The school of hard knocks is tough, but lessons learned can be important and they tend to stick.  I encountered an issue in one of our just-planted, on-farm research blocks yesterday.  Vines were collapsing and dying, and symptomatic vines presented a combination of red leaves, and/or leaves with red to purple leaf veins arising from the petiole; in some cases, bark was splitting and showing dark discoloration.  Since I have not seen this before, I was initially very concerned and confused.  As it turns out, Wayne Mitchem (our grape weed scientist stationed at North Carolina State) has an online slide presentation that allowed me to identify the symptoms as those of gramoxone (paraquat) injury caused by direct application to young vines.  Our IPM guide for bunch grapes does warn us of issues that can arise from applying gramoxone (paraquat) herbicides to young vines: “Do not allow herbicide to contact desirable foliage or immature, uncallused bark. Young vines must be shielded.”  These vines were not protected by grow tubes, and I share with you here the symptoms that were observed — the critical one being vine death (see photos). I always think of gramoxone (paraquat) as being a burn-down contact herbicide, but I learned that it does have some limited systemic activity.  Therefore, contact with suckers or leaves at the base of the plant can allow for uptake and damage as the herbicide moves in the plant — in addition to the direct burn and girdling of the immature trunk. Anyway, I hope that this information will prevent you from making a similar mistake in the real world.

Systemic gramoxone (paraquat) injury on grape leaves. Systemic herbicide damage occurs as the herbicide moves through the leaf veins.
Systemic gramoxone (paraquat) injury on grape leaves. Herbicide damage through the leaf veins could be confused for some type of viral or nutrient issue.
Typical direct contact burn caused by gramoxone (paraquat) injury.
Gramoxone (paraquat) injury can girdle young trunks, resulting in symptoms that are similar to those of crown gall or other trunk diseases — purplish red leaves.