I should also note that this plant went into the raised bed two days ago. Before transplant, this white stuff was not present; in fact, I have two other zucchini plants that are the same age but still in their pots without this white stuff present. The night I placed this particular plant into the soil I did water, but was relatively careful not to wet the leaves. Temperatures at night have been mid 50's.

More likely environmental stress. Was it hardened off prior to planting? Even if you did, what were the weather conditions following planting -- you know, heat, light, wind, etc.

@ Jean001a:

It was hardened off. And as for the weather conditions following transplant: 90 degree weather and very strong winds (30-40 mph).

Powdery mildew on cucurbits where I'm at normally doesn't happen until the plants are adults and in the middle of summer when it's hot and humid.

Kevin

Powdery Mildew is more a problem when the weather is hot and dry then when hot and humid.

Here is a link that might be useful: About Powdery Mildew

Thank you everybody for all the responses!

kimmsr: I'm sure UC Davis knows what they're talking about and PM can affect some veggies without humidity-- much like to my snow peas just within the last month. But I deal with it EVERY year and from my experience with cucurbits, it doesn't get bad until it gets humid along with the heat of summer.

Kevin