Study On The Infection Process Differences Of Diplocarponmali On Malus Domestica Borkh. Cv. Fuji And Malus Baccata(l.) Borkh By Using The Histological And Cytological Technology

Marssonina leaf blotch caused by Diplocarpon mali Y. Harada&K. Sawamura(anamorph Marssonina coronariae (Ellis&J.J. Davis) J. J. Davis)was the main diseases ofthe early defoliation disease of apple. The disease causing defoliation of the apple trees,reduction of quantity and quality of apples and weakness of the tree vigour has become a limitof the sustainable healthy development of the apple industry in our country and increased yearby year in recent years. Searching resistant material and revealing the resistance mechanismcan provide scientific basis for cultivating resistant varieties and then fundamentally controlthe disease. Previous work in our laboratory about the screening and evaluation of resistancedifference to Marssonina coronaria among Malus sp. revealed that: the occurrence of lesionand acervulus on Malus baccata (L.) Borkh was later; lesion expansion speed as well as newacervulus generation velocity was largely slower M. baccata; the incidence was lower on M.baccata. So in this study,we research the differences of infection processs of D.mali onMalus domestica Borkh. cv. Fuji and M. baccata which and the host ultrastructural changesby fluorescence microscope technology, scanning electron microscopy technology andtransmission electron microscope technology to reveal the resistant mechanism and providetheoretical basis for excavating the disease-resistant germplasm resources. The following arethe main results:1. The infection process of D. mali on leaves of M. domestica cv. Fuji was as follows:Mucilage which was beneficial to adhesion was observed between the conidia and the leafsurface and the conidia penetrated leaves by germ tube or by formation of appressoria on thetip of the germ tube6h post-inoculation (hpi). Typical haustoria were observed in theepidermal cells and intercellular hyphae were observed24hpi. The amount of the haustoriacontinually increased with the extension of the pathogen. Colony of the pathogen wasobserved under the cuticle and in the mesophyll tissue, and the hyphae already extend to thesponge tissue cell gap3days post-inoculation (dpi). The lesion occured and the subcuticularhyphal strands (SHS) formed by parallel subcuticular hyphae were observed5dpi. The lesionextended obviously with a good many reproduction structure-acervulus produced on it and a lot of SHS expanded radially7dpi, and at the same time intracellular hyphae were observed.2. The infection process and the development of D. mali on M. baccata was similar with thaton M. domestica cv. Fuji, but they were inhibited obviously. The pathogen developed andexpanded slowly on M. baccata so that not only the lesion occurred late with small areafinally but also the acervulus appeared late and the amount of it was small or neverproduced on M. baccata. From24h post-inoculation to5days post-inoculation (dpi), thepathogen mainly expanded under the cuticle with no SHS observed; produce haustoria inepidermal cells and no colonies observed in the mesophyll tissue. Seven dayspost-inoculation (dpi), lesions occurred, colonies were observed in the mesophyll tissue andthe hyphae expanded in the intercellular space with no intracellular hyphae observed. SHSand limited acervuli were observed until about11dpi.3. Histologic observation of the host response showed that, at the earlier infection stage,fluorescent depositions (callose) were observed on epidermal cells surrounding thepenetration site on M. baccata, but no callose were observed on M. domestica. We thinkthat the formation of the callose have relevance with the resistance reaction of M. baccata.Cytologic observation of the host response showed that,7days post-inoculation (dpi),mesophyll cells adjacent to hyphae on M. baccata displayed a series of pathologicalchanges such as shrinks deformation of the cells, disorganization and aggregation of theprotoplasm, entire disintegration of the organelles structure and died finally; howevermesophyll cells that around the colony of the pathogen enhanced the resistance of M.baccata because of the increasing of vacuoles which benefits to the detoxication of the cells.But on the M. domestica, massive mesophyll cells died with shrinks deformation of thecells, disorganization and aggregation of the protoplasm, entire disintegration of theorganelles structure; or agglomeration of the nucleoplasm and degradation of organelles insuccession.