ABSTRACT
Ratoon stunting disease (RSD) describes a condition where some or all sugarcane plants in a field are infected by the bacterium Leifsonia xyli subsp. xyli (Lxx) (Evtushenko et al., 2000; Suzuki et al., 1999), previously Clavibacter xyli subsp. xyli (Davis et al., 1984). The bacteria inhabit the xylem vessels, occupying the protoxylem, metaxylem, lacunae, tracheids and xylem parenchyma cells (Kao and Damann, 1980). Infected plants produce fewer stalks, which are shorter and thinner than those of healthy plants. There are no specific external symptoms, but the infected plants may develop orange to red spots around the root primordia in the basal nodes when split longitudinally with a sharp knife (Steindl, 1950). However, this symptom is not unambiguous, because the intensity of discolouration varies, depending on the cultivar grown, and a very similar symptom can be caused by other factors (Hughes and Steindl, 1955; Steindl, 1961; Ricaud, 1974). Immature plants may show a salmon discolouration around the growing tip when split longitudinally, but again, this symptom is subject to interpretation because, once split, natural phenolic production starts to occur, resulting in reddening of the tissue (Hughes and Steindl, 1956). The incidence and level of infection varies within a field, producing a characteristic patchiness of the crop. Having no easily recognizable, specific symptoms, RSD is delineated in terms of either infected or non-infected fields.
