Identification of parents for breeding sugarcane yellow leaf and ratoon stunt resistant cultivars.

THE INCIDENCE of Sugarcane yellow leaf virus (SCYLV) infection and ratoon stunt reaction of Stage II CP-clones were evaluated as a means to identify parents for the development of resistant cultivars in the Florida sugarcane breeding program. Over two seasons (2002 and 2003), incidences of SCYLV natural infection and ratoon stunt infection by inoculation were determined in Stage II CP clones. Only one parental clone, CP 94-1528, had a higher incidence of SCYLV than the overall mean of all the parents, Parents that would give rise to progeny with a lower incidence of SCYLV infection could not be identified because of limited data. The ratoon stunt data showed that three cultivars, CP 81-1384, CP 84-1198, and CP 93-1634, used either as a female or male parent over two years, produced more resistant progeny than the other clones used in crosses at the same time. Introduction Yellow leaf and ratoon stunt are two important diseases of sugarcane in Florida and worldwide (Davis and Bailey, 2000; Lockhart and Cronje, 2000). Cultivars differ in their reaction to both these diseases, and resistant cultivars can be selected (Comstock et al., 2001; Lockhart and Cronje, 2000). Sugarcane yellow leaf syndrome was recognised in Florida in 1994 and was subsequently recognised as yellow leaf with the detection of the Sugarcane yellow leaf virus (SCYLV). In Florida, plants of CP-cultivars in most growers’ fields have greater than 85% SCYLV infection (Comstock, unpublished data). Yield losses have also been reported in Brazil, Florida and Louisiana (Comstock and Miller, 2004; Vega et al., 1997; Grisham et al., 2002). SCYLV is transmitted by Melanaphis sacchari (Scagliusi and Lockhart, 2000) and by infected seedcane. Over time most clones in the CP-cultivar development program become infected by SCYLV (Comstock et al., 1999). Uninoculated plants in Stage II have been grown in field plots for approximately 2 to 3 years, and have been exposed to aphid transmission of SCYLV. Ratoon stunt, caused by Leifsonia xyli subsp. xyli (Lxx), is spread by contaminated knives and by infected seedcane (Davis and Bailey, 2000). The CP cultivar development program screens for resistance to ratoon stunt by inoculating progeny of crosses. The recently released cultivars are more resistant than those grown prior to the initiation of the screening program (Comstock et al., 1999, 2001). The spread of Lxx is slower in resistant cultivars; therefore, the incidence in resistant cultivars is lower in commercial fields (Comstock et al., 1996, 1997). Because of the large number of clones screened for ratoon stunt resistance in the CP program, differences in the disease reactions of progeny on a family basis have been observed.