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What is meant by the term “tissue culture”?

In its simplest form, tissue culture (also known as micropropagation) is a technique of plant propagation whereby plants are started from very small pieces under aseptic (sterile) conditions usually on an agar-based medium. Plant parts as small as tiny stem tips, nodes, embryos, seeds or pollen are placed on a special culture medium. The nutrient medium used may vary considerably depending on the growth requirements of the specific plant grown. By varying the medium content, the plant part may be caused to produce undifferentiated callus tissue (which may be further subdivided later on), multiply the number of plantlets produced, grow roots or multiply embryos.

Tissue growing techniques allow gardeners to propagate large numbers of specific clones in a relatively short time in a small amount of space. These techniques can also be useful in producing pathogen-free plants. Tissue culture techniques are widely used by orchid growers and other members of the greenhouse and nursery industry.

We planted 2 linden trees about 5 years ago. They are doing very well, but one of them has suckers that grow every year. By July, they are about 2 feet tall and this is when I have been cutting them off. Is this OK? Is there a better way to handle this? Can the suckers be prevented from growing? Why does one tree have them and not the other?

Occasionally trees may send up sucker growth from mature tissues originating at or below ground level. The best way to prevent recurrence of this is to trim back the suckers as close to the point of origin as possible. Sometimes this may require careful removal of some soil to expose the base of the suckers. Remove the growth by cutting as close to the mature tissue (from which the sucker emerges) as possible, or by twisting and pulling the shoots if they are young and tender. The latter usually provides better control because the latent bud that is causing the sucker is often removed in the process. Once removal is completed, replace and firm the soil.

In some cases, all that can be done is to continue the pruning ritual since no treatment is really available to prevent these suckers from growing. Eventually, some trees will just stop on their own, while others will go on for years…that’s just how nature works.

by Steve Turner

I have a large elm tree in my yard and on one branch the leaves are turning yellow and wilting. Does this mean it has Dutch elm disease? If so, is there anything I can do to save it and what can I do to help prevent my other elms from catching it?

From the symptoms you have described, it does sound like your tree has signs of Dutch elm disease. The yellowing and wilting is called flagging and is one of the first signs of infection. The water-conducting cells in that limb are becoming blocked and the tree can’t supply enough water to keep up with the transpiration (water evaporating from the leaves) in that part of the tree. So, the leaves turn yellow, wilt and then turn brown or drop.

A good way to explain Dutch elm disease, which is a fungus, is to compare it to cholesterol in humans. If your arteries become so blocked that blood cannot pass through them, the organ that is being supplied will fail. The same thing applies with a tree — if the water-conducting tissues become blocked by a fungus build-up, then that portion of the tree will die. All this can happen very quickly under the right conditions.

As far as saving your tree goes, that gets a little more complicated. Dutch elm disease is very hard to control, but it has been done. If the tree is flagging in 5 percent or less of the crown then there is some hope. The first step is to sterile prune out the infected limbs back to healthy wood and then do trunk injections with a systemic elm fungicide at a curative rate. There are several brands on the market. In my experience, I have found Arbortech and Alamo to work the best. Then water, water, water! Your mature tree needs a lot of water since its ability to transport water has diminished. This is especially true during the summer heat which causes the tree to transpire at a much higher rate than in cooler weather. It will need a constant supply to keep up with the increased demand. This is why your tree looked fine in the spring but is now showing the signs of infection. Chances are, your tree has been infected for a while but up until now has not faced a strong demand for water.

In regards to your other elm, there are three ways that elms become infected with Dutch elm disease: 1) by elm bark beetles feeding on diseased and then healthy elms, 2) through pruning with contaminated saws and loppers, and 3) through root grafts connecting elms together by their root systems below the ground. The first way is the main cause of the problem, but the second happens more often than people realize. Look at elms near power lines that have been pruned in the last couple of years; many of them are either dead or infected because the fungus was transferred from one to another. The third is common in neighborhoods with smaller yards or where the elms are planted closely. When one elm becomes infected, the others around it can start dropping like flies. In your case, this is probably the biggest threat to your other elm.

I recommend that owners of large, specimen elms have their tree injected every other year or every year if other trees in your neighborhood are infected. The cost of the injections is less expensive than the removal of a large, mature tree, which can cost up to several thousand dollars depending on the size and location of the tree. The bottom line is to keep your elm in good health through proper watering, fertilizing, pruning, and, on large trees, injecting. Also, to increase the chances for a prize elm’s survival, you may want to remove any small elms in the area, as they seem more prone to infection, especially if they are near power lines and subject to pruning. For those of you who would like to plant an elm, the American Elm Society (800-FOR-ELMS) has a disease resistant variety called the Liberty Elm. They are, however, hard to find in larger planting sizes, but free saplings can be obtained by joining the society, a small price to pay for the beauty of the great American elm.

by Steve Turner

Over the last couple of years, my Norway maple seems to be getting thinner, getting its fall color early, and some limbs are dying on one side of the tree. What’s wrong?

Unfortunately, the situation does not look good. From what you have described, it sounds as if your tree has girdling roots, which is a major problem of Norway maples and ‘Crimson King’ maples, a cultivar of the Norway maple. Many of these trees develop the problem between 25 and 40 years of age, which is also when the tree starts to mature and become the picture perfect tree we all admire. If you look closely at the base of the tree, you will see that instead of your tree having “root flare” as it goes into the ground, it looks more like a telephone pole going straight into the ground or it has a flat side instead of being round. These are sure signs that the roots below are wrapped around the trunk and are causing a slow, but certain death by strangulation for your otherwise perfect maple.

The reasons for this are a little more complicated. The problems begin in the nursery when the trees are dug for transplanting. The process of digging severs tree roots, which in most trees is no big problem. But with Norway maples, this causes a major change in the way the root system develops. In the tips of the roots is a growth regulator that causes all the roots behind the tip to remain smaller than the main root which extends away from the tree. When this root tip is severed, the tree loses its ability to control the size of the lateral roots, and that is where the trouble begins. The lateral roots become dominant and cross the trunk of the tree causing the restriction that eventually brings the demise of the tree. It’s no coincidence that the size of the trunk at the time of decline is usually the size of the original root ball at the time of planting, which is why it takes so long to discover the problem.

Unfortunately, there is no cure. Fertilization will only speed up the growth of the tree and cause it to die sooner. Cutting the roots is expensive and is not a permanent solution; they will grow back in time and then you are back to square one. Keeping the tree well watered will help prolong its life, but it is not a cure.

Steve Turner, Certified Arborist, is from Arboricultural Services in Ferndale, MI.