We are investigating potential causes, but more information would be useful in narrowing-down the possibilities.

What variety of zucchini are these? A specific cultivar, or plants grown from saved seeds of last year's plants? We see part of a "summer squash" tag in the photo, but cannot read all of its information.

Is this growing in a home residential garden or a community garden?

Has the soil been tested by a laboratory? If not, the results would illuminate any nutrient excesses or deficiencies, as well as measure pH and organic matter content. The texture and color of the soil on the surface suggests low organic matter content; did you happen to add compost when planting this season? Here's a lab list and more soil testing information, if needed: https://extension.umd.edu/resource/soil-testing-and-soil-testing-labs

How often are the plants being monitored for watering needs? Much of our area has been quite dry in recent weeks and regular watering would be beneficial for both vegetables and ornamental plants, even if they are not yet wilting. (Since, when plants wilt, they are already suffering from moderate water stress, which can predispose them to other problems.)

You mention soil nutrition should not be an issue - has fertilizer been applied this season, and if so, what kind was used?

Miri

Hello Josh,

Thank you for the additional information. The odd pattern of yellowing (both between leaves and within each leaf) is still a bit puzzling to us, and could be due to multiple environmental stressors coinciding/overlapping. We've had unusual cold snaps late into spring, plus drastic upswings in temperature in passing heat waves as well. Overall, the fluctuation has been pretty dramatic. While drought isn't necessarily the main culprit, it is something to monitor, as waiting until plants wilt before watering means that some root and other tissue damage may already be occurring, even if not yet visible. Feeling the soil a few inches beneath the surface (in the plant's root zone) is the best way to judge how dry conditions are and when to water.

Otherwise, this yellowing could be viral, though we are not completely convinced of this because it doesn't look like a complete symptom match. If it is a virus or virus-like pathogen, it's likely the symptoms will worsen over time and spread to more growth. Young leaves may emerge distorted and malformed. While environmental conditions may cause worsening too, it's unlikely whatever specific combination or alignment of events that started the discoloration will continue for long enough to allow the symptoms to spread for much longer. Viral pathogens also tend to cause overall plant decline and death in time.

You will probably see the plant shed those already-yellow and yellowing leaves. If you see symptoms reappear on new growth or continue to spread (or manifest in different ways), please update us with new photos.

Miri