Cool, wet spring temps cause insect, disease problems

This cool, wet spring was perfect for some early insect and disease problems to appear, namely aphids and powdery mildew. Roses and plants in the rose family, like many of our fruit trees, were rapidly hard hit because of cool, wet spring weather. This will get worse.

Cold winters reduce spring insect populations. Unusually mild winter weather made spring insect problems worse this year. Expect to see a lot more of these pests on new growth as the season progresses. Look for these pests to swarm newly emerging flowers and leaves. This may cause flower drop and distorted and sticky new growth over the next two months.

Hindsight is 20/20. Applying dormant or horticultural oil in December and January would have knocked aphids for a loop this year. Those gardeners who made two winter applications saw far fewer insect problems on their plants this spring than those who didn’t.

Ants leave their nests in the ground as soon as temperatures warm up. These ants search for aphids and relocate young aphids to new leaves, making the problem worse. Ants transport the sugary droppings from aphids to their young in underground nests.

What to do now? Soap and water sprays are lethal to all insects that are sprayed. This includes honeybees, so never spray plants that have open flowers with anything. Either spray these plants before the flowers open or wait until after the flowers are gone.

Two good-guy insects you will see hunting because of the abundance of aphids are ladybird beetles (ladybugs) and the awkward flying green lacewings. These good-guys can’t survive soap and water sprays either. But they can’t keep up with the population explosion that the combination of aphids and ants create. Now, don’t you wish you had applied dormant oil this winter?

Q: You recommended using an oven to kill fungus gnats in potting soil. I’ve been warned that these soils can smell bad when baked in an oven. If I ever attempt this baking method, I’d probably try roasting it in a gas grill (lid down) outside, where nasty smells aren’t as much of a problem.

A: I prefer to recommend placing moist, potting soil in a sealed, clear plastic bag and “solarize” it in direct sunlight for two or three days. This raises the temperature of the potting soil high enough and kills all the insects that might be present in the potting soil.

If temperatures are high enough, it will control plant diseases as well. It’s easy to do, and you don’t have to watch it closely.

The idea that potting soils stink when baked in the oven is true at temperatures above 200 F. High temperatures can also burn up the organic matter in the potting soil.

I ran into a problem this spring because of the prolonged cool, wet weather and the need for insect-free potting soil. We could not generate enough heat to solarize it because of the cold weather.

We experimented and found that if the temperature in the oven was kept below 200 F, there was no unpleasant smell — just “earthy.”

At temperatures of 140-150 F, all insects are killed, along with most weed seeds. Bumping that temperature up to 180 and keeping it there for 30 minutes killed everything in it. There is no reason to raise soil temperatures above 180.

When using an oven to sterilize soils, keep the oven temperature below 200 F. Maintain this temperature throughout the soil for 30 minutes, and then remove the potting soil.

Q: My fruit tree leaves are coming out yellow this spring. Do I need to apply iron?

A: When temperatures are cool and the skies are overcast, it is normal for plant leaves to come out light green or yellowish. They will become darker green as temperatures warm in strong sunlight.

Look at the veins in the leaves. Darker green veins and a light green to yellowish leaf is one indicator an iron fertilizer needs to be applied. Normally, though, yellowish leaves, due to lack of available iron, happen later in the season. It happens early in the season when iron was a problem last season.

Did this tree have yellow leaves last year? If it did, then iron should be applied to the soil along with the first fertilizer application of the season.

Don’t wait for yellowing to happen a second year in a row. It will, guaranteed, if you don’t do something about it now. It doesn’t correct itself. Prevent future yellowing by applying iron to the soil very early in the growing season.

Q: Are you familiar with the Aravaipa avocado tree, and does it grow well in Henderson?

A: There is excitement on the internet about this avocado tree because it is growing in Arizona. The mother tree was growing at an elevation around 2,500 feet in a canyon in southern Arizona, about 1.5 hours from Tucson. Avocados are tough to grow in the desert.

First, I do not recommend growing avocado trees in the Las Vegas Valley. This is because of winter low temperatures and the possibility of losing this tree because of freezing temperatures. You are taking a risk planting it, because you may not see fruit for many years to come.

Avocados handle hot, dry temperatures very well if it doesn’t get too cold during the winter. Most avocado trees do not handle winter temperatures below about 25 F. They also don’t like intense sunlight when young. It is these freezing temperatures, intense sunlight and wet soils that make avocados a challenge here.

All avocados originated from Central America at high, low or altitudes in between. Avocados are divided into three general races based upon these elevations: Mexican, Guatemalan and West Indian.

Most avocados we recognize from the store are hybrids or crosses between these three races. Avocados that handle freezing winter temperatures usually belong to the Mexican race or hybrids related to it.

The other main problem among avocado trees is poor tolerance to wet soils or soils that don’t drain water easily. The main reason avocados are grafted onto rootstocks is because of their inherent low tolerance to wet soils.

If you have a microclimate and you can grow citrus, you might have a chance at growing avocados. Focus on the smaller, semi-dwarf trees such Wurtz, aka Little Cado, that are grafted onto rootstocks tolerant of wet soils and known to produce high-quality fruit. Plant it on the east side of a wall or building, where it gets protection from late afternoon sun. It will be a challenge.

Q: Borers have finally killed my ornamental cherry. I plan to take it out, but is there any way I can plant another one in the same spot? Is there anything I should do to protect the new one?

A: It is not a problem planting the tree in the same hole if the death of the tree was caused by boring insects. The borers we have here never spend any part of their lives in the ground.

Make sure the reason for the tree’s death was not root rot. It is safe to plant in the same location if the reason for its death was not related to root diseases.

The key indicator of root rot is a failure of the tree to get established in the soil. Trees suffering from root rot are easily moved or pushed over when sickly.

Protect the trunk and limbs from intense sunlight that creates sunburn. This damage frequently leads to borer infestations. How to do that? Keep all new growth coming from the trunk until it is pencil-sized in diameter, and then remove it at this size.

Secondly, keep or encourage the lowest limbs on the trunk to help shade it from direct sunlight. This also helps prevent sunburn.

Thirdly, never plant annual flowers at the base of these trees. Plant perennials at the base, if you must, which don’t need to be watered as often.

Fourth, cover the soil surface with wood chips, but keep the wood chips several inches away from the trunk. Wet wood chips against the trunk can cause collar rot quite easily and tree death.

Lastly, when planting, remember the old phrase “Prepare a $5 hole for a $3 plant.” This means put a lot of effort and expense in preparing the planting hole. The hole should be wide. It is not important for the hole to be deep unless water doesn’t drain from the hole overnight.

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

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