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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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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

  • That seems a good match! Will the affected leaves recover? – Ingmar Jul 15 '14 at 23:21
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    @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
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    @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
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    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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Sometimes using too much potassium can lead to magnesium deficiency. but in this type of disorder yellowing can not be seen widely. young leaves develop healthy but older leaves first got glossy violet to brownish and then it turns dried and brown between veins. older leaves will die with sever magnesium deficiency symptoms. Also usually it can be response of upper epidermis to water loss and fast changes in root zone EC (Electrical Conductivity). usually it can be seen after fertilization. flushing the root zone with water is effective way for this problem. many commercial growers are using this common method.

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