Phylogenetic relationships between rice dwarf phytoreovirus isolates from five countries

Rice dwarf virus isolates were collected from several locations in Japan, the Philippines, China, Nepal and Korea. Genomic dsRNA segment profiles in polyacrylamide gel electrophoresis differed among the isolates. There were less differences in the profiles between isolates from Japan and Korea than in those between these two Countries and others. Nucleic acid hybridization was used to examine the extent of genomic variation. Full-length cDNAs to all genomic segments encoding non-structural proteins (S4, S6, S9, S10, S11 and S12) were synthesized from two Japanese isolates, and were used for dot-blot hybridization. Hybridizations using probes generated from the full-length cDNA clones failed to differentiate isolates from different geographical areas. However, cDNA probes covering a variable region of S12 were able to distinguish Japanese and Korean isolates from those of other countries. Phylogenetic tree analysis based on the amino acid sequence of P12 encoded by S12 grouped Japanese and Korean isolates together. The Chinese isolates from two different locations (Yunnan and Fujian) were closely related to each other, and were the most distantly related to Japanese and Korean isolates.     

Effect of soil moisture content on the suppression of Rhizoctonia stem canker on potato by the nematode Aphelenchus avenae and the springtail Folsomia fimetaria

The effect of soil moisture content on the suppression of Rhizoctonia stem canker on potato by mycophagous soil animals was studied in growth chambers. Three soil moisture levels were established in two bioassays, in which potato sprouts grew through a 15-cm soil layer inoculated with sclerotia of Rhizoctonia solani (AG-3). In one experiment two levels of R. solani inoculum were applied. The effect on plant disease of mycophagous soil fauna was assessed by adding the springtail Folsomia fimetaria and/or the nematode Aphelenchus avenae to the soil. In the absence of mycophagous organisms, Rhizoctonia disease severity on potato stems was highest in dry soil. A. avenae and F. fimetaria reduced Rhizoctonia stem canker when applied at populations found in the field. They were effective over a broad range of soil moistures. The stimulatory effect of dry soil conditions on Rhizoctonia stem canker was counteracted by a greater efficacy of the mycophagous soil fauna under these conditions. Mild drought stress did not seem to be a limiting factor in the biological control of stem canker by these two organisms.     

The effects of ultraviolet-B (UV-B: 290–320 nm) radiation on blister blight disease of tea (Camellia sinensis)

Sunlight is known to reduce the incidence of blister blight disease of tea, caused by Exobasidium vexans . The effects of the ultraviolet-B (UV-B: 290–320 nm) component of solar radiation on the disease and the development of E. vexans were studied in the field in Sri Lanka using UV-screening filter materials held over a commercial crop. Exclusion of UV-B radiation by Polyester, which reduces fluxes in the UV-B region by approximately 75–85%, increased both the number of translucent spots (immature sites of infection) and number of sporulating blisters (mature sites of infection), but it had little or no effect on the sporulation of the pathogen. When basidiospores were artificially inoculated on leaves, and were exposed to full or filtered solar radiation, their survival and germination increased when UV-B wavelengths were removed; significant differences were found 62 h after inoculation. However, UV-B did not affect the extension of germ tubes. This study indicates that the UV-B component of solar radiation plays an important role in the natural regulation of blister blight disease in the field.     

Development of a powder formulation of Pseudomonas fluorescens for control of rice blast

Pseudomonas fluorescens strain Pf1, inhibitory to the growth of the rice blast pathogen Pyricularia oryzae in vitro , was developed as a talc-based powder formulation. When rice seeds were treated with this formulation, the bacteria spread to roots, stems and leaves of the plants and protected against leaf infection by P. oryzae . When applied as a foliar spray, the bacteria survived on the leaves. The powder formulation controlled leaf blast under greenhouse conditions. In tests as a seed treatment and foliar spray in four field trials it effectively controlled the disease and increased grain yield.     

A model of plant virus disease epidemics in asynchronously-planted cropping systems

A mathematical model was developed of the dynamics of a plant virus disease within a spatially-referenced lattice of fields of a host crop. The model can be applied to crops in continuous, contiguous cultivation such as tropical irrigated rice. Disease progress in each field of the host crop was assumed to be logistic and determined by incidence within the field itself as well as incidence in neighbouring fields, depending on the gradient of disease spread. The frequency distribution of planting dates (represented by the proportion of the total number of fields planted in successive months) was assumed to follow a normal distribution and the variance of planting date was used as a measure of cropping asynchrony. Analysis of the model revealed that disease incidence within the lattice (i.e. mean incidence over all fields) depended upon the infection efficiency, the slope of the dispersal gradient, and the variance in planting date. Disease endemicity depended mainly on planting date variance and disease persisted in the lattice if this variance exceeded a certain threshold. Above the threshold for persistence, the response of mean disease incidence to planting date variance was non-linear and the region of greatest sensitivity was closest to the threshold. Thus, disease systems that show moderate rather than high cropping asynchrony are more likely to be influenced by changes in the variance of planting date. Implications for the area-wide management of rice tungro virus disease are discussed.     

