It is called powdery mildew. It hits mostly cucurbias and seldom tomatoes get it. I would spray them with NEEM oil spray. Another home formula is like 30 -40% milk and 70 -60% water.

Sey

Sey: seldom tomatoes get it

I actually had a squash vine that ended up climbing up a nearby tomato plant. The squash was afflicted with a pretty bad case of powdery mildew towards the end of the summer, but the tomato plant never suffered from it, even though it was tangled up with the squash plant.

Thanks for the feedback!

Im keen to try the milk spray!

Plus I'm relieved to know it doesn't spread to other plants easily as I have gone a bit crazy with all my tomatoes :)

ta

You can
also use this Garden Fungicide from Safer Brand. The
product is organic (listed OMRI.)

The Neem Oil (Bionide) – recommended by Sey – is a very good product,
and it’s also organic (not listed OMRI.)

As others have said, it's powdery mildew. Unfortunately you've got two different types of PM as the type that affects cucurbits won't affect tomatoes and vice versa. Control is the same. Start using a fungicide.

PM on tomatoes: http://www.ipm.ucdavis.edu/PMG/r783100411.html

PM on cucurbits: http://www.ipm.ucdavis.edu/PMG/r116100711.html

The PM on your cukes is quite advanced already since it appears to be on every leaf so you might want to start over with them.

Rodney