Zucchini leaves are yellowing from the bottom up (w/pic)
I planted a Zucchini in the garden, and it was doing very well until around 2 weeks ago. At that point, I started noticing a yellowing of the bottom-most leaves, starting at the tips. Over about a week, the affected leaves would get yellower and yellower until the only remaining green was along the leaf veins. The rest is dried like parchment, almost a bronze color, and curled up.
The plant as a whole seems to be doing well right now, flowering and producing new leaves. However, it looks like whatever was affecting the older leaves also strikes new leaves about a week after they emerge. The pattern is the same: yellowing around the tips of the leaves, spreading to the base of the leaf over several days, and eventually resulting in a curled-up, dried leaf.
I took a dried leaf to the local garden center to elicit a diagnosis and/or suggestions to fix the problem. Three people looked at it, and none of them knew what the problem was. They said maybe inadequate sunlight, which couldn't be it since the plant gets full sun from around 10 AM to around 6 PM. They thought maybe the soil was too wet, but I don't think this is the problem since the soil feels neither soggy nor powdery when I stick my finger into it (I'm using "square foot gardening" technique: 1/3 vermiculite, 1/3 peat moss, 1/3 assorted composts in a 4' x 4' x 6" raised box). They looked up plant diseases and guessed maybe Cucumber Mosaic virus, but the leaves are evenly dried, there are no dark blotches, and there is no slimy liquid when I cut off a leaf. They said maybe a grub had gotten in and was siphoning off the plant's liquid, but the "trunk" of the zucchini looks pretty healthy to me.
I will attach a picture to give more information.
So, can I cure the plant? If not, should I just tear it out now instead of wasting several weeks waiting for it do completely die and planting a new one? Is it a soil/fungus problem? If so, should I avoid planting a replacement in the same spot?