1a.Objectives (from AD-416)
To determine the susceptibility of commercial geranium (Pelargonium) cultivars to bacterial wilt caused by Ralsonia solanacearum, and whether certain plants may act as asymptomatic hosts; to determine whether susceptibility is correlated with genetic relatedness of the host cultivar; to determine the susceptibility of other greenhouse-grown ornamental crops to Ralstonia solanacearum; to compare isolates of the bacterium from different outbreaks to correlate with geographic origin; to determine freeze survival of the bacterium in soil; to investigate chemical control measures; and to identify genes associated with the survival and pathogenicity at different temperatures, evaluate pathogen populations in relation to their ability to cause pathogenicity to agricultural crops at various temperatures and determine potential environmental fate and movement in the United States.
1b.Approach (from AD-416)
An empirical approach will be used to examine the genetic and physiological capability of exotic strains of bacterial wilt to access their ability to become established and spread within U.S. agriculture. Once introduced Ralstonia solanacearum is know historically to be spread via river systems. The project will further address the survival of distinct genetic populations in various water types and assess the ability to infect and survive under different temperature regimes.
Ralstonia solanacearum is the causal agent of Bacterial Wilt of more than 200 species of plants. This pathogen is distributed in tropical and subtropical areas of the world. R. solanacearum race 3 biovar 2 (R3B2) strains are found in the coldest areas of their range and are thought to be pathogenic at low temperatures. Previously, pathogenicity of R3B2 strains was compared with that of other races of Ralstonia in environmental chambers at 18 degrees C and at 30 degrees C. Both R3B2 and some R1B1 strains isolated from ornamentals were found to be pathogenic on tomato and potato at both 18 C and at 30 C. Growth and twitching motility characterization of selected strains of Ralstonia were completed in order to identify potential phenotypes associated with pathogenicity at low temperature. In addition to profiles of cellular proteins, proteomics experiments were conduced to identify secreted proteins differentially expressed at warm and cool temperatures. Qualitative observations of twitching motility suggest change in behavior at low temperatures; however further expression studies are needed to confirm these results. We also identified an additional set of proteins differentially secreted in contact with plant roots in selected strains with different virulence behavior at low temperatures. Currently, gene expression corresponding to candidate proteins whose accumulation seems correlated with the pathogenic behavior of specific strains is being confirmed and quantified by qRT-PCR. With agricultural globalization, movement of plant pathogens has increased; therefore their introduction to new regions poses risk to agriculture or the environment. Certain races of R. solanacearum such as R3B2 strains have been designated as a “Select Agent” under the Agricultural Bioterrorism Protection Act of 2002 because of historical evidence of their ability to survive and kill solanaceous crops such as potato under cool climate conditions. Strict regulations have been implemented by APHIS/USDA in order to prohibit establishment of R3B2 in the United States. In this work, strains currently present in the United States that have the potential to survive and infect tomato and potato plants at low temperatures have been identified; thus it is important to determine the potential environmental fate and movement of these populations within the United States and to identify their virulence determinants at low temperature. This research will provide information necessary for prudent regulatory procedures and methods of control.
Research activities under this agreement were monitored by e-mails and reports.