Relationship Between Apple Valsa Canker Severity And Contents Of Potassium Content In Offshoot

Apple Valsa canker can cause offshoot and stem cankers,kill the tree,and even destroy the entire orchards.Potassium is one of the main elements required for the growth and development of apple trees.The occurrence and severity of apple Valsa canker are closely related to the content of potassium In this study we characterized the annual dynamic changes of potassium content in secondary year offshoot and its correlation with Valsa canker resistance.A Valsa mali strain was transformed with GFP fluorescenc This study lays a foundation for further studying disease resistant mechanisms.Main results obtained are listed as follows:1.PEG-mediated genetic transformation system was established for apple Valsa canker pathogen.The GFP gene was successfully transformed into the pathogen.After consecutively screening for five generations with 30μg/mL hygromycin,a stable GFP transformant was obtained,which provided an useful material for the study of pathogen infection.2.Field experiment showed that phloem potassium content negatively correlated with Valsa canker disease severity.Slight canker expansion existed when the potassium content of offshoots reached 6.5 g/kg whereas no expansion was observed when the potassium content reached 7.5 g/kg.Based on these results,we suggested that 7.5 g/kg of potassium content of biennial offshoot could be used as a nutritional standard for controlling Valsa canker.3.The effect of potash fertilizer amount on the potassium content improvement of apple tree offshoot was analyzed.Monopotassium phosphate was applied separately at the level of250 g/tree,500 g/tree,750 g/tree,1000 g/tree,1250 g/tree,1500 g/tree and 1750 g/tree,the annual change of potassium content in secondary year shoot was analyzed.It was found that the shoot potassium content would increase in autumn,peaking between December and January,and declined thereafter.The greater extent the decline,the more severe the disease.It was found that the potassium content in the tree in January was the most relevant to the tree potassium content in March.Based on the annual dynamic change patterns,we suggested that it is a good practice to improve potassium content in autum(August to September),so as that the potassium content remain above the threshold required for Valsa canker resistance in March.and4.To analyze the reasons for the resistance of high-potassium apple trees,histological comparison of phloem of biennial apple trees with different potassium contents was carried out.The results indicated that there is a positive correction(y=59.969x+306.1 R~2=0.6137)between the content of potassium and the thickness of periphloem.Accordingly,with a 7.5g/kg potassium content,the thickness of periphloem reached 764.25±78.9μm;but the thickness is only 690.24±94.3μm when the potassium content is 4.5 g/kg.In addition,there is a lignification block occurring in the high potassium shoots(>7.5 g/kg)with a inoculation treatment whereas no lignification was observd with low potassium shoots(<5.5 g/kg).These results showed that high potassium promotes the formation of physical barrier upon pathogen challenge.5.Inoculation was performed with biennial shoots with different potassium contents.Phyroghenol staining showed that the lignin content of the high potassium shoot was significantly higher than low potassium shoot;lignin content measurement showed that the content of glucocorticoids in the phloem increased by 32.0%compared with that in the low potassium content upon inoculation.The results show that potassium can promote lignin biosynthesis;high potassium can promote the production of tree chemicals to prevent the expansion of Valsa canker.In summary,this study has established a potassium fertilization standard for Valsa canker disease control.We also showed that potassium promoted the formation of lignin barriers and ligno-suberied boundary zones which improved apple tree resistance against pathogen.