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.
Effect of wild oats green manure application on Pythium damping off, Rhizoctonia damping off, Fusarium wilt, Verticillium wilt, and bacterial wilt of tomato was examined, using soils infested with one of the pathogens. Slowly progressing diseases such as Verticillium wilt were suppressed. Mycelial growth of Verticillium dahliae was significantly delayed on agar media amended with wild oats extract, but not for the other fungal pathogens. The indigenous soil microflora was altered, as shown by soil dilution plating, fluorescein diacetate hydrolysis and PCR-DGGE, after the application of the wild oats. A combination of plant substances from wild oats suggested to alter the soil microflora and to suppress disease.
Fungicidal activity of a solution of benomyl (wettable powder 50%) against rice blast of seedlings caused by Magnaporthe grisea was investigated. Benomyl had high efficacy after seed treatment or soil drenching. Drenching with benomyl in nursery trays was also highly effective against rice leaf blast. The antifungal activity of benomyl against 60 isolates of M. grisea was almost equivalent, even against those isolates that are resistant to melanin biosynthesis inhibitors that target scytalone dehydratase (MBI-Ds). This implied that none of the 60 isolates was benomyl-resistant strains and that no cross-resistance existed between benomyl and MBI-Ds. An application program that consisted of seed treatment with sterol demethylation inhibitors (DMIs) and drenching with benomyl also had very strong activity against seedling blast and leaf blast on rice, indicating that there was no antagonistic effect between benomyl and DMIs. These results suggested that the application program would be very useful to control rice blast on seedlings.
Alstroemeria plants (Alstroemeria sp. and A. ligtu) with necrotic spots and streaks on leaves were found in Chiba prefecture, Japan. Tospovirus-like enveloped, spherical particles 80-100nm in diameter and filamentous particles 750-850nm in length were observed together in the infected leaves. The enveloped particles were identified as Iris yellow spot virus (IYSV) by RT-PCR. Deduced amino acid sequences of the nucleocapsid gene of the two IYSV isolates (designated CbAlsD1 and Als1) were compared with those of IYSV isolates reported previously; the isolates were genetically similar to an isolate from Dutch iris (Iris hollandica) in the Netherlands. A local lesion isolate of CbAlsD1 reproduced the original symptoms on alstroemeria seedlings without the presence of filamentous virions. These results showed that IYSV was the causal virus of necrotic spots and streaks. Naturally infected A. ligtu plants produced new shoots that appeared healthy when grown under thrips-free conditions. Although IYSV could not be detected in these symptomless shoots by RT-PCR alone, the virus was detected by Southern blotting of the RT-PCR products. Therefore, a low level of IYSV virions exist in the symptomless shoots that developed from infected alstroemeria plants.
Iris yellow spot virus (IYSV) was isolated from onion with streaks on leaves in Saga and Chiba prefectures, Japan. Phylogenetic analysis based on the amino acid sequence identities of the nucleocapsid genes showed the presence of two strains, as represented by an isolate from onion in Brazil (IYSVBR) and that from Dutch iris (Iris hollandica) in the Netherlands (IYSVNL), on onion in Japan. Pathogenicity was tested on 11 cultivars of onion and five of leaf onion by mechanical inoculation with either of the two IYSV isolates, which were genetically similar with IYSVBR and IYSVNL, respectively. Both isolates caused the same type of streaks only on inoculated leaves of all onion cultivars tested, and the virus was not detected in upper leaves. None of the leaf onions developed symptoms on either inoculated or upper leaves, even though IYSV was detected in the inoculated leaves. Onion thrips (Thrips tabaci) transmission to onion, leaf onion, wakegi (Allium wakegi) and shallot (Allium grayi) was examined, and IYSV was detected from onion leaves where thrips fed.