BGRcast: A Disease Forecast Model to Support Decision-making for Chemical Sprays to Control Bacterial Grain Rot of Rice.

A disease forecast model for bacterial grain rot (BGR) of rice, which is caused by Burkholderia glumae, was developed in this study. The model, which was named 'BGRcast', determined daily conduciveness of weather conditions to epidemic development of BGR and forecasted risk of BGR development. All data that were used to develop and validate the BGRcast model were collected from field observations on disease incidence at Naju, Korea during 1998-2004 and 2010. In this study, we have proposed the environmental conduciveness as a measure of conduciveness of weather conditions for population growth of B. glumae and panicle infection in the field. The BGRcast calculated daily environmental conduciveness, Ci , based on daily minimum temperature and daily average relative humidity. With regard to the developmental stages of rice plants, the epidemic development of BGR was divided into three phases, i.e., lag, inoculum build-up and infection phases. Daily average of Ci was calculated for the inoculum build-up phase (Cinf ) and the infection phase (Cinc ). The Cinc and Cinf were considered environmental conduciveness for the periods of inoculum build-up in association with rice plants and panicle infection during the heading stage, respectively. The BGRcast model was able to forecast actual occurrence of BGR at the probability of 71.4% and its false alarm ratio was 47.6%. With the thresholds of Cinc = 0.3 and Cinf = 0.5, the model was able to provide advisories that could be used to make decisions on whether to spray bactericide at the pre- and post-heading stage.

Nematicidal activity of a nonpathogenic biocontrol bacterium, Pseudomonas chlororaphis O6.

Bacterial culture filtrates of an aggressive rhizobacterium, Pseudomonas chlororaphis O6, displayed strong nematicidal activity. The nematicidal activity of P. chlororaphis O6 was markedly reduced in the gacS mutant of P. chlororaphis O6 grown in the presence of glycine, but no reduction of nematicidal activity in the gacS mutant was noted in the absence of glycine. The results of bioassay with P. chlororaphis O6 mutants showed that phenazine and pyrrolnitrin production was not a major factor, but the effects of glycine in the culture medium suggest that formation of hydrogen cyanide might be important. Assessments in greenhouse studies with tomatoes growing in nematode-infested soils confirmed that the application of P. chlororaphis O6 resulted in the control of the root-knot nematode. Our results demonstrated that P. chlororaphis O6 could be employed as a biocontrol agent for the control of the root-knot nematode, and the global regulator, GacS, functions as a positive regulator of the expression of nematicidal compounds and enzymes in P. chlororaphis O6.

Molecular characterization of Korean Pepper mottle virus isolates and its relationship to symptom variations.

The symptom variations among Korean Pepper mottle virus (PepMoV) isolates infecting pepper, tomato and potato were described and the cause of variations in relation to molecular variability were investigated. In addition, the entire genome of the 13 PepMoV isolates, collected from five provinces (Kyonggi, Chungnam, Gyeongnam, Jeonbuk and Jeonnam) in Korea, were determined and compared including the previously reported Korean-Vb isolate and 2 other PepMoV isolates isolated from America (CA and FL). Our results showed that the nucleotide sequence of all Korean isolates tested were nearly identical (98-99%) and only 94% similar to American isolates. In general, the complete nucleotide sequences and deduced polyprotein sequences indicated low genetic variation among isolates showing 0.1-3% nucleotide changes per site. However, based on ratio between nucleotide diversity values in nonsynonymous and synonymous position (dN/dS ratio) surprisingly, P1 and 6K2 genes showed relatively high nucleotide substitution ratio (0.8 and 1.0 nucleotide, respectively). When the 6K2 amino acid were aligned, there were 15 amino acid substitutions found in PepMoV-infected potato and only 1 amino acid change from two isolates of PepMoV-infected bell pepper. Interestingly, three isolates including isolate numbers 731, 205135 and 205136 that possessed different aa changes at 6K2 region also showed distinct symptom differentiation in indicator hosts and cosegregated in the phylogenetic analysis. These results further proved previous studies that P1 and 6K2 genes with other proteins might have some involvement on host specificity and pathogenicity.

Degradation of acyl-homoserine lactone molecules by Acinetobacter sp. strain C1010.

A bacterium C1010, isolated from the rhizospheres of cucumbers in fields in Korea, degraded the microbial quorum-sensing molecules, hexanoyl homoserine lactone (HHSL), and octadecanoyl homoserine lactone (OHSL). Morphological characteristics and 16S rRNA sequence analysis identified C1010 as Acinetobacter sp. strain C1010. This strain was able to degrade the acyl-homoserine lactones (AHLs) produced by the biocontrol bacterium, Pseudomonas chlororaphis O6, and a phytopathogenic bacterium, Burkholderia glumae. Co-cultivation studies showed that the inactivation of AHLs by C1010 inhibited production of phenazines by P. chlororaphis O6. In virulence tests, the C1010 strain attenuated soft rot symptom caused by Erwinia carotovora ssp. carotovora. We suggest Acinetobacter sp. strain C1010 could be a useful bacterium to manipulate biological functions that are regulated by AHLs in various Gram-negative bacteria.