Submitted to: Sugar Bulletin
Publication Type: Trade Journal
Publication Acceptance Date: May 20, 2002
Publication Date: June 20, 2002
Citation: Grisham, M.P., Pan, Y. 2002. Monitoring mosaic [trade journal]. Sugar Bulletin. 80(9):29-31. Interpretive Summary: During the early years of the 20th Century, sugarcane mosaic, a disease caused by a virus, contributed to the near collapse of the sugarcane industry in Louisiana. The solution to the problem began with the importation of sugarcane varieties from Indonesia that were resistant to the disease. These varieties replaced the susceptible varieties and were also used as parents to breed new varieties that were resistant to mosaic and were better adapted to the growing conditions of Louisiana. The problem of mosaic, however, became a chronic one because the virus has the ability to develop new strains that are capable of attacking resistant varieties after they are adopted by growers. Since 1956, only three new strains of the virus have been reported in Louisiana and most of the sugarcane planted in the state is resistant to these strains including the variety, LCP 85-384, that occupies approximately 80% of the area planted to sugarcane. In a survey conducted in 2001 of plants with mosaic, we found that 86% of the plants were infected with two of the previously reported virus strains; however, 14% of the plants were infected with at least two new strains. The new strains have only been detected from varieties known to be susceptible to mosaic or among experimental varieties. The loss of experimental varieties to susceptibility to mosaic is important, but fortunately to date, there is no evidence that these new strains can attack the resistant LCP 85-384 variety. Because the Louisiana sugarcane industry is so dependent on a single variety, it would be very vulnerable to significant yield losses if a new strain emerged that is capable of infecting LCP 85-384. We will continue to monitor commercial and experimental varieties for their susceptibility to these new strains or others that may emerge.
Technical Abstract: Mosaic of sugarcane is caused by strains of sugarcane mosaic virus (SCMV) or sorghum mosaic virus (SrMV). During the first half of the Twentieth Century, mosaic in Louisiana sugarcane was caused by strains of SCMV. SrMV strains H, I, and M were identified in 1956, 1966, and 1973, respectively. In field surveys of plants with mosaic symptoms conducted between 1978 and 1995, more than 90% were infected with SrMV strain H, the remainder with strains I and M. No plant was found infected with SCMV. Surveys were discontinued because the cost of labor and the time required for identifying strains using host differentials was high and the results had changed little in 10 years. A survey conducted in 2001 using molecular tests to identify SCMV and SrMV strains indicated a shift in the population of strains. SrMV strain I and strain H were associated with approximately 65% and 21% of the sugarcane plants with mosaic symptoms, respectively. The shift in which strain of SrMV is predominant suggests we should use a mixture of strains to screen our germplasm for resistance to mosaic, especially the parental varieties used in both the basic and commercial breeding programs. The remainder of the plants (14%) with mosaic symptoms appeared to be infected by a new strain or strains of SrMV not recognized by the molecular tests. Although the new strain or strains of SrMV recovered in the 2001 survey have not been shown to be a risk to Louisiana sugarcane, the potential for the virus to produce new strains was demonstrated. This ability to produce new genetic types or strains also points out the potential vulnerability of an industry that is depending on one variety should a new strain arise that has the ability to overcome the resistance of that variety. Because of the extensive planting of LCP 85-384, the selective pressure is on the virus to produce a new strain that can overcome the resistance in the variety.