Mounding soil to avoid infestations of dogwood borer in apple

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.        

We began a study in our IPM Apple System work at Clarksville in1998 to determine the potential of mounding soil compared to insecticides to control dogwood borer in apple. The paper, authored by Dr. Gut, et. al., published in December 2005 (Gut, L.J., P.H. McGhee and R. Perry. 2005. Soil Mounding as a Control for Dogwood Borer in Apple. HortScienc. HortScience 40(7):2066-2070), recently was awarded the most outstanding Extension paper published in the three American Society for Hort Science journals for 2005 (note abstract below).

In an MSU Fruit Area of Expertise meeting this past winter, I discussed the prospects of recommending the mounding practice to apple growers in Michigan. In the meeting where I discussed the recommended protocol, a question arose regarding the consequences of exposed roots and subsequent herbicide damage. This past week I had a chance to inspect trees in orchards at Clarksville in our research plantings where we have been making applications of herbicides over several years. In all cases, phytoxicity is not apparent and in some cases adventitious shoots (suckers) have sprouted from exposed roots without indication of injury. See the photos of representative trees for review. During our discussion, I speculated that we should not see injury since exposed roots which develop secondary growth develop a cortex and bark tissue not unlike that of stem tissue. Obviously, young roots, like succulent new shoots, are subject to herbicide injury, but usually are covered by soil.

Abstract from the HortScience paper

“The relationship between the extent of burrknots on apple rootstocks and dogwood borer [Synanthedon scitula Harris] infestation, and the efficacy of a cultural management strategy for this pest were studied in heavily infested plots at the Michigan State University Clarksville Horticulture Experiment Station. Spearman rank correlation Rho values of 0.85 and 0.75 in consecutive years of the study substantiated a strong positive correlation between the number of larvae present in the rootstock and the surface area of the rootstock covered by burrknots. Cultivar type affected the level of the dogwood borer infestation in the rootstock. Larval densities were 8- to 11-times higher in Mark rootstocks when the grafted scion was ‘Idared’ instead of ‘Liberty’. This cultivar related difference in larval infestation was associated with a greater number of burrknots on ‘Idared’/Mark compared to ‘Liberty’/Mark trees. Mounding of soil to cover the exposed rootstock was found to be a highly effective alternative to insecticides for dogwood borer control. Under conditions of heavy pest pressure, this cultural control tactic provided 76 to 99% reductions in larval densities. ‘These levels of control are comparable to or better than those reported for trunk sprays with chlorpyrifos, the most effective of currently available insecticides.

Comments and discussion regarding mounding

“Although the practice of mounding or berming of soil can suppress an existing dogwood borer population, it is probably a costly method of treating an established orchard. The practice of mounding and berming are equivalent in effectiveness to suppress dogwood borer. The construction of a berm is more economical and it can be easily constructed in young and newly established orchards. Berming of tree rows can be accomplished at a reasonable cost by simply using a side tool-bar hoe and should be considered a viable practice for dogwood borer control.

Our experience to date suggests that the berm should be formed in the first season, preferably soon after planting while the soil is loose and cultivatable. Inspect the berms during the second growing season to make sure burrknots are covered; follow up with a closer inspection in the third growing season to avoid scion rooting. Should the graft unions be buried by soil, scion rooting may occur within the first three years following construction of a berm.

Our orchard experience indicates that exposure of newly formed adventitious scion roots to air will functionally disable roots without any negative impact on the tree. In the fall of 1998, we formed a berm on newly established trees with the soil level at 7 to 11 centimeters above the graft unions. Inspecting trees in the summer of 1999 revealed that soil had naturally settled to graft union height or below and no scion rooting appeared on trees where some soil still covered the graft union. It is evident that the increasing prevalence of semi-dwarf and dwarf apple rootstocks has fostered an increase in the incidence and severity of dogwood borer infestation.

This insect has a high potential to become an even greater pest of Midwestern and Northeastern apple production systems. Not only are more burrknot-forming rootstocks being planted, but there are few options available for controlling dogwood borer. The availability of the most effective control tactic, insecticide trunk sprays, is jeopardized by worker safety concerns. Even if this approach continues to be allowed, it may not be adequate for suppression of heavy infestations.

We have demonstrated that soil mounding of apple rootstocks is a highly effective alternative to insecticides for dogwood borer control. The practice can be adopted for orchards managed conventionally as well as organically. It is highly recommended for new plantings if the rootstocks are known to be prone to burrknot formation, but can also provide excellent control even with densities averaging over a dozen larvae per tree in exposed rootstocks in established plantings.” Rootstocks which produce burrknots readily include the popular dwarfing stocks such as M.9, M.26, etc.

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