Biological Characterization of Marssonina coronaria Associated with Apple Blotch Disease

Apple blotch is one of the most severe apple diseases known. The disease is widely-distributed, being reported in North America, Oceana, and Asia [1-3]. This disease is caused by the fungus Diplocarpon mali (Y. Harada & K. Sawamura [anamorph Marssonina coronaria (Ellis & J. J. Davis) J.J. Davis, syn. M. mali (Henn.) S. Ito]) [1]. The fungus primarily infects apple leaves, and conidia formed in acervuli causes infection of the leaves and fruits during the growing season. The apothecia produced on overwintered diseased leaves are sources of the inoculum. The disease first appears as dark green circular patches on the upper surface of the mature leaves in mid-summer. As the disease progresses, the leaf spots coalesce and black pinhead-like asexual fruiting bodies (acervuli) develop on the affected surfaces. Severe infections of leaves result in premature defoliation, reducing the quantity and quality of apples produced [4, 5]. The occurrence of apple blotch in Korea was first reported on 1988 and the first disease outbreak happened in 1993 [6]. In 2006, leave defoliation reached 87.7% in an experimental field in mid-September, with nearly all leaves being infected. The diseased has continued and remained serious to the present day. Most pertinent research has focused on the intensive management of this disease through the development of a fungicidal spraying program [7]. However, thiophanate-methyl-resistant strains of D. mali were found in Japan in 1997 [8]. The occurrence of this disease throughout the country prompted the gathering of information on the phylogenetic distribution of Marssonina species. Basic studies on causal fungus and its disease often involve culturing the fungus in media and production of a large quantity of conidia for inoculation of plant tissues [3]. However, Marssonina sp. are particularly difficult to culture in vitro because it grows very slowly on commonly used potato dextrose agar (PDA) medium. Suitable cultural conditions for this fungus have been unclear. This study was conducted to determine the phylogenetic relationship of Massonina isolates based on the sequences of internal transcribed spacer (ITS) region and the partial sequences of 28S rDNA, to determine their pathogenic behavior, and to facilitate the establishment of a suitable culturing method by determining suitable nutrients sources, optimum temperature, and pH for growth.