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My tomato plant's leaves are turning brown in some places (photo).

Tomato leaves in various stages Zoom in

It seems to affect mostly older leaves, but not all of them, as you can see in the image. It starts with a few small brown spots that grow larger over the space of a week or two and eventually most of the leaf turns brown, as shown in the upper left. The plant is otherwise fine, blooming, yielding fruit and producing new shoots.

This particular plant is kept indoors on a windowsill, and therefore not exposed to rain and shielded from many parasites and diseases (this particular window doesn't even open).

The symptoms don't seem to match early blight or late blight.

What could this be?

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I just found this helpful reference for identifying tomato diseases, but I don't see a match for my symptoms either:… – Ingmar Jul 15 '14 at 23:12
Looks like magnesium deficiency. – J. Musser Jul 15 '14 at 23:13

That is magnesium deficiency. It can also cause yellowing between leaf veins. Treat by watering Epsom salts (magnesium sulfate) at a rate of 1 cup per every two gallons, with every watering until the symptoms leave.

See a comparison pic:

enter image description here

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That seems a good match! Will the affected leaves recover? – Ingmar Jul 15 '14 at 23:21
@IngmarHupp The affected leaves may recover if the damage was only superficial, but the markings may remain on some of the worse leaves. – J. Musser Jul 15 '14 at 23:23
@IngmarHupp The marks will not disrupt the plant's growth or production, only the appearance. If you want, you can remove the worst leaves, but that is not recommended if large areas of the plant have been affected. – J. Musser Jul 15 '14 at 23:26
More like 1 Tablespoon per gallon or 2 of water. One cup of epson salts would kill them. – user11296 May 13 '15 at 20:22

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