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One taxonomic characteristic of Bipolaris species is the bipolar germination of conidia, but conidia of Bipolaris oryzae, the causal pathogen of brown spot in rice, are regularly observed to show intercalary germination, a characteristic of Drechslera species. The effect of selection, culture media and culture age on type of conidial germination was determined for three brown spot isolates from Cavinti, San Pablo and Palawan in the Philippines, obtained from infected leaves showing typical disease symptoms. Based on the analyses of their ITS1, ITS2 and 5·8S rDNA nucleotide sequences, the local isolates were clearly identified as B. oryzae. Selection for colonies of the three isolates derived from single conidia with either bipolar or intercalary germination had no effect on the number of spores showing bipolar germination in subsequent cultures. Germination on seven different culture media was tested; of these, rabbit food agar and water agar increased the percentage of bipolar germination of conidia, although this varied between isolates. Incubation of the cultures of all three isolates for longer periods prior to harvesting conidia increased the percentage of bipolar-germinating conidia from c. 40 to c. 90% with 5-day-old and 30-day-old cultures, respectively. [ABSTRACT FROM AUTHOR]

Isolates of Xanthomonas oryzae pv. oryzae (causal agent of bacterial blight of rice) from the Philippines representing two phylogenetic lineages, and five haplotypes within those two lineages, were evaluated for aggressiveness in two glasshouse trials. Aggressiveness was determined by clip-inoculating leaves of a rice cultivar lacking known, effective major genes for resistance and measuring the lengths of resulting lesions. Variance components analysis indicated that 55 and 46% of the lesion length variation were genetic in origin for the first and second trials, respectively. Variation of lesion length among isolates within haplotypes was highly significant in both trials (P = 0·002 and 0·027), but the effects of lineage and haplotype within lineage were not (P = 0·08 and 0·30 for lineage and P = 0·23 and 0·07 for haplotype). These results suggest that substantial heritable variation for aggressiveness exists within Philippine populations of X. oryzae pv. oryzae . This variation appears to be more prevalent within than among known phylogenetic groups, although mean differences among phylogenetic groups may still be of significant biological importance. [ABSTRACT FROM AUTHOR]

A repetitive DNA element cloned from Xanthomonas oryzae pv. oryzae was used to assess the population structure and genetic diversity of 98 strains of X. oryzae pv. oryzae collected between 1972 and 1988 from the Philippine Islands. Genomic DNA from X. oryzae pv. oryzae was digested with EcoRI and analyzed for restriction fragment length polymorphisms (RFLPs) with repetitive DNA element as a probe. Twenty-seven RFLP types were identified; there was no overlap of RFLP types among the six races from the Philippines. Most variability (20 RFLP types) was found in strains of races 1, 2, and 3, which were isolated from tropical lowland areas. Four RFLP types (all race 5) were found among strains isolated from cultivars grown in the temperate highlands. The genetic diversity of the total population of X. oryzae pv. oryzae was 0.93, of which 42% was due to genetic differentiation between races. The genetic diversities of strains collected in 1972 to 1976, 1977 to 1981, and 1982 to 1986, were 0.89, 0.90, and 0.92, respectively, suggesting a consistently high level of variability in the pathogen population over the past 15 years. Cluster analysis based on RFLP banding patterns showed five groupings at 85% similarity. The majority of strains from a given race were contained within one cluster, except for race 3 strains, which were distributed in three of the five clusters.