Molecular Detection of Diverse Leifsonia Strains Associated With Sugarcane

Sampling of Bacteria Associated with Plant Vascular Tissues

The vasculature of plants is typically colonized by a wide-range of bacteria with diverse functions. These bacteria can be sampled by pooling plant biopsies in water and then concentrating cells by centrifugation. When the extracted bacteria are added as a template for the polymerase chain reaction (PCR), sufficient DNA is generally liberated to facilitate the identification of specific taxa and characterization of bacterial community structure. The sampling technique facilitates surveys of multiple plants comprising a single crop, allowing for a more comprehensive understanding of the crop microbiome than what can be achieved when examining single plants. This technique is rapid and cost-effective, and will help researchers monitor microbes associated with vascular tissues at various stages of crop development.

Improved Primers for the Specific Detection of Leifsonia xyli subsp. xyli in Sugarcane Using a Conventional PCR Assay

Ratoon stunting disease (RSD), one of the most important diseases of sugarcane, is caused by the bacterium Leifsonia xyli subsp. xyli (Lxx). Lxx infects sugarcane worldwide and RSD results in high yield losses and varietal degeneration. It is highly challenging to diagnose RSD based on visual symptomatology because this disease does not exhibit distinct external and internal symptoms. In this study, a novel Lxx-specific primer pair Lxx-F1/Lxx-R1 was designed to detect this pathogen using a conventional PCR assay. These primers were then compared with four published Lxx-specific primers and one universal Leifsonia generic primer pair LayF/LayR. Sugarcane leaf samples were collected from Saccharum spp. hybrids in commercial fields (315 samples) and from germplasm clones of five Saccharum species and Erianthus arundinaceus (216 samples). These samples were used for comparative field diagnosis with six conventional PCR assays. Sensitivity tests suggested that the PCR assay with primers Lxx-F1/Lxx-R1 had the same detection limit (1 pg of Lxx genomic DNA) as the primer pairs Cxx1/Cxx2 and CxxITSf#5/CxxITSr#5 and had 10-fold higher sensitivity than the primer pairs Pat1-F2/Pat1-R2, LayF/LayR, and C2F/C2R. Comparison of PCR assays revealed that natural Lxx-infection incidence (6.1%) in field sample evaluation identified by Lxx-F1/Lxx-R1 primers was higher than incidences (0.7 to 3.0%) determined by other primer pairs. Moreover, no nonspecific DNA amplification occurred within these field samples with Lxx-F1/Lxx-R1 primers, unlike with the primer pairs Cxx1/Cxx2 and LayF/LayR. Diverse Leifsonia strains were identified by PCR detection with LayF/LayR primers in the field samples, whereas whether these Leifsonia strains were pathogenic to sugarcane requires further research. Our investigations revealed that the PCR assay with the newly designed primers Lxx-F1/Lxx-R1 could be widely used for RSD diagnosis and Lxx-pathogen detection with satisfactory sensitivity and specificity.

Turning a Blind Eye to Ratoon Stunting Disease of Sugarcane in Australia

The Australian sugar industry has never pursued genetic resistance to ratoon stunting disease (RSD), despite it being widely considered to be one of the most important diseases of sugarcane (Saccharum interspecific hybrids). This is because of a prevailing view that the disease is economically managed, and that no further action needs to take place. However, there is a range of epidemiological evidence that suggests that RSD is having a more significant impact than what is generally recognized. This review traces the factors that have led to an industry stance that is apparently without any scientific justification, and which has tended to downplay the significance of RSD on Australian sugarcane productivity, and thus has led to significant lost production. The consequences of this position are that RSD may be influencing broad but poorly explained issues such as commercial ratooning performance of existing varieties and the "yield decline" that has been subject to much scrutiny, if not much success in resolving the issue. Based on the available information, this review calls on the Australian sugar industry to prioritize selection for RSD resistance in the plant improvement program.