Slime flux is an old problem that can be commonly seen now.

An update on the pests and diseases of trees these days is threefold. As you might expect, a review of the current pest problems is in order, as well as a look ahead to try to anticipate future problems. In addition, a trip down memory lane to look at “year in and year out” problems and either an expansion of existing maladies or ones that will start anew is a good approach. After all, it may have been a few years since you’ve read much about these older problems; hopefully this will serve as a good reminder.

The old

Slime flux is a disease that has been around for many years. It’s best known from the symptoms of vertical stains that run up and down the main trunk of a hardwood tree. Trees infected with slime flux (also known as bacterial wetwood), are easily distinguished by the columns of bleached or lighter-than-normal colored bark; often these columns originate from wounds in the bark or from where lower limbs have been removed in recent years. Trees affected with the greatest frequency include Siberian elm, silver maple, mulberry and hackberry. Occasionally, white barked birches, Osage orange and cottonwood are also affected.

The exudate that is emitted from a tree infected with slime flux causes two problems: it clogs the conductive vessels and stains the bark, causing significantly lower aesthetic appeal, at least in the minds of many property owners. A third outcome that is not always expressed is that it can kill the grass where it falls on the ground. At times, a bit of a foul odor can be detected as well.

The good news about slime flux is that it rarely causes immediate death of the tree. Most of the time, affected trees live for many years after initial infection, especially ones that are fortunate to be growing in landscapes with non-compacted soils, adequate absorptive surface and nutrient/pH ranges that fall into the normal range for the tree species. The bad news is that there are no effective means of treatment other than implementing sound cultural practices that keep the tree in overall good health to minimize the effects of the diminished vascular system.

An old-school way of solving slime flux is to drill a hole into a tree through the sapwood and into the heartwood as a means of facilitating drainage of the exudates. In some scenarios, copper tubing was installed into the hole in an attempt to provide permanence to the treatment. Unfortunately, these attempts did little to lessen the effects of the disease and are no longer recommended.

Sunscald is another malady that has been causing tree service workers (and the rest of the green industry) fits for a long time. Fluctuating temperatures on exposed bark surfaces in winter provide the trigger for sunscald damage. Sunscald can stem from several causal agents. First, physical scarring at the root plate (otherwise known as mower blight) causes a physical separation of bark and sapwood that results in deterioration of the xylem and phloem and major stress on the tree. Soil compaction, soil disturbance and drought stress can predispose trees to this problem. Properly installed mulch near the base of the tree goes a long way toward the prevention of this cause.

Winter warming of the bark during the day followed by rapid cooling overnight is a significant force indeed, as the leaves are not present to protect or mitigate the force of the sun. Thin-barked trees such as Norway maple are more susceptible than others such as oak and black walnut. Susceptible trees can be effectively protected with installation of a light-colored column of PVC tubing. This material also helps prevent damage from critters like rabbits and mice in winter. The drawback to using the tubing is that it may be forgotten and girdle the trunk if left in place too long.


Typical chlorosis symptoms on pin oak.

What trip down memory lane would be complete without good old iron chlorosis. When you look up at a tree’s leaves and the most prominent sign or symptom is a lime green leaf with dark green veins, you’re well on your way to a diagnosis of chlorosis. Chlorosis occurs most often when the preferred pH range of the tree doesn’t match up with that of the soil it is growing in. Many trees can be affected by chlorosis, including pin oak, silver maple, birch and willow.

A popular method of treating chlorosis is to inject micronutrients such as manganese and iron into the cambium and sapwood using a drilling technique into the root flare. These treatments are generally most effective in midspring, when sufficient sap flow occurs to facilitate movement of the nutrients into the conductive vessels. Less-invasive methods of treatment include aeration of the lawn surrounding the tree followed by topdressing application of various nutrients and the placement of columns of NPK fertilizer and micronutrients to encourage uptake and lessening of the effects of chlorosis. Each method has advantages and disadvantages.

The new (or relatively new)


Spiraling whitefly
PHOTO BY DAVID CAPPAERT, MICHIGAN STATE UNIVERSITY, BUGWOOD.ORG

Rugose spiraling whitefly – This pest is quickly becoming a concern across Florida and other southern states, causing damage to trees as well as landscape elements underneath the canopies. Like many other sucking insects, it excretes honeydew, which later solidifies and hardens like sap. The residue poses a dual threat in that it is difficult to remove from cars, children’s play equipment and other items, and it serves as the food for an unattractive black mold.

