Intense sunlight can damage grapes

Q: I need confirmation of a problem with my grapes. A gardening Facebook group is certain my grape berries have thrip damage this year. I sent you a picture to confirm it or not.

A: I looked at the picture you sent and it does resemble damage done by thrips. But I don’t think it is. I think it is most likely sunburn damage to the fruit.

Thrips are tiny insects that rip and tear at soft plant tissues so they can lap up plant juices that come from the damage they create. This ripping and tearing causes scarring on the skin that makes the fruit unusable or undesirable.

They are very troublesome to rose fanciers and people with nectarine trees. Occasionally I get reports of them on other plants.

Western flower thrips are so small they are nearly invisible but in large numbers cause extensive damage to flowers and expanding fruits such as nectarine and other crops. In nectarine, their damage causes extensive scarring of the fruit.

Your grape berries have scarring like thrip damage found on nectarine fruits. But I have grown grapes 50 feet from nectarine trees full of thrips and never saw this problem on table or wine grapes. But one thing we can agree upon. The skin of your grapes is severely scarred.

So, what can cause this kind of damage to the skin of grapes? Sunburn of the berries can cause what you’re seeing. In our climate with our very high light intensity, it’s very important for most fruit, including grapes, to be shaded from direct sunlight.

When growing grapes in our hot desert climate, I like to grow them in a way that encourages shading of the grape bunches with their own leaves. I don’t like grape bunches to be exposed to full sunlight even for 10 minutes. This is OK in cloudier environments.

A passing infection of powdery mildew might also cause fruit scarring. We seldom see bunch rot diseases on grapes, but they can occur under the right conditions.

Powdery mildew disease would appear after rain followed by warm temperatures. This disease would damage grape berries if they are present. It would then disappear as soon as our dry desert weather returned. If powdery mildew remains in the vines, it would destroy grape bunches.

Bunch rot diseases like powdery mildew are discouraged by removing leaves that surround the grape bunches on the sides. Leave the leaves that shade them. This encourages air movement and rapid drying of the berries after rain. This can be enough to stop this disease development in its tracks.

Dormant oil sprays on the vines during the winter should help. Spinosad is a very effective organic insecticide against flower thrips. You should confirm that it is thrips before spraying or taking this kind of action.

Q: I have an apricot tree that seems to be perfectly healthy, but it has not flowered or produced apricots for two years. Is this normal and is there anything I can do?

A: Young apricot trees should begin flowering in their second or third year after planting. They will continue to flower in the spring every year unless they have been pruned incorrectly. If they are pruned incorrectly and produce no more fruit, I would replace them unless you want them only because they look nice.

Most apricots produce flowers and fruit on tiny stems along older branches. These tiny stems are called spurs. I have seen spurs removed by very tidy people who are not gardeners but do not like the look of them. If spurs are removed, then this also removes future flowers and fruit.

Once removed, they will not grow back. They are gone forever. New growth coming from the ends of the branches will produce new spurs but may take two to three years before fruit is produced. And then fruit will be produced only at the ends of pruned branches.

Some older varieties of apricot require another tree to provide the correct pollen to produce fruit. They still produce flowers in the spring, just no fruit.

In cases like these, a tree supplying the correct pollen could be in a neighbor’s yard. If the neighbor removes the pollinating tree, and with it the correct pollen, your tree would flower but produce no more fruit. Nearly all newer varieties of apricots are self-pollinating and don’t require a second tree.

Q: We are removing 2,000 square feet of grass and there are 40-year-old trees close to it. We do not want to lose them. Do I build large moats around the trunks, or put drippers in the new rock area? How much water do they need?

A: We get very little rainfall here in the desert so roots of your trees are well-established in the lawn you are removing. Removing the lawn will also remove their primary source of water. It is best to do this from late September through February.

Expect your trees to go into shock when the grass is removed. That is normal. Wherever the lawn is removed, roots of trees will die. Death of the roots will be seen in the canopy of the trees as limbs begin to die also from a lack of water.

It might take two or three years or longer for these trees to recover. There are two things that you should do to save them: properly reduce the size of these trees and give them enough water to recover. Large trees should never be pruned like a “crew cut” or a hat rack.

Reduce the canopy of these trees by at least one-third of their original size. I strongly suggest hiring someone knowledgeable about reducing the size of the canopy, such as certified arborists.

Certified arborists receive education and training on how to correctly prune in these types of circumstances. They know procedures like “drop crotching” when pruning large trees and oppose “topping” or cutting all limbs to the same height.

Apply enough water after pruning. Supply trees with plenty of water while they produce new roots in new locations closer to the trunk. Roots of older trees need years to recover and adjust to this new pattern of irrigation.

Large trees consume a lot of water. If these trees are large, they will require more water than six or eight drip emitters can provide. The soil under the new canopy should be wet to at least 18 inches deep each time you water.

This can be done by filling a “moat” or saucer under the trees’ canopy. It can also be done with drip irrigation but emitters should be 18 inches apart and cover the same area.

Q: Should Mexican primroses be pruned around November or December like lantana? Mine are getting leggy, pale and small. They haven’t been pruned.

A: Prune them back to the ground in late December or even January. November is a bit early for winter pruning in our climate. Prune them back as soon as they no longer look good. Fertilize them when you see new growth in the spring. Use a rose-type fertilizer.

People either love this plant or hate it. There is not much in-between. The reason people hate it is that it can become invasive; it can spread into areas where it’s not wanted. In cases like these, dig them up when they get too crowded or threaten to invade an area.

These are not maintenance-free plants. They should be planted 12 to 18 inches apart and fertilized. When they are not flowering to your liking, cut them back and apply compost or high-quality fertilizer. Dig them up and separate them when they become too clumpy or dense.

Bob Morris is a horticulture expert and professor emeritus for the University of Nevada, Las Vegas. Visit his blog at xtremehorticulture.blogspot.com. Send questions to Extremehort@aol.com.