Effects Of Apple Leaves In Disease Cycle Of Apple Bitter Rot

Apple bitter rot is one of the major diseases of apple fruit, which cause fruit rot duringboth growth and storage period. Our previous research indicated that Colletotrichum spp.could infect apple leaves under the condition of in vitro inoculation. In this research, weanalysed the role of apple leaves as the primary infection source in disease circle byhistological observation after indoor inoculation and field inoculation and the pathogendetection of field leaves. The main results are as follows.Indoor inoculation of in vitro apple leaves showed that both C. gloeosporioides and C.acutatum had the ability of infection, presenting symptoms after seven days and producingsticky spore ball. According to microscopic examination, it was found that conidia of both ofthe pathogens could germinate and produce germ tubes on the leaves, then appressoria wereformed on the germ tubes, and finally secondary conidia were formed on conidia and germtubes. Along with the completion of infection, conidia, germ tubes and appressoria wouldgradually degrade and only the penetration pore were left on the leaves. Some differencesexisted bettween the two fungi in the number of appressoria and growth rate of germ tubes.It was observed by inoculating the field leaves that no symtoms were shown frominoculation (Aug.) to the late growth period (Nov.). Microscopic inspection indicated thatconidia of both of the pathogens could germinate on the leaves, forming appressoria andinfection pegs, and the penetration pore were left after the appressoria fell off. Secondaryconidia could be also formed on conidia and germ tubes. It was found that the inoculatedleaves of no symptom would present symptom after being treated by paraquat, which theyexisted in the host tissue by latent infection.According to inoculation tests on cultural apple leave, we found that that C.gloeosporioides was able to infect younger leaves and showed symptom within4d afterinoculation. Irregular inflated infection vesicles were produced on penetration pores followedby thicker primary hyphae differentiation. Primary hyphae expanded to adjacent plantepidermal cells and constricted when passing intercellular cell walls. This infection processwas biotrophic and the infection vesicles and primary hyphae can be observed on both greenand brown mesophyll cells. However, thinner secondary hyphae only existed in mesophyll cell in brown and cannot be discovered in healthy tissues which indicated that pathogenstransferred form biotrophic to necrotrophic with secondary hyphae producing. The resultsshowed that C. gloeosporioides was typical semi-biotrophic in apple leaves.To understand fungi carrying status of apple leaves in the field, health leaves wascollected in conventional management orchards and treated with paraquat to promotesporulation. Induced apple leaves were transferred to culture medium directly and identifiedby molecular methods. The result revealed that fungi carrying status of apple leaves in thefield was as high as14%. Morphology and ITS nucleotide sequence analysis showed that ofthe14isolates we obtained,13were C. gloeosporioides complex and the remained one was C.acutatum complex.C. gloeosporioides and C. acutatum were found as the pathogen of anthracnose on applefruit in the study above. They can infest apple leaves under the natural condition; conidiaspores can be produced before infection to be the source of secondary infection. The pathogenexists in healthy leaves in a latent infection, could serve as a source of primary infection in thefield in the coming years.