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PLANT HEALTH CARE The Science of Systemic Bark Sprays By Brandon M. Gallagher Watson S ystemic applications were a revolution in plant science when they became available in the mid-20th Century. For trees, we most often think of systemic treatments as those applied to the roots.This is usually performed by applying a product within a foot of the trunk where the fibrous roots absorb it and move it up the vascular system to the canopy. Similarly, tree injection treatments place the treatment directly into the vascular system through the root flares and the treatment goes up from there.Within the last few years, a third option — systemic bark spray applications — has been gaining popularity with arborists. How does this method work? Is it as effective as other application methods? And, perhaps more importantly, why have so many tree health care professionals been adopting this method for their client's trees? First, let's step back and look at systemic treatments in general. Systemic treatments are any applications that utilize the vascular system of a plant for distributing the treatment to other parts of the plant. Systemic treatments are the go-to treatment option arborists most often employ for managing a number of different tree health issues. Although they are widely used in every area of horticulture and agriculture, systemic treatments are especially valuable to tree care as the height, locations in the landscape, and long windows of susceptibility make many other treatments difficult in practice. Systemic treatments have many advantages over other modes of action. Chemicals that are known as "contact treatments" are quite effective, but This photo shows a tree treated for Emerald Ash Borer using a bark spray application (dinotefuran + water) on the left, compared with an untreated tree on the right. Photo provided by Rainbow Treecare Scientific Advancements 14 Arbor Age / May/June 2013 require the pest to be present at the time of treatment. If, for example, you spray a contact insecticide for Japanese beetles, it will kill any beetle that the spray touches. It will not, however, stop any that were on the underside of the leaf if you did not hit directly, nor will it control any that land on the tree a week after you sprayed.With a systemic application, the treatment is contained inside the tree, and, if timed correctly, the protection for the tree is already present when the first pest arrives. Additionally, once they have been absorbed, these treatments cannot be washed off by rain or broken down quickly by exposure to sunlight.This not only provides longer windows of efficacy, it also reduces the total volume of chemicals that are released into the environment by eliminating the need for multiple applications. So taking advantage of the vascular system is a good thing,but how do we get the systemic treatment into the vascular system? As mentioned earlier,roots and drilling holes directly into the root flare or trunk are common, but spraying certain types of treatments directly on the bark of a tree is an option as well.Those new to this method are skeptical that this could work, often because of long-standing misbeliefs on the structure of a tree. In dicot flowering plants and conifers, bark is basically anything on the outside of the vascular cambium.Without going into too much detail on tree anatomy, bark can basically be thought of as a hard, dead protective tissue whose primary function is defense from pests, fire, physical damage, and the sun. However, interspersed throughout that protective tissue are myriad little pores, known as lenticels. Every one of these tiny pores on the surface of the bark is linked directly to the flow inside the tree — no different than the pores on your skin.And, taking that analogy further, just as there are many medical treatments applied directly to human skin that absorb though our pores, bark spray treatments work essentially the same way. Bark spray treatments are not considered "new"in tree health management. Spraying the bole of a tree has been the primary defense against bark beetles on conifers for a number of years.Those treatments absorb through the lenticels and kill the insects feeding directly below the bark. Newer chemistries, such as dinotefuran (Transtect, Safari) have the ability to not only move through the lenticels into the tree, but can move systemically to the rest of the tree.This allows an applicator to simply spray the trunk, from about eye height down to the base, and protect the entire tree. So, do they work as effectively as applying a treatment to the soil or directly injected into the vascular system? Systemic bark spray applications began to be evaluated in the mid-2000s as a management option for emerald ash borer in several different studies. In head-to-head trials of www.arborage.com