Magnaporthe grisea produces a toxin(s) which is important in the establishment of basic compatibility at the species level between fungus and plant. The effects of the toxin(s) on host (rice and barley) and non-host (finger millet) plants were examined by ultrastructural observation and morphometric measurement. The first ultrastructural changes observed after 1 hr of toxin(s) treatment were a decrease in the number of mitochondrial cristae and in the electron density of the mitochondrial matrix in rice leaf cells. Further, such modifications were observed in both compatible and incompatible inoculated with the fungal strain used for preparation of the toxin(s). However, morphological and ultrastructural changes were not observed in other organelles. Such phenomena were observed in all leaf tissues (epidermis, mesophyll and vascular bundle) of host plants. On the other hand, the effects of toxin(s) were not observed in leaves of the non-host plant. These results suggested that the mitochondrial modifications induced by the toxin(s) were a host-specific phenomenon and possibly, on essential event for M. grisea to establish basic compatibility with host plants at the species level.
The effects of supplementary UV-B radiation (290-320 nm, +UV-B) on the development of wilt disease of spinach (Spinacia oleracea) caused by the fungus Fusarium oxysporum f. sp. spinaciae were investigated under natural light condition in a glasshouse. The disease incidence in these plants greatly increased at low inoculum concentrations in soil inoculation experiments compared with plants without UV-B supplementation (-UV-B). In needle inoculation experiments with F. oxysporum f. sp. spinaciae, wilt occurred in plants under +UV-B as severely as in soil inoculation experiments. F. oxysporum f. sp. spinaciae was isolated from the roots of all wilted plants, as well as some symptomless plants in both soil inoculation and needle inoculation experiments. Re-isolation frequency under +UV-B was ca. 3.5 times higher than that of plants under -UV-B during the first three days of growth under UV-B supplementation. However, the re-isolation frequency in inoculated plants in both experiments irrespective of UV-B supplementation was over 80% by the end of both experiments. The results indicated that supplementary UV-B radiation incited disease development.
We examined the distribution and multiplication of Ralstonia solanacearum (Synonym Pseudomonas solanacearum) in roots and stems of tomato plants of resistant rootstock cultivar LS-89 and susceptible cultivar Ponderosa. In a growth chamber at 30°C, 30-day-old seedlings were inoculated with R. solanacearum strain 8107S after trimming lateral roots and immersing the root ball in a bacterial suspension of 107 cfu/ml. In inoculated Ponderosa plants, bacteria were recovered from all taproot and stem samples tested. Pathogen density increased in root and stem tissues to 108 or 109 cfu/g fresh matter in 10 days. Inoculated LS-89 plants were latently infected with the pathogen. Recovery of the pathogen in these LS-89 plants at 14 days after inoculation was about 90% in taproots and the upper hypocotyls, and about 80% in the first internodes, but the percentage decreased higher in the stem (30% recovery at the fifth internode). Pathogen density increased to 106 cfu/g fresh matter in taproots, upper hypocotyls and the first internodes in the first 4 days, but it was steady afterwards. Transverse sections of the upper hypocotyls of infected plants showed bacterial masses in the primary and secondary xylem tissues in Ponderosa, but only in a portion of the primary xylem tissues in LS-89. These results indicate that the bacteria are localized in the primary xylem tissues and their movement is limited in resistant LS-89 plants.
A viroid was detected from European pears cultivated in Fukushima and Yamagata prefectures, Japan. Nucleotide sequence analysis of each of the Fukushima and Yamagata isolates revealed that they were isolates of pear blister canker viroid (PBCVd) reported in France. The Fukushima isolate consisted of 312 nucleotides, whereas the Yamagata isolate had 313 nucleotides. The only difference between the consensus sequence of the two isolates was an additional A residue in the Yamagata isolate at the site between no. 54 and 55 of the Fukushima isolate. The Fukushima isolate differed from the French isolate P2098T (Hernandez et al., 1992) at 26 sites in the sequence, the Yamagata isolate at 27 sites. These changes included 17 base exchanges and six deletions for both isolates, with three insertions for the Fukushima isolate and four insertions for Yamagata. PBCVd was detected from six of twelve (50%) European rough bark (Sohi-byo in Japanese) pear samples in Fukushima prefecture and three out of eight (37.5%) dimple pit (Kubomika-byo in Japanese) samples in Yamagata, as opposed to none of nine European and eight Japanese symptomless pears.
Several conditions were examined to optimize the detection of hop stunt viroid-plum, a causal pathogen of plum dapple fruit disease, by reverse transcription-polymerase chain reaction (RT-PCR). The best primer pair of the six examined was HSVc-1 (5'-GGCTCCTTTCTCAGGTAAG-3')/HSVs-2 (5'-CCGGGGCAACTCTTCTCAGAATCCA-3'). By using this primer pair, the RT-PCR assay was about 10, 000 times more sensitive than two-dimensional polyacrylamide gel electrophoresis, judging from a dilution end point experiment. Treatment of CF-11 cellulose column chromatography before RT-PCR was effective in removing inhibitors present in plum sap extracts. SepaGene RV-R was satisfactory as a simple extraction method to extract the viroid from barks, roots and leaves of plum for RT-PCR assays. The extracts from barks and roots gave better PCR amplification than leaf extracts. The direct nucleotide sequencing of PCR-amplified cDNAs of the viroid isolated from dapple fruit plum (cvs. Ooishiwasesumomo and Soldam) revealed that the viroid was identical to HSVd-plum reported in plum (cv. Taiyo) in Yamanashi Prefecture.