| Valsa canker,caused by the fungus Valsa mali Mayabe & Yamada,is one of the most destructive diseases of apple in China.This disease is epidemic in the primary apple-producing areas in China,and is a severe threat to the economic viability of apple production.Although numerous management approaches have been implemented to try to limit Valsa canker,including attempts at physical,chemical and biological control,the disease remains prevalent.It is clear that new strategies to manage Valsa canker are needed and a better understanding of host/pathogen relations is required.Nutrient status is often a determining factor in disease development,and plants with optimal nutritional status often have the highest resistance to disease.Our earlier studies investigated the occurrence of Valsa canker in 24 apple orchards in Shaanxi Province in concert with foliar nutrient analysis,and found that there was no significant linear relationship between disease index and leaf N or P content;however a significant inverse relationship occurred between potassium(K)content and the severity of Valsa canker.On that basis,this research was to study the effects of potassium nutrition on Valsa canker from the perspective of “nutrition-plants-pathogens”interaction.The main results are summarized as follows:1.Fertilization experiments with potted and field-grown trees proved there was a significant negative relationship between apple tree K content and the severity of Valsa canker.After 6 months of fertilization,leaves K content of potted trees was 0.45%,0.67%,1.30%,and 1.75%;and the branch K content was 0.25%,0.34%,0.53% and 0.65% in treatments 1 through 4,respectively.Seven days after inoculation,trees with 0.45% K had the longest average canker length,about 3.84 cm,and the trees with 0.67% leaf K had 1.93 cm average canker length.When K content was 1.30% or higher,trees showed very limited infection and,when infection occurred,it was limited to the inoculation site.At 14 days after inoculation,average canker length on trees with 0.45% leaf K had extended to 13.46 cm compared to 7.38 cm for trees with 0.67% leaf K.The trees with 1.30% and 1.75% leaf K had not developed canker lesions.At the same time,there was a similar phenomenon on the inoculated leaves and shoots.2.Among the treatments in the field experiment,the content of N and P was similar.In contrast,foliar K was 0.75% in the no-fertilizer treatment,which showed the longest lesions,approximately 7.06 cm.There was no significant difference in leaf K content between fertilizer application to leaves alone and root application,but the K content was higher in comparison to no fertilizer.The K content of treatments 2 and 3 were 1.06% and 1.07%,and canker length was approximately half of that in treatment 1,about 3.64 cm and 3.40 cm,respectively.K content in treatment 4(foliage fertilizer plus root application)was 1.37% and lesion length was 0.19 cm.This further corroborated that Increasing tree K content could enhance the resistance to pathogen colonization and establishment.It is noteworthy that when leaf K content was greater than 1.30%,infection was limited or absent altogether,indicating that apple trees have relative resistance to the canker pathogen until a low K content threshold occurs,below which the trees become increasingly susceptible.3.Fertilization experiments showed that increasing tree K content enhanced resistance to pathogen colonization and establishment.Field trials demonstrated that increasing K fertilization could significantly reduce disease incidence.Improved management of tree nutrition,especially K content,could effectively control the occurrence and development of apple Valsa canker.Potassium deficiency caused by unbalanced fertilization is probably the main reason for the epidemic of Valsa canker in China.3.The annual periodic variations in tree mineral nutrition and Valsa canker occurrence were investigated in Yangling,which indicated that there was no effect of apple tree N and P content on the occurrence of Valsa canker.However,there were two fast declining periods(May and September)in the K content of apple branches,which were consistently associated with the peaks of Valsa canker development.4.The survey of the annual change of apple branch soluble sugar and phenolic compounds shown that,the soluble sugar content of apple branches began to declining from March to September.On May and September,the phenolic content was at its lowest level of the year.Meanwhile,there was a positive relationship between annual change of soluble sugar and phenolic compounds content and apple tree K content.As a consequence,the annual fluctuation of tree K content caused the changes of soluble sugar and phenolic compounds accumulation in apple branches in different periods.5.Through potted-tree fertilization experiments,we analyzed the changes of soluble sugar and phenolic compounds content in different K content apple branches before and after inoculation.The results showed that potassium application not only improved the inherent soluble sugar and phenolic compound but also significantly increased induced soluble sugar and phenolic compounds of apple branches.6.Based on the investigation of wound healing rate,and the histological observation of apple branch exophylactic periderm and wound periderm,it showed that before inoculation the wound healing rate of different K content branch trees was 6.4,12.5,55.8%,and 57.1%,respectively.And 14 days after inoculation,the wound healing rate of different K content branch trees was 0,0,61.3%,and 66.2%,respectively.So,improved K nutrition helped in increasing rate of the wound healing apple branch before and after inoculation,and a more significant influence on the wound healing of apple branches after inoculation.In additionally,the increase in K content of apple tree enhanced the exophylactic periderm thickness of healthy apple tree(R2=0.966)and significantly improved the wound periderm forming ability of inoculated apple branch,thereby the resistance to Valsa mali colonization and establishment was enhanced.K application not only improved the inherent lignin content before inoculation,but also significantly increased the induced lignin content after inoculation.7.In the potted-tree fertilization experiments,we found that K could not influence the activities of defense-related related enzymes(phenylalanine ammonia-lyase,PAL;peroxidase,POD;polyphenoloxidase,PPO)before inoculation,but significantly enhanced the enzyme activity after inoculation.8.Compared the protein expression profile with the normal K level(0.53%)and K deficiency(0.25)apple branches,there are three soluble sugar related enzymes(SS,XTH and AGPase),three phenolics synthesis related enzymes(PPO,CHS and ANS)and 3 key enzymes in the process of lignin synthesis(PAL,CCoAOMT,CAD)were up-regulation expression,and the up-regulation expression was more significant after inoculation.In conclusion,there was a significant negative correlation of K content with the severity of Valsa canker,and improved K application was expected to reduce or control the occurrence of Valsa canker.During the year,K reduction and transfer from apple branches reduced the content of soluble sugar and phenolic compounds in apple branches,which maybe significantly influence Valsa canker development.Improved K nutrition could increase the content of soluble sugar,phenolic compounds and lignin,as well as the activities of defense-related enzymes,which could enhance the exophylactic periderm thickness and the wound periderm forming ability,and therefore increased the resistance to Valsa mali colonization and establishment.Consequently,our studies provided an effective method and a theoretical basis for the control of Valsa canker. |
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