Histopathology Of Leaf Infection By Marssonina Coronaria On Resistant And Susceptible Apple Cultivars And Genetic Diversity Analysis Of Marssonina Coronaria Population

Apple blotch disease, caused by Marssonina coronaria (Ellis&J. J. Davis)J. J. Davisis an important disease of apple that affects mainly leaves and causes severe prematuredefoliation on apple trees in China.The use of resistance cultivars offers the most economic and environmental friendlyway to control the disease. To elucidate the mechanisms of apple and M. coronariainteractions to appropriate deployment of resistant cultivars to achieve sustainable controlof the disease, the infection and the main cellular events during the the pathogencolonization of Marssonina coronaria was investigated on leaves of susceptible andresistant apple cultivars by means of light and electron microscopy. To understand thegenetic diversity of M. coronaria, genetic diversity of13apple Marssonina blotch isolates(M. coronaria) from nine counties in Shaanxi, Shandong, Liaoning province, wereinvestigated by the Inter-Simple Sequence Repeat (ISSR) technique. The follow resultswere obtained:1. Infection and colonization by the pathogen could be divided into six stages: sporegermination and germ tube formation, appressorium formation, penetration, haustoriumformation in mesophyll cells, expansion of mycelium between and in mesophyll cells, andacervulus formation. Cell wall swelling and degradation, plasmolysis, accumulation ofstarch grains and osmiphilic globules, disruption of grana lamellae and vacuolization ofmitochondrion was observed during mycelial expansion in host tissues. Branching rate ofgerm tubes, formation rate of appressoria and expansion of mycelium were significantlydifferent between susceptible and resistant cultivars; however, the conidial germination rate did not show significant differences. For resistant cultivar, the pathogen’s penetration andformation of subcuticular hyphae, haustoria in mesophyll cells, and acervuli were clearlydelayed, and the number of germ tube branches and haustoria in the mesophyll cells wereall decreased. Hypha grew more slowly and deposited more electron-dense material in theleaves of resistant cultivars. It is concluded that the resistance of apple cultivars against M.coronaria is related to inhibition of fungal infection, colonization, and mycelial expansionprocesses in host tissues.2.13apple Marssonina blotch isolates (M. coronaria) were investigated by theInter-Simple Sequence Repeat (ISSR) technique, using7selected specific and stableprimers. A total of50sites were generated, among which72%were polymorphic anddemonstrated hingh genetic diversity among populations, The tested isolates could beclassified into2ISSR groups, the ISSR clustering groups had certain correlation with thesymptom of the isolates, but had no significant correlation with geographical origin of theisolates.