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Spider mites can be a problem during hot, dry weather

I will be giving a series of four classes about establishing a desert orchard starting the first Saturday in November and continuing through Nov. 24. The class will be held 1-3 p.m. at Ahern Rentals Academy, 1722 W. Bonanza Road.

The focus will be on high and medium density fruit production for residential landscapes including how to grow multiple fruit trees in a single hole. Class size is limited. Registration can be found online at Eventbrite.

Q: I have a fruit tree with leaves that are brown on the edges and covered in dust. Some of the leaves are turning gray-green on branches, and the tree does not look healthy.

A: Most likely the tree damage was caused by spider mites. Spider mites are a summer pest problem during hot, dry weather.

Another telltale sign that you mentioned is the dusty appearance of the leaves. This dust results from dead spider mites left behind as the population grows.

Look for feeding damage to the leaves caused by spider mites. This feeding damage causes tiny yellow dots scattered all over the surface of the leaf and accompanied by tiny black dots the size of this period.

By the way, the tiny black dots are mite poop. Even though mites feed on leaf bottoms, the yellow dots can be seen on the top of leaves.

A common misconception is that webbing must be present if mites are the problem. Spider mites leave behind webbing, like the weak form of a spider’s web. But not all mites that cause plant damage spin webs.

So, finding webs when there is damage is not a dead giveaway that the damage is from mites. It could be one indicator, though. There are spiders that are beneficial to plants.

Mite attacks to plants may come after the application of a hard pesticide. Bad mites are always present on plants, but their numbers are controlled by predatory insects and even “good mites.” So, applying a hard pesticide to control borers, for instance, could lead to an outbreak of spider mites because the predators were killed.

What can you do? Confirm that spider mites are the problem. Use a white paper test. Shake or slap an infested branch on a white piece of paper or paper plate. Closely look at the white surface for tiny dots, the size of a period, crawling along the paper.

Smear them with your fingers if you aren’t sure. If you see lots of them, along with plant damage that I described, you have confirmed mite damage.

Multiple applications of soap and water sprays do a good job controlling small outbreaks. Hosing the leaves of plants monthly, or after a dust storm, removes dust from the leaf surface which can increase spider mite populations.

Severe infestations of spider mites may require a pesticide application. If spider mites were a problem during the year, be sure to apply two dormant oil sprays during the winter months.

Q: I have two Mexican fan palms in my backyard that are 16 to 18 feet tall. One is 3 feet from a retaining wall and 6 feet from a pool. Another is 3 feet from the corner of my house, which is on a slab. Do I need to worry about the roots damaging the wall, pool or slab?

A: The short answer is you should be concerned anytime something that can get large is growing close to anything that can be damaged. The good news is that palm roots don’t typically damage walls, swimming pools or concrete slabs as much as other types of tree roots. But they can cause damage.

Palms, in general, are not a good choice around pools, but Mexican fan palms get huge and should never be planted close to structures. Smaller palms, such as windmill and Mediterranean fan palm, would be a better choice. The closest large fan palms should be planted to walls, swimming pools and house slabs is perhaps 10 feet away.

There are a couple of options if you don’t want to remove these palms. One is installing a root barrier so that palm roots are deflected away from structures that could be damaged. Root barriers are installed and extend about 30 inches deep or more and reside slightly above soil level.

The second option is to use water for directing root growth. In deserts, soils are normally dry due to a lack of rain. Tree roots grow where water is available. By placing irrigation water away from walls, cool decking or a house foundation, root growth can be directed away from these problem areas.

Water does not need to be applied evenly under plant canopies. I recommend keeping water 2 to 3 feet away from these potential problem areas.

Q: My Italian cypress trees started to lose their color and then die. Upon inspection of the trunk, I saw borers in them. Borers seemed to take place within two months or so and killed a few mature trees. I treated them with Bayer Tree and Shrub Insect Control but wanted to know if there is anything else I can do to help save them.

A: Damage from borers or boring insects is typically a slower progression than a couple of months unless the plants are small. You are right; the progression in many plants is a change in color from a vibrant green to a dull, gray-green color as the limb and leaves are dying from a lack of water.

My guess is borer damage to your trees started before this year. Internal damage to the trunk from borers can be present for several years before enough damage has accumulated to result in the death of older trees. If trees are growing vigorously, they can recover from light borer damage on an annual basis.

This color change due to damage starts in midsummer when air temperatures begin to heat up and plants require more water. Damage from boring insects begins in midspring (March) but gets progressively more intense as these insects become larger and more voracious feeders. By midsummer (June and later) internal damage to limbs and the trunk can be extensive, unrecoverable by the plant, and the supply of water from the roots is cut off.,You chose the correct insecticide to use, but it was probably applied too late. The best time to control boring insects with these types of insecticides is in midspring when they first become active.

Consider using pesticides as a last resort because they can contribute to other pest problems. Death of boring insects can happen in a few days when systemic insecticides are applied as a liquid drench to the soil.

Q: I have a greengage plum tree that is 18 years old and produces plenty of plums each year. In August I saw sap coming out of one branch and along the trunk in several places. Last week the bark started to separate, and it looks like the trunk is starting to split. The leaves on that limb are starting to die. This side of the tree receives the west sun. All the other branches on the tree appear to be fine.

A: Greengage is a good plum for our desert climate and 18 is not old for a plum tree. Plums can be sappy compared to other fruit trees but from your description, it sure sounds like borers. I would take a very sharp knife that has been sanitized and start actively looking for borers.

It is possible to remove the outer layer of bark covering the trunk and limbs and reveal the immature form of this insect, called the larva, causing the damage. Once exposed like this, it will die. Expose all the damage caused by borers down to the healthy wood. Leave this exposed area open for healing. I would say that about 80 percent of the time this is an effective way to remove the borer and allow the limb to recover.

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

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