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Aleppo Pine Blight - Treating the Mites

 

Egg cases at leaf base                     Close up of egg cases                  Close up of mite & egg

Aleppo pine blight is a serious disorder that has devastated many of our largest pine tree specimens in the low elevation deserts of Phoenix, Las Vegas, San Diego, Palm Springs and Tucson. Until recently, this disorder was thought to be a stress-related condition with no suitable remedy. However, as described in last month’s issue of Southwest Trees and Turf (April 2004), the culprit behind Aleppo pine blight now appears to be a mite that can be controlled. These mites colonize small stems, burrow into needle bases and feed on needles that eventually dry out in large clusters that are randomly dispersed throughout the tree crown. In this article I describe our efforts in treating these mites and controlling the blight.

Mite Diagnosis

Before embarking on a treatment, it is vital to first verify the presence of mites (see photos above & below). When viewed from a distance the needle blight caused by the pine mites superficially resembles other disorders. We routinely use a dissecting microscope in our examination because the Aleppo pine mites are tiny (0.2 mm long) and they do not produce the webs that are typically associated with spider mites. We examine stems, twigs and needles to look for active adults, nymphs, eggs and empty egg cases (see photos). Active mites range in color from orange, yellow and brown to black or dark green. When searching for mites it is best to select branches with dark green or fading green needles instead of dead needles. Active mites typically vacate branches with dead needles and are not a good place to search. With practice it becomes relatively easy to select tissue where mites exist. We have found that the severity of the blight symptoms usually correlates well with the degree of mite infestation.

When viewed up close the needle discoloration from mite damage is distinct from damage by other disorders such as excess soil salts and drought. Under the microscope mite feeding typically results in stippling; leading to the formation tiny, yellow circular spots, that run the entire length of the needle. To the naked eye these needles have a light yellow green color unlike salt damage that results the top half of the needle to turn yellow while the bottom half remains dark green. When viewed under the microscope, salt-damaged needles are not stippled. Drought-damaged needles tend to be gray-green in color, not yellow-green, and are also not stippled. We have encountered cases where salt damage and mite damage occur together. Here stippling is present along with tip yellowing.

Aleppo Pine mite symptoms: Blighted branches are dispersed randomly in salt and pepper fashion; most severe in winter; tree can remain this way for many months


 

vs

Phytophthora symptoms: Blighted branches first appear at top of tree and blight works its way down; most severe in summer
(photo on left shows tree with Phytophthora)                         (photo on right is same tree after 3 weeks)
 

 

Note: Blighted branches are not randomly dispersed in salt and pepper fashionand; blight sweeps evenly through tree crown to kill tree within a few weeks.

The overall appearance of the tree crown is also critical. Damage from mites causes a checkerboard of chlorotic throughout the tree that lasts from December to April. In contrast, damage from root rots, such as Phytophthora, begins by chlorosis at the top of the tree that sweeps down and consumes the entire crown within a matter of weeks, usually in mid-summer. Other root conditions that lead to overall crown chlorosis are soil compaction and root girdling. These usually develop slowly over several seasons and do not kill the tree as rapidly as root rots.

One key feature of this blight is that the denuded stems are often still flexible and viable. This is important for both diagnosing the condition and for restoring the health of the tree because these stems possess buds that can regenerate new shoots. It is vital to not remove these denuded stems unless it is verified that they are dead and brittle.

Pine Mite Treatments

Treatments to control mites can be applied via trunk micro-injections or foliar sprays. Trunk injections are the preferred method for most situations, especially for large trees in confined sites or in heavily populated areas. Most landscape companies do not have the large sprayers capable of the coverage needed. Moreover, trunk injections employ far smaller doses than sprays and they utilize systemic compounds that specifically target plant-feeding pests and avoid killing beneficial predators that keep the mite populations under control. We suspect that the severity of mite damage in the past few years was linked to conditions that favored increased mite populations and reduced predator populations. We also believe that the lack of rain, both in winter and in summer monsoons, led to elevated mite populations simply because rain water tends to wash off destructive mites, their eggs and the dust particles they use for hiding places in the tree. This may explain why dry weather in summer favors the incidence of pine blight.

We have observed good mite control with Abasol (see photo), a Mauget trunk injection product, that contains abamectin, a miticide and debacarb, a fungicide. Abasol is effective yet requires one year to obtain good results as new shoot growth is required to replace the old damage. Effective control is also observed with Abacide 2, a product that does not contain any fungicide. We are not sure if the fungicidal properties of Abasol are essential. Both Abasol and Abacide 2 should offer a good preventative as well as a therapeutic control of mites.

We have also had success controlling the pine mites with foliar sprays of Talstar, both with and without neem oil. This results in a rapid kill of the mites. However, we do not generally recommend using foliar sprays over trunk injections unless certain criteria are met. Although Talstar is not restricted, the public’s perception of any spray application is an issue. We only recommend sprays when the blighted area of the tree exceeds 50% of the crown. This is because green needles are required to draw up the trunk-injected material with water flow in the xylem. If the tree is denuded then little material will be taken in. Another concern is that trees chosen for spraying should be in large, open areas free from people, buildings and neighbors. Most of the trees affected with mites tend to be larger than 50 ft. and require large spray rigs with enough pressure and coverage to reach this height. In confined areas it is often impossible to avoid spray drift. Another problem with sprays is that they act as a weapon of mass destruction that kills all of the insects in the tree, including the beneficial predators that keep the mite pests under control. Without them, over time, the problem could become worse allowing sprayed trees to become re-colonized with destructive mites ahead of the beneficial predators. It is known that the Aleppo pine blight symptoms disappear in the summer and re-emerge in the winter. The reason may be that during high summer temperatures mite predators are greatest keeping the mite populations in check. In winter pine mites become abundant when these predators disappear.

We also caution using dormant spray oils, including neem oil, that are thought to help suffocate existing mite eggs. These should not be applied when temperatures exceed 90 F or tissue damage can result.

Proper Pruning of Mite-Affected Trees

Knowing what we now know, it has been an absolute tragedy to learn that many of our most stately Aleppo pine trees were unnecessarily disfigured or cut down and removed because of Aleppo pine blight. This is like hearing about a beloved family dog being put to sleep because it was suffering from mange. Like Aleppo pine blight the culprit behind mange is also tiny mite that burrows under the skin of susceptible dogs that eventually causes their hair to fall out. How would you feel about amputating a poor dog’s leg because it had mange on his paw? Yet, this is what we’ve seen happen to pine trees where massive limbs were remove because large patches of needles turned brown and fell out. When mangy dogs are treated with a systemic or topical miticide their symptoms usually disappear within a few months and their health is restored with proper nutrition

Likewise, when pine trees are treated with a systemic miticide their health can be restored at fraction of the cost of removing them. We usually also recommend other forms of plant health care treatment that improve soil health and eventually reinvigorate shoot growth. It is important that blighted branches are not removed until after growth resumes, usually in summer. This way we are sure not to remove healthy, viable limbs. It is important to keep as much green wood a possible so that the structural integrity of the tree can be restored once the mites have been controlled.

For many years it was assumed that stress was the sole cause of Aleppo pine blight. While this appears to not be the case, stress most likely contributes to the severity of mite infestations. Soil compaction, drought and excessive pruning can all weaken trees and make them more susceptible to attack by mites.

Using this information as a guide, we hope that most of our wonderful, mature desert pine trees can be preserved from unnecessary destruction in the future.

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