Bark Inclusions in Canes of Southern Highbush Blueberry and Their Impact on Cane Union Strength and Association with Botryosphaeria Stem Blight

Bark inclusions are an understudied structural defect in trees and shrubs. They consist of areas of bark on adjacent parts of stems or scaffolds, typically on the inner faces of a narrow fork, which become overgrown and internalized to occupy part of the wood between the stems. Here, bark inclusions are described for the first time to occur in cane unions at the crown of southern highbush blueberry (Vaccinium corymbosum interspecific hybrids) cultivars 'Farthing' and 'Meadowlark', both of which are characterized by a narrow, vase-shaped architecture at the base of the plant, leading to crowding of the canes. When affected canes were dissected at their bases, bark inclusions were visible internally as a line of compressed bark within the wood of adjoining canes, or as bark invaginations and fissures across part of or the entire cross-section of the cane. Externally, blueberry crowns with included bark were characterized by either an inward ridgeline of bark between canes of similar diameters emerging from the crown at a narrow angle from each other, or by the presence of girdling roots. Bark inclusions were observed in plants of all ages, from the nursery to mature production fields. The internal length of the bark inclusion correlated strongly with the external length of the inward stem bark ridgeline symptom as measured by destructive sampling in the field (r = 0.916, p < 0.0001, n = 20). When plants with and without bark inclusions were subjected to a winch test in the field, the probability of breakage for canes without included bark was significantly lower (p < 0.0002) than for those with included bark, and at the maximum applied force of 972.4 N, 95.2% of the canes with bark inclusions failed (i.e., broke at the crown), compared with only 52.6% for canes without included bark. In a survey across three fields, the number of bark inclusions per plant was significantly associated with an index of cane crowding (r = 0.286. p = 0.0267, n = 60), suggesting that plants with tight, crowded bases had more bark inclusions. In addition, there was a significant association (p < 0.0001) between the presence or length of bark inclusions and the intensity of Botryosphaeria stem blight in these fields. This study showed that bark inclusions occur commonly in certain southern highbush blueberry cultivars in the production conditions of Georgia and Florida, with negative implications for cane integrity and plant health.