[Mr Mole]

I have grown half a dozen or so tomato plants for a good many years and never had any problems.  Today I went to pot them on and it looks as though my luck has run out.
The only way I can think to describe the white marks is that they look like dabs of white emulsion........they have only been watered with rain water so that would rule out lime scale marks from our hard water.
Hopefully some pics will appear bellow and hopefully one of you knowledgeable people will be able to help.

[Aunt Sally]

Have you sprayed or painted your greenhouse windows with white shading ?

[Mr Mole]

No Aunt Sally.  All the other plants in the greenhouse are clean, it's just the tomatoes

[mumofstig]

bird droppings? did you leave the door open for a while 

[savbo]

the blotches are at the end of the leaves or leaf-tooths...I think it could be some damage as the leaves were developing that has grown and become more evident as the leaf has matured...

[DD.]

Not just simply sun-scorch through magnification via water droplets?

[DavidT]

Wooly aphid?

[8doubles]

Not just simply sun-scorch through magnification via water droplets?

This would be my guess, it has been very bright on occasions.

[Ice]

If you can prise the white bits off I would say it's a spill of some kind.  Otherwise I have to agree that it could be sunscorch as the droplets would gather there on the leaf fringes.  Never come across any tomato problem that causes those symptoms.

[Mr Mole]

Mumof stig...........that would make the birds very selective........only bombing tomatoes .

Savbo........it could be, but I am fairly careful when handling young plants and never had it happen before.

DD .....the pots stand in a trays and are watered  from the bottom .

D avid T......doesn’t look like an insect yet..........must get magnifying glass out.

8doubles.......It just looks to white to be a scorch mark.

Ice....it will rub off so maybe this would indicate a fungus of some sort.

Thanks’ for everybody’s suggestions, I will keep you updated.   

[8doubles]

Maybe , -iEebxCkJwE
 

[PennyS]

Maybe , -iEebxCkJwE
 

Blimey that brings back memories.  I think it was on Sunday lunchtimes? 
I used to think it was really scary sometimes!!