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1.
Four rice cultivars, susceptible or partially resistant to Pyricularia grisea were evaluated for their apparent infection rates (r) and for terminal severity values in seedlings grown in seedbeds. Susceptibility of partially resistant cultivars decreased when seeded in wet seedbed compared to those grown in raised or upland seedbed. The degree of blast reduction due to flooding varied with rice genotype, seeding rate, and environmental conditions. Flooding was effective in managing seedling blast of partially resistant cultivars but not of susceptible cultivar. Use of a lower seed rate in comparison to a higher seed rate also led to reductions in r-values and terminal disease severities. However, in case of the susceptible cultivar IR50, effect of seed rate was more pronounced, when low amount and short duration of rainfall occurred. Flooding of wet beds with low seed rate further reduced the blast incidence. Results of this study suggest that with appropriate use of water, seed rate, and partial host resistance there are prospects for effective control of leaf blast in the tropics. 相似文献
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The phylogeny of Greya Busck (Lepidoptera: Prodoxidae) was inferred from
nucleotide sequence variation across a 765-bp region in the cytochrome
oxidase I and II genes of the mitochondrial genome. Most parsimonious
relationships of 25 haplotypes from 16 Greya species and two outgroup
genera (Tetragma and Prodoxus) showed substantial congruence with the
species relationships indicated by morphological variation. Differences
between mitochondrial and morphological trees were found primarily in the
positions of two species, G. variabilis and G. pectinifera, and in the
branching order of the three major species groups in the genus. Conflicts
between the data sets were examined by comparing levels of homoplasy in
characters supporting alternative hypotheses. The phylogeny of Greya
species suggests that host-plant association at the family level and larval
feeding mode are conservative characters. Transition/transversion ratios
estimated by reconstruction of nucleotide substitutions on the phylogeny
had a range of 2.0-9.3, when different subsets of the phylogeny were used.
The decline of this ratio with the increase in maximum sequence divergence
among taxa indicates that transitions are masked by transversions along
deeper internodes or long branches of the phylogeny. Among transitions,
substitutions of A-->G and T-->C outnumbered their reciprocal
substitutions by 2-6 times, presumably because of the approximately 4:1
(77%) A+T-bias in nucleotide base composition. Of all transversions,
73%-80% were A<-->T substitutions, 85% of which occurred at third
positions of codons; these estimates did not decrease with an increase in
maximum sequence divergence of taxa included in the analysis. The high
frequency of A<-->T substitutions is either a reflection or an
explanation of the 92% A+T bias at third codon positions.
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