HORTICULTURE
HORTICULTURE
Questions about 2,4-D and Grapes
I received these questions in an email from a backyard grower and thought I would post them for the benefit of other hobbyists.
Are grapes
very sensitive to 2,4-D injury?
Yes! Grapes are very sensitive to 2,4-D. In SW Michigan we have restrictions on the use of 2,4-D in the grape growing regions. The immediate symptom is wilting of the shoot tips. Leaf symptoms are visible as cupping upward on grape leaves that were partially formed when the application took place. All the new leaves will show a fan shape as in the picture below. Eventually the new leaves will return to a normal shape.
Do they grow out of it?
Depends on the dose. If the shape of the new leaves return to normal then I think they have grown out of it. The effect on the fruit will linger for the rest of the year.
Can a plant show injury from last year?
I would think that if they were hurt that bad they would die or be severely stunted.
Are symptoms always from recent sprays?
No. You can tell by looking at the position of leaves on the shoot when the
application took place. The cupping and leaf deformation can only occur on growing leaves
not on those that are already fully expanded. Once the leaf is fully-grown it cannot show
symptoms. Conversely a leaf that is deformed while it is growing cannot stop showing
symptoms because that is the way it was formed. It is possible to trace the history of
growth that year by looking at the leaves. I saw a grape vine once that showed 3
distinctly different episodes of 2,4-D injury.
How far away can the grapes be and still be damaged?
Miles, if the culprit is an ester formulation of 2,4-D. These materials easily volatilize into a vapor. The vapor can move over a large area or even jump from one area to another as the cloud moves along touching the ground here or there. Worst case I ever saw was ester 2,4-D applied in hot humid weather an entire 10 acre vineyard was wilted the next day and showed no difference between the upwind or downwind side but the culprit was right next door. The vineyard recovered. I usually suspect something close by, e.g., Weed-Be-Gone at the neighbors or and ester form of 2,4-D on grain.
Does it affect fruit set?
Yes, fruit contaminated at bloom usually develop smaller berries and with increased dose will develop shot berries i.e. tiny berries that stay green but do not fall, and/or a lot of fruit may fall off so you end up with only a few berries. I would not blame poor fruit set on 2,4-D injury from fruit symptoms alone. If the leaves, about 10 leaves up the shoot from the fruit cluster were affected by 2,4-D then I would believe the application occurred near bloom. Injury after bloom often results in uneven ripening of the cluster and green berries that will not ripen in the cluster (hard to tell on green grapes).
