Irregular patches of yellow tissue on leaves in a clear indication of chimera, a genetic disorder.
Chimera is caused by a mutation in a growing part of the plant which leads to discolored areas.
The irregular patches can be white in color also as seen in squash in this picture.
Entire bleaching of the leaves is another symptom with only the tip with green color as seen in squash.
Bleaching of many leaves and growing sections on cucurbits can also been seen.
The bleaching pattern may also show intermittent spread of white and green areas as seen on watermelon here.
Bleached appearance of leaves of a squash plant caused by chimera. Chimera usually occurs only on a few plants.
A view of a squash field with a few plants in the entire field with chimera (check bottom left).