Looks like a condition called Silverleaf. Check out the info here....
http://edis.ifas.ufl.edu/pp160
Silverleaf
Silverleaf was first observed in squash in Israel in 1963. It was then described as a distinct disorder differing from the genetically controlled leaf silvering that is common in zucchini-type summer squash. Although observed in Florida, and the cause of squash silverleaf is not known, but does not appear likely that drought stress causes the condition, as reported in Israel. However, stress induced by other factors may be related to the disorder. Extremely high populations of the sweetpotato whitefly are frequently noted in association with severe silverleaf. Therefore, it is thought that this insect may cause a physiological imbalance that leads to silverleaf symptoms by (a) direct feeding, (b) introduction of a toxin of some sort, or (c) by vectoring a heretofore unknown disease. No other biotic or environmental factor has been consistently related to the occurrence of silverleaf.
Silverleaf symptoms appear first at the leaf veins, as opposed to interveinal silvering that is genetically controlled, and is common in many zucchini cultivars. Symptoms develop in the interveinal area so that the entire upper leaf surface is distinctively silver. Symptoms do not occur on the leaf underside.
Fruit symptoms are not noticeable in cases of mild leaf silvering, but when leaf silvering is severe, fruit color is lighter than normal. Yellow summer squash is very pale-colored, zucchini squash is light-green to yellowish-green, acorn squash is mottled green to yellow, and golden acorn squash is white. Yield reductions and poor fruit quality are usually associated with leaf silvering.
Silverleaf symptoms have been noted on all types of squash but have not been observed in muskmelons, cucumbers, or watermelons.
Figure 21. Silverleaf on squash (foreground) and normal squash (back)
Credits: Photo by P. Gilreath