Geographical cline of DNA variation within the F intersterility group of Heterobasidion annosum in Italy

Eighty-six Heterobasidion annosum isolates, mainly belonging to the F intersterility group and obtained from 32 different geographical localities in Italy, were subjected to genetic analysis by the Random Amplified Polymorphic DNA (RAPD) markers. The similarity between F and S groups was higher than that between F and P. In UPGMA Cluster Analysis, the F isolates originating from the same locality usually grouped in the same cluster. The isolates also showed a tendency to group at the level of larger geographical areas. Within the F group, isolates from the south of the Italian peninsula showed the highest genetic variation and northern isolates from the Alpine regions showed the lowest. This indicates a gradual cline along the peninsula. The genetic variability in the Italian F group is discussed in relation to the past and present distribution of the host species in Italy and Europe.     

Transfer from Erwinia herbicola to Escherichia coli of a plasmid associated with biocontrol of fire blight

Strains of Erwinia herbicola effective in the biocontrol of fire blight of hawthorn were used to investigate the possibility that the antagonistic activity is coded by plasmid-born genes. Agarose gel electrophoresis of isolated plasmids from four antagonistic Erw. herbicola strains showed a band of a supercoiled 12 kb plasmid in each strain, with a second band greater than 16.2 kb consistently seen in two strains. Erw. herbicola strains showed resistance to penicillin-G, which could be conferred on penicillin-G sensitive Escherichia coli TG1 by transformation with a pure Erw. herbicola plasmid preparation. Transformed strains of Esc. coli appeared to contain the Erw. herbicola 12 kb plasmid, but not the > 16.2 kb plasmid. In an agar plate assay, Esc. coli transformants produced an inhibition zone against Erw. amylovora similar to those produced by the original Erw. herbicola strains. In two biocontrol assays, the transformed Esc. coli strains had a suppressive effect on disease development on infected pear fruit slices and hawthorn blossoms.     

Germination behaviour of dormant oilseed rape seeds in relation to temperature

Seeds of oilseed rape ( Brassica napus L., cv. Liglandor) were induced into secondary dormancy by imbibing them for 2-4 weeks in an osmotic solution at -1500 kPa in darkness. Treated seeds were then transferred into pots or Petri dishes, where they were given adequate water for germinatton. These pots and Petri dishes were exposed to various temperature regimes during the course of the following 6–14 months. During that time, seed samples were taken at 2- to 6- weekly intervals to monitor survival of seeds and to study their dormancy. The ntimber of seeds that had not germinated in pots or Petri dishes remained constant in the majority of treatments. but it declined when seeds were exposed to pronounced daily temperature alternations. This decline was due to germination and not to death of seeds from other causes. Germination tests on subsamples of seeds showed that they were light sensitive, more seeds germinating in the light than in the dark. The seeds'germination behaviour differed between samphng dates. This variation in germination behaviour appeared not to display the classical dormancy:non-dormancy cycle. Part of the variation seemed to be due to the changes in temperature that the seeds experienced when taken from pots and set up in the germination tests. Seeds exposed to a constant temperature for 1 year appeared lo become more dormant with time. The relevance of this work in relation to the persistence of volunteer oilseed rape is discussed.     

Effects of neemgard on phytophagous and predacious mites and on Spiders

The effects of Neemgard, an acaricidal and fungicidal formulation obtained from neem (Azadirachta indica) seed kernels, on the phytophagous miteTetranychus cinnabarinus, the predacious mitePhytoseiulus persimilis, and the predatory spiderChiracanthium mildei, were investigated in laboratory experiments. Neemgard was highly toxic toT. cinnabarinus but had no toxic effect onC. mildei orP. persimilis. Another neem formulation—the insecticidal Neemix 45—caused conspicuous repellency, but no mortality, inT. cinnabarinus.     

Auxins increase the occurrence of leaf-colour variants in Caladium regenerated from leaf explants

Leaf explants of Caladium ‘Pink Cloud’ were cultured in vitro on MS medium containing various auxins (NAA, IBA, IAA, 2,4,5-T and 2,4-D) in combination with cytokinin (BA). NAA gave the most vigorous in vitro propagation of this plant, and only 15% of the plants were leaf-colour variants on the medium containing 0.5 μmol NAA. Leaf colour variation was observed in all plants regenerated on the medium containing 2,4-D at 0.5–4.5 μmol. In hormone-free medium, only a few leaf-colour variants (6%) occurred, but the rate of plant regeneration was very low. Application of 0.5 μmol NAA together with 4.5 μmol BA seemed to be the most appropriate for in vitro propagation of Caladium ‘Pink Cloud’ with only a few leaf-colour variants.