differently than other cucurbits.

Good point there.

My comment re pH was based on 30-some years of gardening experience in SoCal, plus 15 years experience teaching horticulture there.

- It's an arid climate.
- People seldom add compost to the garden before planting. (Don't know why.)
- People often use "organic" methods and add lime to soil -- a recipe for disaster in SoCal.
- People seldom use mulch, whereas the folks up here in the NW often do -- go figure.

Missing info from OP:
- Nothing about soil prep, if any
- No info about additives, if any
- No info about how OP determines when to water -- those plants need water, and./or temporary shade.
- No info about "how much water is enough but not too much." And that's no matter how the watermelon is doing.

Basically, any "remedy" suggested by forum members is purely a guess unless OP supplies solid info about the environment -- you know: light, temperature, water, fertilizer elements & soil prep.

Oh yes, and how the cukes were started -- seed or transplant.

skootsi," Basically, any "remedy" suggested by forum members is purely a guess unless ..."
I do most respectfully beg to object !
I first decide whether I can offer an answer or add anything constructive to what has already been posted. Then I compose the best answer based upon what the poster has written and drawing from my own knowledge and experience.
All of us have a bit of both assets.
Reading the answers offered by other posters; and reading their linked references lead me to believe that there is a core of people on this forum who enjoy the challenge of a plant problem and they genuinely strive to cast light on the issue.
It is quite unfortunate that some posters prefer to generate heat.

Thank you for your comments.

My main point is that little info has been offered by the OP.

Yellowing of a plants leaves, Chlorosis, means the plants leaves have lost the ability to manufacture the Chlorofil they need to photosynthesize for some reason. That can range from lack of water to any number of nutrient deficiencies caused from lack of them in the soil to the wrong soil pH so the plant cannot uptake those nutrients, or properly utilize them.
The first thing many people think of is lack of Nitrogen although they seldom think that the lack of N may be related to a wrong soil pH. Others often think of an Iron deficiency and some of those think of soil pH.
Too much of one nutrient can also affect a plants ability to utilize one or another nutrient, for example excess Potash in the soil can interfere with the utilization of Nitrogen.
Generally, California soils tend to be alkaline because of lack of rainfall, and soils with a high pH often have plants that display a myriad of nutrient deficiencies.
A good reliable soil test is needed here to begin diagnosing the problem.

Thanks for all the comments and sorry that I'm a little late in responding. I'm in San Jacinto, which is mostly sandy soil to a considerable depth. The pH of the soil is 6.5. I don't know the origin of the compost, but it was spread 2" deep over almost the entire 3500 square foot garden and tilled in. Only one area experienced the problem.

The soil is sandy, although drainage isn't what one would expect on parts of the garden that hadn't had organic material added and worked in before. Then earthworms go to work. Until then, the soil is compacted at 7" depth.

Irrigation is by buried (3") soaker hose, 6" to either side of the plants. I initially water heavily to ensure that the soil is moistened where it counts. Then the frequency and duration is adjusted until just a bit of moisture shows over the hoses following a watering.

Fertilization is by 14-14-14 Osmocote and bone meal under the hoses. I use half strength high phosphorus fertilizer early on as a foliar spray to aid root growth. Growth in other parts of the garden is excellent, even though planting was a month late (trying to garden while rebuilding the garden is something I won't do again).

I lean toward toxicity of some sort since that seems to fit the symptoms, but without a soil test I have no idea what the chemical culprit might be. Too bad soil tests aren't reasonably priced.

Will probably replant with watermelons and see what happens. It's warm into December here, so I have a lot of time.

If you worked "compost" in to the soil this spring and that "compost" was not finished then possibly your plants are suffering from a Nitrogen deficiency. The best time to prepare your garden for planitng this spring was last fall.

kimmsr...

Thanks for responding. Judging by the appearance of plants in other parts of the garden that received the same amount of compost, nitrogen is the least of my worries. Also, the pattern of yellowing doesn't match nitrogen deficiency.

I agree that last fall was the best time to prepare the garden, but this is a community garden with an extremely small active community (two people), so optimum is a goal we seldom achieve.

Perhaps this will be of some help.

Here is a link that might be useful: causes of plant chlorosis