Ideas?

Lots of possible reasons for yellow leaves in squash. One is nitrogen deficiency. I see your mulch soil seems to have a lot of uncomposted stuff. Maybe wood chips? That would do it. Your supplemental nitrogen may just have been eaten up by the composting that is going on.

First photo could easily be N deficiency given the color of the new leaf. Could also be over-watering.

But the second photo is something else. Possible downy mildew.

Dave

Dave, it's kinda early for DM up in Ohio, isn't it? It does look like it, though. But, looking at the mulch it seems a bit waterlogged, too....

Unlikley it is over watering. Mulch is black. I also do not think it is downey mildew - not getting worse and up close does not look like it. Seems odd it is occurring more often in my mulch beds then in regular garden.

Is that egg shells beneath the plant in the second picture?

The reason I mentioned waterlogged is the mildew/algae(?) at the base of the wooden structure and the darkness...dampness is hard to tell from a photo, though. You might want to look at this: https://www.gardeningknowhow.com/plant-problems/environmental/leaf-chlorosis-and-iron.htm. It pertains to iron deficiency. Iron deficiency doesn't mean there is a shortage of iron, just that the plant isn't able to access what's there.

Two different growing mediums so I would imagine that they would have different problems and requirements. Have you checked the pH? Did you add "new" mulch to the mulch beds this year? In other words, mulch from a new source? I'm beginning to get paranoid about using OM that I don't know the history of...so many pesticides out there being used.<sigh>

Yes eggshells in one pic. I get a lot of moss growing even in places you would not expect it. I did not add new mulch this year. I did not soil test and all new leaves look ok so confused:0

"Dave, it's kinda early for DM up in Ohio, isn't it?"

Not if temp and humidity conditions are right for it to develop.

"I also do not think it is downey mildew - not getting worse and up close does not look like it."

So what does downey mildew look like up close?

Dave

Don V, if the new leaves are looking ok I'd let it roll and see what happens. If they start looking like the first ones then I'd have to dig a bit deeper.

Dave, I thought DM traveled on the wind from southern latitudes, basically hopscotching northward as summer progresses...???

Dave, I thought DM traveled on the wind from southern latitudes, basically hopscotching northward as summer progresses...???

True to a degree but as weather currents have changed - predominately the southern shifting of the primary jet stream in the US - over the past several years it moves north in a much more direct manner and earlier too.

I think it was 2012 or 2013 when there was a huge influx of the spores found in fields in Michigan as early as mid May. And subsequent years have also seen early development of symptoms.

Give it cool and wet air currents to travel and from what I have read it seems to be increasingly developing earlier and earlier up through the Mississippi Valley and eastward.

Dave

Conditions *don't* seem to be getting any better for life as a whole, do they? Garden-wise I battled a horrific disease in my tomatoes last year...14 plants over 7' tall and healthy-looking as you'd want produced tons of blooms and tomatoes...and every single tomato rotted. Those were all open-pollinated/heirlooms. A very frustrating adventure. Being a rookie I did not spray or other wise try disease control other than trimming the lower leaves. Last year was a hot and wet year.

This year I planted around 90% hybrids and they're looking good and large tomatoes are developing nicely...so far. I also planted a couple of Bumble Bee Mix "artisan" op plants. I've already pulled one BBM due to "something" starting to kill it...irregular brown spots on leaves, shriveling leaves, tops dieing out, etc.,...the green fruit looked good, though. After pulling that one a couple of days later I've found the other BBM exhibiting the same symptoms (no surprise as it was planted beside the other one). I'll be pulling it this afternoon. I have one other OP plant which is a Brandywine Red that is planted at the other end of the row. So far it appears to be (mostly) dodging blight, fungus, disease, whatever. All the plants are showing a touch of yellow in the lower leaves. The plants are trimmed up to around 12" from ground and I will probably trim them a bit more. I sprayed (first time) with Serenade last Saturday...only to have a thunderstorm move in a few minutes later and eventually dump about 1/2" of rain on us.<sigh>

The weather has been very hospitable to diseases...hot and very humid. The weather forecast is for sunshine this week and dropping humidity, then rain this weekend. The sunshine is *most* welcome!!! My goal for after work today is to pulled the remaining Bumble Bee and to respray the Serenade...I feel the first application pretty well got washed off.

But, yes, I can see how pestilence and disease is becoming more widespread as time goes on. A sign of the times...