Research ArticleArticles

Principles of Eradicative Pruning

Pavel Svihra
Arboriculture & Urban Forestry (AUF) September 1994, 20 (5) 262-272; DOI: https://doi.org/10.48044/joa.1994.20.5.262

Abstract

Excision of infected or infested branches from woody plants was recommended as a control tactic more than 150 years ago. The term “eradicative pruning” was first introduced for Dutch elm disease control. Eradicative pruning can eliminate local and regional branch (limb) infection or infestation to prevent further spread of a pathogen or pest in the tree. When combined with other chemical and cultural treatments, eradicative pruning fits well to the concept of Integrated Pest Management (IPM) or Plant Health Care (PHC). Timing of pruning, severity and extent of symptoms, location of pathogen or pest attack, stage of infection or infestation, and contamination of pruning tools all influence the success of eradicative pruning. This report presents a guide for selected disease and pest attacks that can be successfully eradicated from woody plants in California.

Pruning of trees is one of the oldest plant treatments (27). Excision of an infested or infected branch from a tree was first recommended as a control tactic in 1841 by T.W. Harris (14). The success of excision as a tree cure varied greatly, mainly because the life histories of insects and pathogens were not well known and, additionally, because appropriate chemicals were not available. Ironically, with the advent of synthetic insecticides and fungicides and a high reliance on their use after 1945, excision as a control tactic was abandoned. In 1972 Brown (5) revived pruning as one of the “weapons of defense in the fight to prevent the spread of 21 pests and diseases.” But he warned that many factors would influence the effectiveness of such pruning.

The public is now demanding decreased use of chemicals to control pests and diseases in the urban forest ecosystem (23). A great opportunity thus exists for arborists to specialize in eradicative pruning to control local insect infestations or pathogen infections in landscape plants. The variety of pruning tools, the arborist’s climbing skills, hydraulically operated aerial lifts capable of carrying the operator near the symptomatic branch, and advances in tree care and proper pruning cuts (27) have made it possible to save many trees by eradicative pruning.

Evolution of “Eradicative Pruning”

Several decades of experience with the pruning of elm branches infected by the Dutch elm disease (DED) fungus, Ceratocystis ulmi, clearly demonstrate the concept. At the National Shade Tree Conference in 1944, Zentmyer and Wallace (33) reported the survival of American elms, Ulmus americana, after branches infected with the DED fungus had been pruned. May and Douglas reported similar observations (25). More careful studies of the progress and remission of fungal infections in diseased elms disclosed that pruning of symptomatic branches in some cases stopped and in others greatly retarded further progress of C. ulmi (24). In 1976 Campana (6) established that the success or failure of pruning was related to the length of the discoloration in the xylem. Since then pruning as a control technique has received serious attention by plant pathologists and arborists (16). Later, in 1978, Campana (8) defined this specialized control tactic as “eradicative pruning” and concluded that the subsequent survival of an elm heavily depended on the population dynamics of the pathogen’s vector, the European elm bark beetle, Scolytus multistriatus.

Despite these major advances in determining why some cuts were successful and others were not, it soon became clear that excision of branches alone had a limited effect on recovery of trees infected by the DED fungus (7,16). Researchers (12,18) took the approach that eradicative pruning could not be blamed for failure when the discoloration traced beneath the bark had spread beyond the pruning cut (e.g., the infection had penetrated into the main trunk) or, in some cases, the progress of the pathogen was invisible in the tissues as a stain. They hypothesized that an additional therapeutic procedure had to be employed to affect the pathogen beyond the cutting area and the survival of the elm.

Gregory (12) first injected a fungicide into American elms that had 10 percent or less of the foliage manifesting DED symptoms and then one or two weeks later removed each symptomatic limb beyond the discolored wood whenever possible. After pruning, 71 to 79 percent of the trees survived. However, if discolored wood could not be pruned to clearwood, only 45 to 61 percent of the elms survived.

Kondo (18) recommended cutting flush at the limb origin, followed by root injection. His results showed that if the tree possessed an acceptable rating index to become a candidate for eradicative pruning and fungicidal injection (about 5 to 10 percent of foliar symptoms) and if it was injected annually, it would remain free of foliar symptoms and have a healthy external appearance. Only elms with less than ten percent foliar symptoms were candidates for the pruning and therapeutic treatment.

