"David Hare-Scott" writes:

Dan Espen wrote:
gleb writes:

Two years ago I grew tomatoes on the bathroom windowsill. The
seedlings soon developed some leaves which became papery and white
and the plants had to be discarded. Last year I disinfected the
windowsill and tried again, same result. This year I am growing my
tomatoes elsewhere but am growing celery on the bathroom windowsill
and the seedlings are suffering the same fate. Does anybody know
what is wrong and how to fix it?

Too much light for young plants?
Is this a south facing window?

More likely too little.

I'm more familiar with too little light causing yellow leaves.
Not to argue though. The main point is the OP should give more info.

Otherwise inspect top and bottom of leaves with magnifier.
Look for fungus or insects.

D

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Dan Espen