Verticillium wilt cause death of single branch

Q: I had a 2-year-old desert willow that had one branch die. What could cause this?

A: Off the top of my head I didn’t know anything that could cause a single branch to die in desert willow other than that branch breaking or being damaged for some reason. Borers that can cause this type of problem are not common to desert willow in my experiences. Seeing the branch die in midsummer would be a telltale sign that borers might be the problem. So, I rather doubt that borers are the problem with this tree and in this case.

A search on the internet of reliable websites, like the University of California’s Integrated Pest Management site, told me that desert willow does at times get a disease called Verticillium wilt. It’s not often, but it does. Verticillium wilt disease does cause a single branch to die. Going on the information you provided, my best guess is this disease.

Verticillium wilt disease plugs the internal tubes that carry water from the roots to the leaves. It commonly infects a single branch, causing it to die and appear like it is not getting enough water. This branch is not getting enough water, but it’s because of the disease, not because the entire tree isn’t getting enough water. Watering the tree more would not solve the problem and might worsen the problem.

Verticillium is a disease commonly found in soils, but it also can be spread from plant to plant with dirty pruning shears. Whenever pruning, disinfect the blades of pruning shears with Lysol, bleach or alcohol before making cuts.

If you suspect a problem, always disinfect pruning shears before and after the pruning cut is made or between each cut. Never lay pruning equipment on the ground where it can become infected with about five different plant diseases. Typically, pruning shears are disinfected before and after pruning when plant diseases are not seen.

Q: In a previous article you talked about whiteflies. What do they look like and how do I know if my plants have them?

A: Whiteflies are commonly found during summer months on many different vegetables like tomatoes, squash and melons. But they are not as common to ornamental trees and shrubs. As far as ornamentals go, they can be a problem for ash trees, citrus, pomegranate, gardenia, hibiscus, iris, flowering annuals, gladiolus and some others.

Whiteflies are small insects, brilliant white in color as winged adults, that suck plant juices from leaves and young stems much like aphids and leafhoppers do.

If you are observant and constantly tinkering around your plants, you will notice them during the heat of the summer swarming in the air when disturbed from plant leaves like white “dandruff.” This is an indicator to look at the undersides of leaves where you will find a massive number of unwinged young ones sucking juice from the plants.

If you are not as observant, you might notice sticky, sugary sap dropping on plant leaves from the feeding just above it. Or you might see columns of ants attracted to this sugary sap, going back and forth to their underground nest carrying this sugary food.

Sometimes a black mold will grow on this sugary sap in more humid climates, just like it will with aphids. This black mold is called “sooty mold,” which can cause lots of plant damage if left unchecked.

The adults can fly, but the young ones can’t. So that swarm of white adults you see flying are adults only and a sure sign that feeding damage is underway.

It’s winter now, so you will not see whiteflies on plants unless they are on plants in warm spots like greenhouses. They are tough to control because they are resistant to many chemical controls.

If you find them early enough, removing infested leaves might keep them in check. Repeated soap and water sprays will kill them. Winter applications of dormant oil to woody trees and shrubs in January will help suppress their numbers in the summer from overwintering adults.

Q: My father’s lawn is tall fescue that completely died this summer. What did we do wrong? We are anxious to plant a new lawn and want some advice on how to plant seed and the best seed to use. The lawn gets full sun.

A: It’s important to discover what caused your father’s lawn to die suddenly and prevent it from happening again. If the entire lawn died soon after it was planted, then it was probably watered too often, or had poor soil preparation resulting in bad drainage, or both. If it died when it was older, leaving green patches behind, then it could be a poorly designed irrigation system or a disease problem. Such problems can be prevented.

Tall fescue lawns need adequate amounts of sunshine, at least six to eight hours of full sun every day or a minimum of 50 percent sunlight all day if grown in shady areas. Tall fescue lawns require good soil preparation and drainage to a depth of at least 8 to 10 inches to be successful in the Mojave Desert.

Designing a good irrigation system requires knowledge in water pressure and water flow, correct sizing of pipes and sprinkler heads, and matching irrigation products so they operate effectively together. Most homeowners require help.

Many companies that supply irrigation products have knowledgeable staff that will help you or will design the system for you as a service if you buy their products. Ask and take advantage of the services.

Irrigation systems for small lawn areas are commonly pop-up stationary sprinklers operated from 12 to 15 minutes each time an irrigation is needed. It is important that this water penetrate the soil 10 to 12 inches deep each time an irrigation is done. How often or the days each week and times of operation are frequently dictated by your water company. But daily irrigations should only be done during the heat of summer.

Establishing a tall fescue lawn is done in the spring or fall months from seed or sod after the soil has been prepared sufficiently. Whether to amend your soil with a starter fertilizer depends on the quality of the soil. If a rich soil mix is used, then a starter fertilizer is not needed.

When buying seed, the cost of seed is directly related to its quality. Typically, expensive seed is better quality seed if the blend or mixture of seed is suitable for the climate.

Cover the seed with a 1/8-inch layer of rich compost, or “topper,” and water twice a day until you see it emerging — once in the morning and once in the afternoon. Mow the grass no closer than 2 inches tall when the grass is 3 inches tall or more. Mowing causes the lawn to become denser.

Q: Should I add anything extra to the raised beds I use for growing vegetables?

A: Raised beds use compost mixed with soil or sand. Adding more compost depends on the quality of the soil mix you began with and how long it has been used to grow vegetables. Raised beds for growing vegetables should have fresh compost added every one to two years. I usually recommend a 1-inch layer of compost on top of the soil in the raised bed and then “double dug” into the soil in the raised bed so that it’s mixed.

Compost and soil mix qualities are judged by their appearance — dark color, fine texture and aroma — and their nutrient content. Nutrient content can only be assessed through a compost analysis done by a reputable soil or compost testing laboratory.

Most people only judge a compost or soil mix by its appearance and cost, never considering a laboratory analysis. That can be a big mistake.

Quality composts are dark in color, fine-textured and smell like a forest. That’s easy to judge. But there can be problems lurking in a good-looking compost that can be detected only through a laboratory analysis.

When buying a compost, ask to see its laboratory analysis. The seller should have a copy to show you.

When buying a soil mix, judge it by its appearance and the quality of the compost used to make it. A good soil mix should be dark in color and incorporate a good quality compost.

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.