[erainn]

Noticed on lower leaves of marrow plants what to all purposes looks like specks of white powder, is it a fungal infection of some sort, or a midnight flour shaker ?

[JayG]

Very likely to be powdery mildew, which affects most squashes eventually, although usually later in the season than this.

It's part of nature's way of recycling dead or dying plant material, so will affect the older leaves first.

As long as there are plenty of leaves left, it's best to take off the mildewed ones; keep the plants well watered but try not to wet the leaves more than necessary.