Often this type of discoloration is indicitive of a sucking insect such as
thrips or aphids. Look underneath the leaf for small black specks like
pepper; if you see the specks, that is the insect's excrement.
David J. Bockman, Fairfax, VA (USDA Hardiness Zone 7)
I'd also check for spider mites. When there's a really bad infestation, you
can see the fine webbing. Otherwise, as David Bockman said in a earlier
post, check the underside of the leaves for tiny black specks -- this also
can be a good indication of spider mites.
In any event, whether aphids or spider mites, an insecticical soap product
should knock them out. Follow the label recommendations and be sure to
treat the underside of the leaves.
Suzy O, Zone 5, Wisconsin
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