I sent pictures of my onion plants to Dixondale and they suggested it was a fungal disease, specifically Downy Mildew, and sent me this link of onion diseases:
https://www.seminis.com/SiteCollecti...ease-Guide.PDF
My symptoms do not appear, at least to me, to match Downy Mildew, Basal Rot, Powdery Mildew, or any other diseases that I have looked at online. There is no mold, spores, fungus or other deposits on the leaves or bulblets, and the roots look white and healthy. It is just the leaves that are affected.
I do not use any herbicides on my lawn so the grass clipping mulch should be fine. Likewise neighbors as I am surrounded on three sides by cow pastures and the forth is woods.
At first I only saw the white patches at the base on the leaves, but now I noticed some patches higher up the leaves (see new photos).
Interestingly, if appears some of the affected plans continue to have healthy leaves, and some might even be putting up new leaf growth - hard to say
for sure.
About a hundred or so plants are randomly affected out of the 450 I planted or about 25%. 90% of the affected plants are Copra with only maybe a dozen Candy.
Dixondale was very fast to respond and offered to send me new plants for free, even though this was not their fault. Great customer service! My new onion plants are arriving tomorrow and I'll plant them in a remote location. Mean time I haven't given up totally on the existing bed. We'll see.
Tom