It's nitrogen deficiency, probably exacerbated by nitrogen "tie-up" from undecomposed wood in the mulch. Fish emulsion fertilizer will solve it if you want to stay organic. All purpose miracle-gro will produce quick results if you're okay with synthetics. I'd supplement that with foliar feeding from either source as often as you can until the problem is abated. Use a more dilute solution for foliar feeding. Squash are very heavy feeders, especially when young. The only explanation I can give for peppers and tomatoes is that peppers grow more slowly to start out with. Tomatoes are also heavy feeders, so I'm not sure why you're not seeing problems there. If they are transplants you bought, it's likely that they were already heavily loaded with fertilizer when you bought them, and that's carrying them over for now. I'm not familiar with soils in California, but it may be prudent to add a source of magnesium and/or calcium to your raised bed soil. Dolomitic lime provides both, but can raise the pH. Lime was probably mixed into the soil you bought, but not the compost. Gypsum is pH neutral and provides calcium. Epsom salts provide magnesium, but are very soluble and therefore must be included in irrigation water. All-purpose miracle grow doesn't contain calcium or magnesium (few fertilizers do because of formulation issues).

Before adding anything, I'd wait to see if letting it dry a bit helps. Too much water isn't usually a problem in the intermountain west, but it can be if you water too much. Too much water can cause yellowing in leaves pretty quickly. On the bright side, it doesn't often take very long for things to dry out around here.

Cora, it sure does look like root rot (Phytophthora). I would treat right now with AgriFos (phosphorus acid), 1 Tbsp. per 5 gals of water. Then, treat again in 2 weeks, and a 3rd treatment in another four weeks (so now, 2 weeks, and then 6 weeks). It is not uncommon for one tree to be just fine, and the one right next to it to be affected, based on where the organism is growing in your soil, and what rootstock your tree is on (some are much more susceptible than other rootstocks). You should see recovery in the next few months. Older leaves may green up some, or simply fall off to be replaced by new, healthy flush. Continue to treat every 6 months. after you've done your first 3 treatments. You can buy Monterey's AgriFos on amazon.com, or may be able to find it at your better garden centers. In February, go ahead and fertilize as usual. And, you're right in keeping the soil drying during our winters. That is the time the organism will flourish, based on too wet soils being the right temperature range for optimal growth. Patty S.

Its definitely not overwatering (i think), im very sensitive to the watering schedule and always allow them to just start to wilt, and i can see the medium turning a dry color at the top. I have a very low nutrient medium that ive custom made from scratch(mostly aeration). its kind of a cross between a 5-1-1 and a basic potting soil. Lotsa perlite, a little dolomitic lime, lots of orchid bark. The problem may be from a specific set of plants I repotted in a frenzy and I basically just used straight potting soil. I also planted some seeds in that particular batch of soil and it completely frapped out, perfect germination but stunted growth from the outset and turned all yellow. I believe it was "greenall" brand potting soil. total crap! I replanted all the seedlings into new MG seed start mix to hopefully salvage a few of them but im not holding my breath. About 20 varietals all barely putting out true leaves after 2 months. well see if those get any better. I have a HUGE amount of leftover GH flora series nutes that I have been using. they have been sitting for a few years. 15 gallons total, 5 gals each nute. Its possible that I overfertilized last week since I have so much its hard to get a "teaspoon" amount out of the 5 gallon containers. but Im pretty sure I got it right. @ottawapepper why do you use your nutes at 1/4 strength? ive read this elsewhere but im unsure why I would go that route if my plants are 2-3 months old? I figured this would require a fairly rapid uptake. Im currently growing around 300 pepper plants and about 40 varieties. all hot and superhots. Im running out of room in my greenhouse, Everyone asks my why I dont grow other plants, all i can say is I really like hot peppers! @habjolokia Hi Mark, thanks for the input. I have some supplemental light (led's that are very weak but designed for plant growth) that are more of an experiment than anything else. that is why some of my plants look purple. its the blue/red LED's. Eventually im planning on adding an array of LEDs to the entire greenhouse and then really trying to push my growing season out. So im curious what type of ferts you use Mark? any name brand? why not use the epsom salts? have you had any bad experiences? Im one of those guys who tends to overdo things so the less is more philosophy is usually very applicable for me.