why are my plants turning yellow?

Went out this morning to check the garden and noticed dramatic yellowing on several plants. The summer squash:
www.reddawn.net/temp/squash.jpg
And tomatoes:
www.reddawn.net/temp/tomato.jpg
In both cases it seems to be spreading up the stalk of the plant, not yellowing at the tips. Anyone know what this is? I've never seen it happen before.
Strawberries, bell peppers, broccoli and brussels sprouts are so far undamaged.
Dawn
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wrote:

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http://ftsg.ifas.ufl.edu/TYVDBC.HTM
Tomato yellows...perhaps?
Penelope
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