Grape Leaves Cupping

Q. Some of the leaves on my Thompson Seedless grapes have started to curl/cup …see attached pictures.  There is more curling on the newer leaves; the older leaves are OK and the curling is only on one side of the plant.  The Red Flame grapes planted adjacent to the Thompson Seedless grapes are OK and no sign of curling or cupping.  These 5-gallon plants that I planted 2 years ago.  All of the plants are on a dripline system and are watered three times a week, one hour each time.  Each plant has two 1-gallon drippers so each plant is receiving 6 gallons of water per week … is that sufficient?  The plants are in a raised bed approximately 18 inches deep of premium mulch/soil.  Both of the plants have several bunches of grapes and I have already pinched off the bottom 1/3 of each grape bunch.  Any advice would be appreciated.

Leaf cupping on grape. Possibly from 2,4-D or a close relative, a phenoxy herbicide known for its effects on regulating growth of plants at very low concentrations but acting like a weed killer at higher doses.

A. Sounds like a great job and from the looks of it your plants are thriving. Six gallons a week is a bit light in my opinion but watch the plants and they will tell you. If you are getting some good vigorous growth from six gallons then it is enough and I would not change it.

Grapes are normally deep rooted plants and can have roots that go down dozens of feet. In our landscapes this doesn’t make much sense to water deeply to accommodate roots like this so your raised bed sounds like a good depth for the plants. You are watering only to a depth of maybe 18 inches or so, so watering three times a week right now on grapes with this rooting depth makes sense to me. After grape harvest, you could cut back on the frequency of your application (times per week) if you want to but it is not necessary. But you need to maintain good soil moisture during and up to fruit harvest.

It sounds like you are doing everything right. We do have two types of grape thinning; one is removing grape bunches that are too small and spacing bunches so they are not too close together (8 to 10 inches apart) AND reducing the bunch by pinching off the bottom third of the bunch. This is done as early in bunch formation as possible.

You didn’t mention any pest control such as grape leaf skeletonizer, leafhopper or hornworm control so I guess you haven’t seen any.

There is a fourth pest of grapes we see sometimes but not very often and that is the fleabeetle. They are small, dark blue/black rather round looking insects that chew holes in the leaves. They are usually not that devastating so we just ignore them but once in a while they can cause considerable leaf damage in the spring and fall.

Now regarding the leaf cupping. Leaf cupping has to occur on developing (young) leaves since the cupping results from leaf growth in the center of the leaf while the leaf edges either don’t grow as fast or are damaged so don’t grow at all. There are three primary reasons leaf cupping can occur. First is damage from a chemical growth regulator that drifted on to your vines.

Thompson seedless from reader next to affected grapes but no signs of damage.

Grapes can be quite sensitive to these chemicals. One common growth regulator used by homeowners for lawn weed control is 2,4-D and sister compounds that are also growth regulators. Commonly these chemicals are used to control dandelions and other “broadleaf weeds” in lawns. If this chemical were applied to a nearby lawn (could even be 100 yards away) and there was a wind that blew this chemical from the lawn to your grape vine then that would explain the cupping. This type of damage is usually not deadly but just causes leaf distortion. The leaves are not just distorted but weirdly distorted. The leaves will not un-distort or grow back normally. They will be like that until leaf fall. The fruit is safe to eat.

A second possibility are insects that cause plant leaf cupping when they are feeding. The most common insects that do these sorts of things are aphids. The feeding of aphids on plant leaves that are still growing can cause the leaves to cup downward (this is called epinasty in horticultural terms). It is thought that this type of plant reaction to the feeding of aphids is a protection for these insects from predators. Aphids of course secrete honeydew, a sugary substance made from the plant juices they suck, which attracts ants. The ants in turn use the honeydew as a food source and help protect the aphids from predators in exchange for “harvesting” the honeydew. Aphids are not common on grapes.

Aphids feeding on new growth of plum causing the cupping of the leaves due to their feeding on expanding new growth. Aphids are also covering the stems.

This leaves the third possibility and the one I am leaning towards. If the leaves were just coming out and expanding and if there was suddenly a very hot wind OR it got hot quickly and the grapes were tender enough to get some damage to the leaf edges then this would explain the cupping. The leaf edge would dry out from high temperatures or a hot wind and dessicate or dry out. The damaged leaf edge would not be able to grow or grow slowly. The rest of the leaf would be unaffected and would expand or grow. The growth of the center of the leaf while the leaf edge remained unable to grow or grow as fast would begin to cup. The cupping would be worse as it grew more. And of course this would happen only to young leaves which are still growing or expanding. Again, all will be well but you should check to make sure the vines are getting enough water. If they were droughty, the leaf edges would scorch and cupping would result as well.

It might be a good idea to change your drip emitters to two gallon per hour emitters or higher and apply a surface mulch to reduce water loss from the soil.