There are a number of reasons why a plants leaves display that yellowing, caused by a lack of chlorophyll and it is known as Chlorosis, but nutrients are the usual cause. It could be the soil pH which influences the plants uptake of micro nutrients such as Iron, Zinc, Manganese, it could be an imbalance of magnesium to calcium, it could be the soil has too much of certain nutrients or too little of those nutrients.

The only way to know would be with a good reliable soil test and taking a good, in depth look at the soil these plants are growing in.

Perhaps this article may be of some help, http://extension.illinois.edu/focus/index.cfm?problem=chlorosis

kimmq is kimmsr

It could also be lack of nitrogen; old leaves; and /or end of the season

Thanks for the input. These plants are very young, so they are not yellowing from nearing their life cycle completion. I'm worried more and more that this is Yellow Mosaic Virus or Cucumber Mosaic Virus since the cukes went that way. These zucchini are making small, misshapen, discolored fruit and the dropping them prematurely. I will look into soil sampling on the off chance that it is a lack or over abundance of nutrients. But I am fearing the worst.

Can you tell us where you live? Late October is an odd time to have young zucchini plants for lots of us. Are you in the S hemisphere?