Such experiences with this deadly disease of shade trees suggest that eradicative pruning has a great potential for removing infection, but it requires more than just cutting off the symptomatic branch. Eradicative pruning is an ecologically demanding control tactic, because it:

  • – may inadvertently increase the disease or insect threat by cutting into healthy living tissues, allowing reinoculation of the pathogen if the open wound becomes attractive to bark beetles (2);

  • – requires knowledge of the tree’s biology;

  • – requires knowledge of the pathogen’s life cycle and that of its vector, S. multistriatus;

  • – requires assessment of the visible external foliar symptoms as well as the extent of internal vascular browning that quantitatively and qualitatively determines a candidate tree suitable for treatment; and

  • – must include other preventive techniques and cultural practices to boost a tree’s defenses and its chances of survival.

The word “eradicative pruning” has entered the vocabulary of arboriculture but the arborist has not always “eradicated, wiped out, eliminated, destroyed, or exterminated” the Dutch elm disease fungus from the tree. As studies by Shigo (26) have shown, a susceptible species like the American elm can confine and survive an infection of the tissues, with the DED pathogen remaining alive in the system. Such examples are well documented from California (29). “Eradicative pruning” in the arboricultural context is hard to define and even harder to use well. It is doubtful that the term could be described in any other way that would make it more useful in practice, e.g., therapeutic pruning or tree surgery (4,9).

While the primary goal of eradicative pruning is the complete elimination of local or regional branch infection or infestation to prevent fu rther spread of the pathogen or pest in the tree, the goal of this technique must be to boost plant defenses through a combination of cultural and chemical treatments. Eradicative pruning deals with the situation in a particular tree and has little or no effect on the disease or pest population in the urban forest ecosystem where the tree is growing. For this reason, the arborist must consider the influence of many other factors alluded to by Brown (5), which can affect pruning success.

Sanitation vs. Eradicative Pruning

Sanitation is also designed to eradicate known sources of disease and insect breeding by destroying host material where either one can reproduce, take refuge, or overwinter. In horticultural and fruit tree crops it has been particularly effective through destruction of tree pruhings (11). Sanitation is a continuous operation around the plant—raking leaves from under plants susceptible to the Entomosporium leaf spot, pruning of dying and dead branches from elms, disposal of elm firewood to reduce breeding sites for S. multistriatus, and the like.

While very important, sanitation practices eradicate neither the causal fungus nor the insect vectors and may reduce their populations only nominally (9). But if the arborist were to perform successful eradicative pruning on an elm displaying the very early stage of DED infection but neglected sanitation by leaving dead branches on the same tree or others in the vicinity (where the insect vector can breed and the DED fungus survives in the saprophytic stage), such an effort would almost certainly be futile.

Sanitation addresses dieback and known breeding sites that began or were established in the previous growing season and their elimination may lead to the killing of many individuals in the population, but remnants remain around the plant. Eradicative pruning responds swiftly to the just-developing symptoms on twig, branch, or limb to excise them and stop the progress of dieback.

Eradicative Pruning as an IPM or PHC Tool

Eradicative pruning is compatible with IPM or Plant Health Care (PHC), because it affects local infection or infestation in a tree, uses pesticides locally or not at all, and requires the concerted application of several cultural practices. Knowledge about the development of wound resistance to infection has practical IPM consequences, e.g., as wounds age, the tissues become increasingly resistant to infection (22).

The dynamics of wound resistance or susceptibility to reinoculation by the pathogen (or insect infestation) is an important consideration for establishing the period during which prophylactic sprays should effectively protect wounds (3,10). As infection or infestation is prevented by these sprays, the induced natural production of plant defensive chemicals (lignins, phenols, tannins, alkaloids, amino acids, terpenes, etc.) is fully developed (21). The tree thus becomes less predisposed to an attack as the wound ages and the sprayed chemical becomes ineffective. What is required is accurate information about the tree or shrub, an understanding of the pathogen or insect life cycle and interaction with the host, the diagnosis, and corresponding treatment(s) timed according to phenological periods of the plant and pathogen/pest.

