Role Of Surface Sturcture On Susceptibility Of Apple Fruit To Botryosphaeria Dothidea

Botryosphaeria dothidea (Moug.) Ces. et de Not. infects both branches and fruit of apple causing bot canker and ring rot, respectively. And it has brought great harm to apple industry. Major commercial cultivars, such as’ Red Fuji’,’ Golden Delicious’,’Gala’, are highly susceptible to B. dothidea fruit infection. Young fruit can be infected by B. dothidea. In general, because of a long latent period, the incidence of fruit ring rot can explode any time from the rapid growth of the young fruit until postharvest storage, so the fruit ring rot is very difficult to control. Therefore, utilization and breeding of new disease-resistant cultivars will become one of the effective ways to control fruit ring rot. Then it is important to understand the mechanism of fruit resistance to B. dothidea.In the experiment,11cultivars, including four cultivars known to be resistant,’Jiguan’,’Starkrimson’,’Jonathan’and’ Ralls Janet’; three cultivars of intermediate resistance,’Jonagold’, NY543and’ Kuihua’; and four cultivars known to be susceptible,’Jinhong’,’Red Fuji’,’Golden Delicious’ and’Golden Spur’, and9germplasm accessions were used as material. Apple ring rot latent period, infection rate, lesion length of Malus domestica cultivars and Malus germplasm accessions were investigated14days after in vitro inoculation with B. dothidea mycelia of strain Lsl at different developmental stages. Area per fruit surface (APFS) of lenticels and microcracks and thickness of cuticular wax were observed by scanning electron microscopy (SEM) and light microscopy (LM). Then susceptible cultivars treated with GA4+7, oil emulsion and fruit bagging in order to further study the relationship between susceptibility to B. dothidea and natural openings and exocarp structures. The results are as follows:1. The natural opening (including lenticel and microcrack) APFS varied significantly and was closely correlated with disease severity, not only between M. domestica cultivars, but also among Malus germplasm accessions. Positive correlations were found between lesion length and natural opening APFS among11cultivars and9germplasm accessions. Coefficients of determination were0.836,0.693,0.872,0.934in11cultivars at120,135,150days after full bloom (DAFB) and9mature germplasm accessions, respectively (p<0.01). Hyphae were observed penetrating through natural openings from both the resistant and susceptible cultivars by SEM.2. Lenticels were the main source of natural openings in most apple cultivars. By SEM observation, except for’Golden Delicious’ and ’Golden Spur’, lenticels were the main natural openings on the exocarps of11apple cultivars and all nine Malus germplasm accessions. Positive correlations were found between lesion length and lenticel APFS among cultivars; the coefficients of determination at120,135and150DAFB were0.750,0.751and0.767, respectively (p<0.01).3. Except large lenticels, numerous microcracks on the fruit exocarp of ’Golden Delicious’ and ’Golden Spur’ were observed under SEM. Microcracks APFS was10%-20%, accounting for over90%of the total natural opening area in these two cultivars. Microcracks were a causal factor, but not the dominant one, related to the high susceptibility of ’Golden Delicious’and’Golden Spur’ to B. dothidea. 4. The thickness of cuticular wax differed significantly between cultivars. There was a significant negative correlation between cuticular wax thickness and lesion length (R2=0.907, p<0.01). The cuticular wax of’Golden Delicious’ and ’Golden Spur’ were thinnest, only about1μm. We speculated that thickness of cuticular wax may be a key factor involved in microcrack formation. The thickness of cuticular wax all decreased ranging from1.17to1.49μm in resistant cultivars’Jiguan’,’Starkrimson’,’Jonathan’and ’Rails Janet’ from60to75DAFB, accompanied by the appearance of microcracks by SEM observation. The thickness of cuticular wax all increased ranging from1.96to2.39μm from75to150DAFB. We speculated the thickened wax fill microcracks, fruit surface became smoother. On the contrary, the thickness of cuticular wax of’Golden Delicious’declined sharply from30to60DAFB, the thinnest wax only was0.64μm and1.20μm at mature. The deposition of cuticular wax could not filled microcracks.5. Susceptible cultivars ’Golden Delicious’,’Red Fuji’,’ Jinhong’ sprayed with GA4+7, we found that the cuticular wax became thicker, miacrocrack APFS was smaller, fruit surface of ’Red Fuji’ and ’Jinhong’ became smoother comparing to fruit bagging. It confirmed that wax may fill microcrack, then the natural opening APFS reduced accompanying by disease-resistance increased. The thickness of cuticular wax all decreased after fruits bagged of3cultivars, the susceptibility to B. dothidea became stronger with natural opening APFS increased.