Role of lenticels and microcracks on susceptibility of apple fruit to Botryosphaeria dothidea

Apple ring rot, caused by Botryosphaeria dothidea, is one of the most serious diseases affecting apple industry in East Asia. To study the correlation between fruit natural exocarp structures and susceptibility to B. dothidea, lesion length of Malus domestica cultivars and Malus germplasm accessions was measured 14 days after in vitro inoculation with B. dothidea mycelia at different developmental stages. Area per fruit surface (APFS) of lenticels and microcracks and thickness of cuticular wax were observed. Lesion length, lenticel APFS and cuticular wax thickness varied significantly among the 11 cultivars studied. A positive correlation between lesion length and lenticel APFS and a negative correlation between lesion length and cuticular wax thickness were detected among cultivars and during fruit development. In addition, similar correlations were validated in nine Malus germplasm accessions. Numerous microcracks were observed on surfaces of Golden Delicious and Golden Spur fruit, accounting for 90 % of the natural openings. The hyphae of B. dothidea penetrated into exocarp through both lenticels and microcracks. Fruit bagging with double-layered paper bags and exogenous GA(4+7) reduced lenticel APFS, thickened cuticular wax and elevated resistance to disease. Natural openings on fruit exocarp are an important factor affecting susceptibility of apple to B. dothidea.