By Ute Albrecht, Larissa Nunes and Igor Silva
Trunk injection of oxytetracycline (OTC) requires drilling a hole into the trunk and injecting a foreign substance into the tree. Not only does the physical damage from drilling cause injury, but the application of chemicals can exacerbate both the external and internal damage.
BARK CRACKING
Externally, aside from the drill hole, bark cracking is often observed after OTC injection, especially in young trees. Bark cracking can occur naturally as trees expand in diameter and the outer bark splits to accommodate the new growth. It can be exacerbated by periods of rapid growth, for example, after rainfall following a dry period.
University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) lab studies have shown that bark cracking after OTC injection is more severe the smaller the trunk diameter of the injected tree. Therefore, young trees are especially vulnerable.
To reduce bark cracking, the drill bit and applicator tip need to be suited for the trunk size. For small trees (< 4 inches in diameter), the applicator diameter should not exceed 13/64 inch. Due to the larger size of the rootstock, rootstock injection is recommended for small trees, at least until they get larger.
Figure A shows the trunk of a two-year-old citrus tree that was injected six months after removal of its individual protective cover. An 11/64-inch applicator tip was used in year 1 and a 13/64-inch tip in year 2. Using this strategy, little bark cracking occurred.
It is important to use sharp drill bits. Handle the drill without excessive force, and insert the applicator properly. Leaking of OTC at the injection site exacerbates trunk injury because it damages the cambium, which is responsible for wound wood formation. While normally new bark forms underneath the cracked area to protect the inner wood, cracks may still provide entry points for insects and diseases. For this reason, place microjets away from the trunk to prevent moisture accumulation and fungal infection.
WOUND TREATMENTS
UF/IFAS researchers have tested different wound treatments, including the fungicide Ridomil and a petroleum-based pruning sealant. Neither provided any benefits, and in some instances, even exacerbated trunk injury.
Recently, Bordeaux paste (a mixture of copper sulfate, quick lime and water) and a European wound sealant known as Lac Balsam or ‘artificial bark’ were investigated. While neither provided any benefits in terms of bark cracking, Lac Balsam effectively closed the drill hole and reduced the discoloration inside the trunk caused by the OTC (Figure B). Whether this provides any benefits to the tree in the long term will need to be established. Most arborists do not recommend using wound sealants, as they may cause more harm than good.
Ute Albrecht is an associate professor, Larissa Nunes is a doctoral candidate, and Igor Silva is a master’s degree student — all at the at the UF/IFAS Southwest Florida Research and Education Center in Immokalee.
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