Spider mites! Look at it with a magnifying glass and you will likely see the tiny arthropods moving around sucking the life out of the leaves.

Fine webbing is probably present as well. Look on undersides of leaves and at the growing tip.

Mites are often a serious problem in hot, dry weather.

Hopefully others will tell you the best way to eliminate them. Regular hard sprays of water on a regular basis is a the safe way to cut them down a bit.

Leaves won't 'recover' their green-ness, but new leaves should look better.

This post was edited by dave_in_nova on Mon, Aug 25, 14 at 9:58

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