Emerald ash borer – Most pests and plant pathogens are opportunistic in that they take advantage of favorable weather conditions or weakened plants. This is not the case with these culprits. Because emerald ash borer (EAB) larvae tunnel inside the tree, hidden from view, it is often difficult to detect their injury. This is especially true for newly infested trees.


Emerald ash borer
PHOTO BY DEBBIE MILLER, USDA FOREST SERVICE, BUGWOOD.ORG.

One sign of EAB infestation is the presence of jagged or irregular holes in the bark, the work of woodpeckers seeking food. After a year of infestation, the D-shaped exit holes will be present on various trunk and lower limb bark surfaces. Because of disruption in the cambium layer, the bark may split vertically above larval feeding galleries. Removal of this bark will reveal the characteristic S-shaped galleries filled with frass. It is common for the sapwood to be discolored as a result of secondary infection from fungal pathogens in these locations.

Control of EAB is not an easy matter. However, by increasing species diversity in the landscape and the judicious use of insecticides, an acceptable level of EAB control can often be achieved. In a landscape setting, university research has shown that insecticidal control of EAB is also possible using bark and canopy sprays (bifenthrin, carbaryl, cyfluthrin, dinotefuran, permethrin), soil injections/drenches (imidacloprid) or trunk injections (acephate, dicrotophos, emamectin benzoate, imidacloprid).


Eastern ash bark beetle
PHOTO BY DAVID CAPPAERT, MICHIGAN STATE UNIVERSITY, BUGWOOD.ORG

Japanese beetles – Not so new to much of the U.S., but relatively new to certain areas, Japanese beetles are heavy-duty pests of turf and ornamentals. This pest has a wide host range and feeds in the larval and adult stages. Easy to identify because of the shiny green wing covers, Japanese beetles are leaf feeders in the adult stage, skeletonizing trees such as crabapple and cherry. All forms of rose (climbing, tea, rugosa, miniature and shrub) are a preferred shrub host.


Japanese beetles are easy to identify in the adult stage.

Japanese beetles are difficult to control in the adult form due to their nearly impenetrable wing covers and good flying ability. Larval forms are much more susceptible, especially in turf, where most common insecticides applied for annual white grubs offer a reasonable degree of control.


Tent caterpillar
PHOTO BY MILAN ZUBRIK, FOREST RESEARCH INSTITUTE – SLOVAKIA, BUGWOOD.ORG

Mountain pine beetle – The mountain pine beetle has killed millions of acres of pine trees in the western U.S. and portions of Canada. Scotch pine is highly susceptible to the beetle; Ponderosa and Austrian pines are less susceptible. Mountain pine beetles infest pine trees from July through October, leaving marble-sized masses of resin called pitch tubes on the trunk. Adults (beetles) are black, about .25 inch long. Females lay their eggs in the tunnels between the bark and wood. Larvae hatch from the eggs and tunnel in the tree until July of the following year. These larvae can kill the tree if sufficient numbers are present.

To reduce the risk of the beetles spreading to nearby pines, heavily infested and dead trees should be removed and either chipped or burned before the end of May. High-value and recently infested trees with only a few pitch tubes on the trunk can be treated with insecticide trunk sprays containing bifenthrin, carbaryl or permethrin. Sprays should be applied in the late spring but before mid-June.

Thousand cankers disease – Caused by the fungus Geosmithia morbida, thousand cankers disease has been on the rise in many western U.S. states. The fungus is spread by the walnut twig beetle, a tiny black beetle that feeds under the bark. The beetle and the fungus are readily transported in infested walnut wood. As a result, quarantines are being set up to limit movement of the wood from state to state in an effort to contain infestations locally.

Thousand cankers disease symptoms include yellowing foliage followed by brown wilted foliage, branch dieback and tree death. Black walnut is highly susceptible, while English walnut, butternut and other walnut species show varying degrees of susceptibility. Pecan and hickories are resistant. Tree death occurs two to three years after initial symptoms appear. Trees may be infected for many years without showing visible symptoms, eventually dying as a result of the collective effects of many individual cankers, hence the name of the disease. Unfortunately, at this time there are no effective chemical controls.

John C. Fech is a horticulturist and certified arborist located in Omaha, Neb.
The author gratefully acknowledges the assistance of Laurie J. Stepanek, Nebraska Forest Service, with the preparation of this article.