Sticky trail a sure sign of aphids

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Q: We have two yellow Lady Banks roses that are about three years old. However, they are secreting a sticky substance similar to that of an aphid infestation, but I cannot find any aphids on the plants. The bees and wasps are drawn to the substance. We sure would appreciate any input on this problem.

A: I am sure you have aphids. You just have not seen them. It is a common problem this time of year and a very susceptible host for aphids.

The other possibility might be whiteflies. Regardless, it is an insect that is sucking plant juices and excreting what we call honeydew. It is too cold for the whiteflies to be flitting about, but you should be able to see either of them on the undersides of leaves.

Because it is so late in the season there is no reason to apply a pesticide now. Wait until leaf drop this winter and apply a dormant oil application twice, once in December and a second one in January. This is applied directly to the dormant woody plant.

Also remove any weeds where aphids might overwinter. Apply the dormant oil to nearby plants in the landscape to help keep insects under control in the early spring. As soon as new growth occurs in the spring, winged aphids will fly or crawl to the new growth and begin producing young. These young infest new, soft succulent growth when the weather is cool and reproduce very rapidly.

Their feeding usually causes distorted growth when new leaves are young and expanding. Along with the feeding is the sticky, sweet, shiny excrement that attracts bees and ants.

Aphids do very well in the spring and fall months when it is cooler. They become inactive during the summer heat. In the fall they become active again causing exactly the problems you are mentioning, minus much leaf distortion.

If they really become bothersome, try a soil-applied systemic insecticide for roses but please be cautious when applying these “harder” pesticides.

Q: Is it too late to apply magnesium sulfate, sulfur, fertilizer and iron where needed to my roses? I didn’t have the chance to do it earlier.

A: Fertilizer and iron is applied just before new growth in the spring so I would apply that in about mid-January to the roses. Applying it now is wasteful. The fertilizer and iron I am talking about is applied to the soil, not a liquid applied to the foliage. Of course, liquid iron and fertilizer for foliar applications is applied when leaf growth is present.

Magnesium sulfate, which is found in Epsom salts and dolomitic limestone, is also applied just before new spring growth. Many rosarians believe that magnesium sulfate is important in the growth of so-called “magnesium hungry” plants such as roses, potatoes, tomatoes and peppers.

I have always thought that most of our soils here are rich in magnesium (as well as our tap water that originates from the Colorado River) and so I have never been a proponent of its use here. That being said, many rosarians here in the valley swear by it for their roses. When it comes from people who grow award-winning roses, that kind of information should not be discounted. Certainly it will cause no harm if you use it.

As far as sulfur is concerned, it’s a different story. Sulfur could be applied for a couple of different reasons. First, sulfur is an essential plant nutrient; plants would die if they didn’t get it. However, there is plenty available for plants in most soils and applications are usually not needed for that purpose.

The other reason for applying it is to help lower the soil pH or reduce its alkalinity. When sulfur dissolves in water and is broken down by certain sulfur-loving bacteria, the reaction can be quite acidic. This helps to counteract our very alkaline soils.

For this to happen, the sulfur must be in the form of pure sulfur (not in the form of sulfates), easily dissolved in water (very finely ground), and the bacteria have to be active (warm, wet soils). Applying sulfur in large chunks or granules on cold soils will have no impact until the sulfur dissolves (may take years) and the soil is warm and wet.

You can get the same lowering of soil pH with the decomposition of organic matter like wood surface mulches and compost. In fact, it is better than just plain sulfur because of all the other nutrients released.

Q: I have two palo verde trees that are about 15 years old and stand about 15 feet tall. Sap is oozing from several blisters on larger branches where branches were pruned several years ago. Can they be saved by removing the affected branches?

A: Most likely the sap is oozing due to sun damage resulting from the pruning. Palo verde trees have a very thin bark that is easily sunburned if pruned in a way that allows the sun to shine directly on the trunk and limbs all day long.

The biggest mistake people make in pruning palo verde is removing the lower branches that shade the trunk and limbs. This results in the sun scorching the newly exposed thin bark.

Another mistake made is to open the canopy of the tree for better light penetration. This results in sun damage to previously shaded limbs. A natural response of plants to damage is to wall off the damaged area. Bubbles of sap can be released near the damaged areas in response.

Damaged limbs can attract boring insects, which can cause further dieback. Increased dieback opens more of the canopy and allows more light to penetrate thus causing more sunburn and subsequent attacks by boring insects. If not careful, the tree damage can spiral out of control because of the cycle of sun damage, branch dieback, open canopy, more sunburn, more branch dieback, etc.

Of course, the obvious answer is to be extremely careful when pruning palo verde and not remove lower limbs unless their removal will not cause any sun damage. Also do not thin the canopy excessively.

I am not sure what you can do to interrupt this cycle except to encourage as much new growth and shade on these limbs as possible. Palo verde will respond to fertilizer and increased watering at the time it is growing. Fertilize the trees in very early spring and increase the amount of water to the tree in hopes that it will shade itself. Be careful not to water this tree too frequently as it will suffer when watered too often.

Bob Morris is an associate professor with the University of Nevada Cooperative Extension. Direct gardening questions to the master gardener hot line at 257-5555 or contact Morris by e-mail at morrisr@unce.unr.edu.