Myrotoxin B detection from mulberry leaves infected with Myrothecium roridum, cause Myrothecium leaf spot of mulberry, and possible roles in pathogenicity

Abstract

Although myrotoxin B has been isolated from cultures of Myrothecium roridum, a pathogen of Myrothecium leaf spot of the mulberry, whether the toxin acts as a pathotoxin was unknown. Myrotoxin B was isolated from lesions on mulberry leaves infected with M. roridum strain M9403 using a thin layer chromatography (TLC)-bioassay and high performance liquid chromatography (HPLC) to examine its role in pathogenicity. Extracts from the center, margin and outer parts of the lesions induced necrosis on mulberry leaves. Toxicity of the extract from the center of the lesion was higher than extracts from other parts. A toxic component with the same Rf value as myrotoxin B was detected from all extracts with the TLC-bioassay method, and it from the lesion's center was the largest of three parts of the lesion. Using HPLC, myrotoxin B was detected from extracts of the leaves inoculated with conidia of strain M9403, which were washed with water, one day after the inoculation. The concentration was the highest (11.5ppm) at 14 days after the inoculation. Conidia of the fungus alone did not infect mulberry leaves, but leaves were infected when 10^-6ppm myrotoxin B or higher was mixed with the conidia. Usually avirulent fungi infected mulberry leaves when myrotoxin B was dropped on the leaves before the inoculation. Myrothecium roridum strains that produce the toxin and have high virulence on mulberry leaves also produced high amounts of the mycotoxin. Resistant cultivars developed smaller necroses by dropping of myrotoxin B than did susceptible cultivars. Based on the results, myrotoxin B may be a pathotoxin of M. roridum that causes the disease.