COMPARISON OF RSD ASSAYS FOR DIAGNOSIS AND SCREENING VARIETIES FOR RESISTANCE

RATOON STUNTING DISEASE (RSD), caused by the bacterium Leifsonia xyli subsp. xyli, has no specific external symptoms and internal symptoms are often unreliable. Immunoassays are widely used to diagnose the disease because they are sensitive, relatively inexpensive and are suitable for bulk screening. The most widely used immunoassays for RSD diagnosis are the enzyme-linked immunosorbent assay (ELISA) and the stalk tissue blot immunoassay. Phase-contrast microscopy is also used for diagnosis but one person can only process 100–200 samples in a day. These assay methods all use extracts from stalks of mature cane. Collecting stalks from mature crops of cane is heavy work and requires bulky stalk samples to be transported to the site where they are processed. RSD bacteria are known to occur in roots, stalks, leaf sheaths and leaves. Bacteria occur at low concentration in leaves. There have been two previous studies that have identified moderate to high levels of bacteria in leaf sheaths. Samples of leaf sheaths can be easily and quickly taken in the field and can be transported in small plastic bags. In this paper, we compare five different diagnostic methods for RSD; phase-contrast microscopy of extracts from stalks and leaf sheaths, ELISA on extracts from stalks and tissue blot immunoassay of stalks and leaf sheaths. Tissue blot of stalk tissue was the most sensitive method followed closely by ELISA. Tissue blot of leaf sheath tissue and phase-contrast microscopy had similar sensitivity but were less sensitive than tissue blot of stalk tissue and ELISA. The ELISA and tissue blot of leaf sheaths were compared for screening varieties for resistance to RSD. Twenty seven varieties were screened for resistance with both methods. The absorbance from the ELISA was highly correlated with the percent infected vascular bundles in the leaf sheath. This paper discusses the advantages and disadvantages of the various diagnostic assays.