Looks fungal, spray in cool of day with Daconil (Garden Disease Control). Have to wait at least a day to harvest, wash well.

Thanks for the daconil suggestion but that stuff seems pretty toxic. Trying to go the organic route so i'll use an oranic fungicide i already have. Didnt want to spray anything until i found out the problem first.

Sorry about your cukes! and the deer. You might put a good, tall fence around your gardens and keep it close to the raised beds. The deer won't be able to jump the fence that way. Just make sure the fencing is taller than an avg deer would be! They will chow down on stuff growing along the fence but that could also be solved by surrounding the fence w/ a good mesh like you have in your pic. It will cost a little at first, but then you'll have that yr after yr. A good dog will help chase deer away! lol! That yellowing leaf could be a lot of things other than fungus. Might be the cukes are getting too much water, maybe not enough plant food, nutrients, could be the cucumber beetle, but the upper leaves look healthy. Cut off those yellowing leaves and see what happens!