Research progress in pathogen-host interaction mechanism and integrated control of apple valsa canker

Abstract: Valsa canker, caused by valsa mali (Vm) , is the most destructive disease of apple productionin the Eastern Aisa. In China, this disease occurs in almost all apple-growing areas, and results in can-kers on branches, trunks and diebacks of twigs. Fungicidal applications are not always effective becausethe mycelium is able to extensively spread to the xylem. Vm is a necrotrophic fungi, usually invades tis-sues through the wound caused by pruning, frost damage, sunscalds, and other mechanical injuries. Most new lesions on the infected tissues appear in spring, rapidly expand between spring and early sum-mer, and then slowly spread during the middle and late summer and the whole winter. Recently, manystudies have focused on the pathogen-host interaction mechanism and integrated prevention and con-trols. Genome sequencing indicates that Vm has a large number of protein kinases, suggesting a verycomplex pathogenic regulation mechanism. To date, degradation enzymes, secondary metabolisms, effecter proteins and several transcription factors have been confirmed, which correlated with pathogenicity of Vm. Compared with other pathogenic fungi, a great amount of genes involved in cell wall degrada-tion enzymes biosynthesis, secondary metabolisms and secretory proteins were found in Vm genome.Among these, genes involved in pectin and phlorizin degradation and encoding Polyketide Synthase (PKS) and Nonribosomal Peptide Synthetase (NRPS) play crucial roles in Vm pathogenicity. Some secretory proteins, such as the necrosis-inducing protein Nep1-like, the necrosis-inducing factor Hce2, theserine protease inhibitor I9 and the LysM domain-containing protein, participated in the pathogenic process. Besides, some transcription factors (TFs) , including PacC and seb1, have been confirmed to playcrucial roles in the process of Vm infection. Lastly, a large number of membrane transporters, such asmultidrug resistant transporters (MFS superfamily) and siderophore-ion transporters (SITs) , can helpVm overcome limitation of antifungal compounds and ion, indirectly contributing to Vm virulence. Toinvestigate molecular mechanism of apple trees against Vm infection, several types of resistant germ-plasm were screened, including rootstock species'Sanyehaitang' (M. sieboldii) , 'Deqinhaitang' (M.sikkimimensis) , 'Taishanhaitang' (M. hupehensis) , 'Pingyitiancha' (M. hupehensis) , 'Yajiangbianye-haitang' (M. toringoides) , 'Linzhihaitang', 'Lushihaitang'and'Kelegou Baccata LF (H) ', and culti-vated varieties'Jonathan''Qinguan''Yuhuazaofu'and'Youjin'. To withstanding Vm infection, vari-ous resistant responses were wakened in apple, and these were mainly involved in chitin signals, hor-monal homeostasis, as well as resistant genes and TFs. RNAseq analysis showed that cell apoptosis, transcription regulation, IAA signal pathway, ATP-, DNA-and protein-binding activity were involved in'Fuji'resistance. Based on re-sequencing, three SNPs in an RNA-binding protein gene, a serine/threo-nine-protein kinase gene and a MYB transcription factor gene showed a close relationship to apple resis-tance. Although more than 1 800 resistant genes were discovered from apple genome, a few of thesewere differently expressed in'Fuji'responses to Vm signals. Additionally, polygalacturonase-inhibitingprotein, cytochrome P450 and phytoalexins synthetic genes might be involved in Vm resistance. Foliarnutrient analysis and fertilization experiments exhibited that increasing tree potassium (K) contents en-hanced resistance to Vm colonization. However, further investigation is needed to explain the molecularmechanism of K on apple resistance. Integrated control measure was recommended to effectively con-trol the occurrence of the disease, including rapid and effective detection systems, orchard management, resistant breeding, diseased lesion treatment and biological control. To effectively control the disease, the national apple industry system has established China Apple Pest Control Collaboration, which couldmonitor the occurrence of apple diseases and insect pests in time. simultaniously, several rapid and ef-fective detection methods have been established, like nested PCR and quantitative Real-time PCR as-say. Development of resistant cultivars is one of the most effective and durable practical approaches tocontrolling the disease. Establishment of efficient genome editing in apples provides a new approach toresistant breeding. Strengthening orchard management plays an important role in the prevention andcontrol of the Valsa canker, including strengthening the tree vigor, managing wound with antifungal andhealing drugs and house-cleaning the orchard in time. Diseased lesion should be clean removed and thewound should be painted with 2.12% copper humic acid and 3.315% thiophanate-methyl ∙ 1-naphtha-lene acetic acid. Besides, Wrist-bridging Rejuvenation, Phloem Graft Method and Mud Paste Methodcould effectively prevent the recurrence of Valsa canker. Biological control of fungal disease hasemerged as an effective practice. Various antifungal microorganisms, such as Trichoderma longibrachia-tum, Sphaeropsis spp., Streptomyces longissimus, Streptomyces aureus, and Bacillus amyloliquefaciens, and botanical fungicides such as Ozone Oil, Psoralea corylifolia Linn and Polyhydrooxy Dinaphthalde-hyde, have been confirmed, which could significantly inhibit the growth of Vm. With the rapid spread ofthe disease, Valsa canker of apple has become the focus in apple production and research. In the pastfew decades, through the continuous research and exploration by scientists in many apple-producingcountries, pathogen-host interaction mechanism and integrated control measures have been partly under-stood, some progress has been made in control of the disease and resistant breeding. At present, break-throughs in several aspects are urgently in need: 1) resistant breeding, 2) pathogen-host interaction in-vestigation and resistant gene screening, 3) development of environmentally friendly and efficient fungi-cides, 4) establishment of early detection systems for diseases. With the continuous exploration of scien-tific researches and the rapid development of molecular biology and bioinformatics, it is believed thatthe efficiently defeating valsa canker of apple will be around the corner.