H Negishi1, H Murakami2, K Suyama1 and S Wakimoto 1
1Tokyo University of Agriculture, Tokyo, Japan; 2Tomono Agrica Co. Ltd. Shizuoka, Japan

Background and objectives
From 1985, declining syndrome including decreasing of blossom and fruit production, yellowing and dwarfing of leaf and bad growing of twigs was seen on mume (Prunus mume) in Tanabe-city, Wakayama Prefecture, Japan. The syndrome was named as decline although its cause was still unknown. Although pruning, fertilizer application, soil pH adjustment, antibiotic's injection, isoprothiolane treatment and soil amendment were carried out, there was no effect to prevent from occurring and developing of the decline. The healthy mume plant that was grafted with the twig of declined tree did not appear any above mentioned symptom. We suspected that soil-borne fungi were the causal agents of the decline and succeeded to isolate and identified the causal organisms.

Materials and methods
The small segments of the root surface including both healthy and diseased tissues were disinfected by ordinary methods and incubated on water agar plate or potato sucrose agar medium (PSA) contained 100 ppm chloramphenicol respectively. Incubated at 25C for 2 - 3 days, emerging mycelial tips were isolated. Pathogenicity of the isolates on mume plant was tested several times in the following methods. The mycelium was suspended in disinfected water to pour into small transparent plastic pipe (inner diam. 3.4 cm) that was stubbed by cotton plug, filled with disinfected vermiculite, planted with 2-year old mume root stock and covered with aluminium film. Some isolate was grown on PSA containing a certain agrochemical to test the sensitivity.

Results and conclusions
Among many fungal isolates, 5 and 2 isolates of genus Cylindrocarpon and Fusarium, respectively, caused browning and/or blackening of root surface, inhibition of rooting and germination and/or wilting of newly emerged leaves. In case of the inoculation after rooting, they sometimes caused total death of the plant. In each case, reisolation was succeeded. The pathogenic isolates of the two genera were morphological uniform in each other. The macroconidia of Cylindrocarpon were produced on polyphialides, hyaline, 2 - 4 celled, cylindrical and 10 - 49 x 2 - 8 pm in size. The microconidia were also produced on polyphialides, hyaline, mainly having no septum, ellipsoidal and 5 - 16 x 1 - 4,um in size. Dark colored spherical chlamydospores were produced in chain. From the morphological characteristics, the isolates were identified as C.destructans. The isolates belonged to Fusarium were suspected as F.oxysporum but not confirmed because of their poor ability of macroconidia production even on carnation leaf agar medium. Above mentioned disease on mume plant was the first report and we proposed the disease as mume root rot decline caused by C.destructans and Fusarium sp. It was suggested that the control of the disease was very difficult because the causal fungi grew well on PSA added many kinds of agrochemicals except only organic copper, benomyl and thiophanatemethyl whose minimal effective concentrations were more than 10 ppm.

1. Murakami,H., Tokita,T., Negishi,H., Suyama,K., Arimoto,Y., and Wakimoto,S. 1996. Arm. Phytophathol. Soc. Jpn. 62:608(Abst.).