THE INTRODUCTION OF A SEROLOGICAL METHOD FOR LARGE SCALE DIAGNOSIS OF RATOON STUNTING DISEASE IN THE SOUTH AFRICAN SUGAR INDUSTRY

Ratoon stunting disease (RSD). caused by the bacterium Clovibacter xyli subsp xyli, is widely distributed i n the South African sugar industry and, if common, severely reduces the yield of most sugarcane varieties under both rainfed and irrigated conditions. It is important that intended seedcane sources and plough-out fields are checked for freedom from RSD before fields are planted. Phase contrast microscopy (PCM) has been used i n the South African sugar industry to diagnose RSD on a large scale since 1977. The diagnostic service offered to cane growers has provided valuable information on the RSD situation in the sugar industry and has contributed greatly to success in reducing the levels of the discasc in most areas. PCM is a rapid and acceptably reliable method of diagnosis but it requires the urgent transportation of bulky stalk samplcs, often from distant parts of the industry, for examination in laboratories at the South African Sugar Association Experiment Station (SASEX). An evaporativc binding-enzyme immunoassay (EB-EIA) replaced PCM in April 1998. T h ~ s method of diagnosis has a number of advantages over PCM. The main one is that extracted xylem sap is transported to SASEX in small tubcs when convenient. Additionally, the test is less subjective than PCM and more samplcs can be processed at a lower cost. A number of largc scale comparisons between PCM and EB-EIA using samplcs from growersi fields have shown that the two methods arc similar i n sensitivity, and in the last comparison there was 97% agreement. Staff of Local Pcst & Disease Control Committees (LP&DCCs) arc now closely involved in RSD diagnosis. Stalk samples are delivered to the local offices ot LP&DCCs, whcre technical staff perform the initial extraction of sap and forward these samples to SASEX for serological diagnosis. In the first year of implementation of EBEIA, the number of samples tested increased from approximately 6 000 to more than 9 000. Introduction Ratoon stunting disease (RSD) is caused by the xylem-limited bacterium, Clavibacter xyli subsp xyli (Cxx) (Davis et al., 1984). It is generally considered to cause greater losses in yield world-wide than any other sugarcane disease. The extent of yield loss varies with environmental conditions and the susceptibility of the varieties grown. In South Africa, losses as high as 30 to 40% have been recorded in susceptible varieties grown under typical rainfed and irrigated conditions (Ba~ley and Bechet, 1986; 1995). RSD causes no obvious external symptoms other than a nonspecific stunting and is thereforc difficult to diagnose with accuracy i n the field. Up to the mid1970s diagnosis was bascd on the observation of internal stalk symptoms, but these vary both between and within varieties, are often inconspicuous and are unreliable. A bacterium was first found to be associated with RSD in 1972 (Gillaspie et al., 1973) and this soon led to diagnosis based on the microscopic cxamination of the xylem vessel contents. This method of diagnosis was adopted in the South African sugar industry in 1976 (Bailey, 1976) and i n 1977 a large-scale diagnostic scrvicc for the sugar industry was introduced using phase contrast microscopy (PCM) to detect Cxx (Bailey and Fox, 1984). The diagnostic service has provided valuable information on the RSD situation in the South African and neighbouring sugar industries. It has also lead to an increased awareness of thc discasc and this has been an important factor in reducing the incidence of RSD in most parts of the South African industry (Bailey and Tough, 1991). Although PCM is an acceptably I-eliable and rapid method of diagnosis, i t has some didvantage5. In the South African sugar industry, bulky samples of stalks have to be transported, often over long d~stances, to SASEX laboratories at Mount Edgecombe or Pongola. The samples need to be processed within 24 hours of collection in the field to ensure reliable results (Bailey and Fox, 1984). Additionally, microscope diagnosis requires skilled technicians and is time consuming. The number of samples processed per day is thercfore limited and the results can be subjective. A serological test, the evaporative binding-enzyme