Phenologies (27) of both plants and associated organisms are important and the role of the arborist is to take action at the time when the tree benefits the most, while the destructive activity of the pathogen or insect is at a minimum. Such favorable conditions are not always achievable when the life of the plant is threatened and eradicative pruning must be done without delay. A pathogen or insect’s most destructive effect on a tree’s health occurs during the following phenological periods: onset of growth, storage of energy, and dormancy (27). The arborist’s job is to consider phenological periods, collect appropriate data about tree management and life history of the pathogen/insect, assess the magnitude of infection or infestation, and then organize all the above information for optimal long-term IPM or PHC practices to benefit tree defenses and survival.

Some Practical Aspects of Eradicative Pruning

Case studies: In an 11-year-old apricot tree, dieback occurred in eight branches, each of which was less than 1 inch in diameter scattered in the canopy. The homeowner noticed the symptoms for the first time in the previous growing season. Each instance of dieback was associated with several rough cankers and amber-colored gum (Fig.1). The causal pathogen was bacterial canker, Pseudomonas syringae.

Figure 1.
Figure 1.

Branch infected with Pseudomonas syringae.

For diagnostic purposes a cut was made 2 inches below the canker next to the lateral. Staining was still quite visible near the cut line (Fig.2). The third cut 6 inches below the stain eradicated the infection (Fig.3). Unfortunately, this cut was done in the dormant season, not in the summer when conditions for bacterial re-infection are least favorable in California’s hot and rainless summer.

Figure 2.
Figure 2.

Pruning cut made two inches below the canker has not eradicated infection. Discolored-infected wood is clearly visible at the pruning cut.

Figure 3.
Figure 3.

Cut made seven inches below the canker, next to the lateral, eradicated infection from the branch.

In this case we only partially fulfilled the requirements of eradicative pruning by gathering information about the tree species and manifestation of the disease symptoms, consulting the diagnostic laboratory to determine the causal pathogen, and executing appropriate treatment.However, by pruning in the dormant season we disregarded the pathogen’s life cycle and related proper time of pruning that favors the host’s defense and not that of the pathogen.

The techniques of eradicative pruning cuts are nearly identical to those of general pruning except that, by eradicative pruning the arborist traces the pathogen or pest to excise an infection or infestation to prevent a tree’s death or further progress of branch dieback. This technique differs from other types of pruning, such as for maintenance, health (sanitation), size control, flowering, fruiting, and vigor (15), in which predictable tree growth, form, and habit are the factors and not a life-threatening pathogen or pest. Thus, the recommendation of timing for these other types of pruning (15,19,27) must be adjusted to the life history of the pathogen/pest threatening the tree in the area. For example, spread of certain fungal or bacterial diseases is reduced by preventing branch crowding. The dormant season is the best time to distinguish and correct overcrowding by thinning the tree canopy, but at that same time the bacterium P. syringae is active in some deciduous trees in California. In contrast, if done in the summer, thinning will stimulate growth of succulent new shoots susceptible to pathogen infection (19). Nevertheless, canopy thinning to prevent branch crowding must be done in the summer when there is minimal risk of rain that helps to spread P. syringae in California.

The arborist must objectively evaluate the severity and extent of symptoms and then assess whether pruning can succeed while also preserving an acceptable aesthetic appearance. Location of infection is also very important. Eradicative pruning cannot benefit Monterey pine, Pinus radiata, infected on major stems with western gall rust caused by Endocronartium harkenessii. But if one or two infections are detected on lateral branches, eradicative pruning will be successful.

Another major factor is the stage of infection or infestation. Clearly, if all major limbs of white alder, Alnus rhombifolia, are infested by the flatheaded borer, Agrilus burkei, it would be pointless to prune such a tree for both aesthetic and health reasons. For example, in an Alamo, California experimental site with 28 white alders infested by A. burkei, 14 alders were selected at random for eradicative pruning treatment (30,31). In one instance (Fig.4), the alder was infested on 64 places, requiring the removal of 29 branches in the winter (Fig.5). Prophylactic sprays by Dursban 4C prevented further beetle attack. However, I disregarded that the white alder is susceptible to sunscald. Such radical opening of the crown (Fig. 5) brought sunscald damage to exposed limbs and the main trunk. Drawing from this experience, the arborist is advised to shield the post-prune exposed trunk and limbs with a shade cloth on their southwestern sides.

Figure 4.
Figure 4.

Flatheaded borer infested 29 branches of white alder.

Figure 5.
Figure 5.

Eradicative pruning maintained natural character of white alder, but it radically opened canopy, making the tree susceptible to sunscald.

Each woody plant species responds differently to pruning cuts through which the pathogen can re-enter the plant system (3). Also, host volatiles attractive to some borers are released by pruning (2). Appropriate planning for a wound treatment is essential, because

a) Open wounds become attractive and can predispose the tree (such as elm, pine, alder, ash, birch and eucalyptus) to attack by boring insects. Spray the wound and its vicinity with insecticides to prevent further attack by borers.

b) Open wounds are an invitation for re-infection by a pathogen via contaminated tools or naturally when climatic conditions favorable to the pathogen develop. Prevent inoculation of a disease organism into pruning wounds by sterilizing cutting tools (bleach, Physam, 75% alcohol, or alcohol-based Lysol), and spray pruning wounds with a fungicide or bactericide when recommended by a plant pathologist.

c) Tree care should promote tree defenses and rapid wound closure (“healing”). Water deficits may impair wound healing in bark (3). Vigorous trees accumulate sufficient carbohydrates to heal injuries and maintain physiological processes at levels necessary to sustain life (20). In many cases, tree care will require ancillary sprays of insecticides or fungicides to control other fungi or insects that may subsequently attack the tree treated with eradicative pruning. For example, spraying the canopy of Siberian elm, U. pumila, is justified after a branch infected by the DED fungus has been removed in the spring, if the arborist detects even a low infestation of the elm leaf beetle (ELB), Xanthogaleruca luteloa. ELB can complete two to three generations annually, resulting in partial or complete defoliation by the end of thegrowing season. Such loss of leaves reduces production of carbohydrates that elms use in their defense against invasion by pathogens and insect attacks (20).

Pre-planned visits to assess tree responses to eradicative pruning and to cultural practices aimed at boosting plant defenses must be incorporated into the post-treatment program.

Eradicative Pruning for Control of Some Diseases and Insects in California

Tables 1 and 2 provide information about some destructive disease and insect problems in California that can be controlled by eradicative pruning. It is hard to develop a guide, since so many variables can affect execution. For example, one Quercus kelloggii, infected by the leafy mistletoe (Fig.6) required the removal of only eight branches to eradicate the parasite from the tree. In contrast, if the infection has been neglected long enough to allow its spread to all major limbs of Modesto ash, Fraxinus velutina ‘Modesto’, the result is not eradicative pruning but undesired maiming of trees which then become an aesthetic disgrace in the neighborhood (Fig 7 and 8).

View this table:
Table 1.

Fungal and bacterial diseases that can be controlled by eradicative pruning.

View this table:
Table 2.

Insect infestations that can be controlled by eradicative pruning.

Figure 6.
Figure 6.

An initial stage of infection by leafy mistletoe in black oak.

Figure 7.
Figure 7.

Modesto ash heavily infected by leafy mistletoe.

Figure 8.
Figure 8.

This is absolutely not eradicative pruning.

The guide (Tables 1 and 2) highlights important aspects of the biology and natural history of the host plant and the pathogen or insect as well as timing of eradicative pruning described in a variety of sources (1,6,8,13,17,28,30,32). However, it will be the arborist’s experience and knowledge of local conditions that effect a successful application of eradicative pruning.

Acknowledgment

I thank Dr. Carlton S. Koehler, Entomologist Emeritus, University of California, Berkeley for his helpful critique of the article.

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Principles of Eradicative Pruning
Pavel Svihra
Arboriculture & Urban Forestry (AUF) Sep 1994, 20 (5) 262-272; DOI: 10.48044/joa.1994.20.5.262
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