USDA, ARS, Southern Regional Research Centre, Sugarcane Research Unit, Houma, LA 70361 USA

Background and objectives
Ratoon stunting disease (RSD) of sugarcane (Saccharum interspecific hybrids) caused by Clavibacter xyli subsp. xyli is believed to cause yield loss in most sugarcane-growing areas of the world. In a study conducted between 1978 and 1986 in Louisiana, yield losses caused by RSD among sugarcane cultivars tested ranged from 5-31% across the three-crop planting cycle [1]. The first objective of this study was to determine the yield response to RSD among cultivars currently recommended for planting in Louisiana, and to demonstrate the importance of control practices.

Steib and Chilton [2] found that planting more stalks of virus-infected stalks of cane than recommended reduced the effect of sugarcane mosaic on yield. The second objective of this study was to determine the influence of increased planting rate on yield loss caused by RSD.

Materials and methods
Yield-loss experiments were arranged in a split-plot design with four replications. The main plots were plants infected with C.&nbso;xyli subsp. xyli or uninfected, and the subplots were sugarcane cultivars. Plots were 5 ;m long and 5.8 ;m (three rows) wide planted with 18 stalks. The experiment was planted nine times between 1985 and 1994. Each planting cycle consisted of three crops harvested annually.

Four field experiments to determine the influence of planting rate were planted annually in a split-split plot design. The main and subplots were as above and the sub-subplots were either a two-stalk (18 stalks per plot) or a four-stalk (36 stalks per plot) planting rate (the number of whole stalks placed side-by-side in the furrow with an overlap of two mature internodes with the next set of stalks). Each experiment had four replicates and up to four cultivars. Three annual crops were harvested from each planting.

Results and conclusions
Yield loss was detected infrequently in the plant-cane crop. Yield loss varied from year to year, and cultivars did not always express the greatest yield loss in the same year resulting in year by treatment interactions. Among the nine recommended cultivars which occupied 97% of the land area in Louisiana planted to sugarcane in 1997, eight (CP 65-357, CP 70-321, CP 72-370, LCP 82-89, HoCP 83-153, LCP 85-384, HoCP 85-845, and LCP 86-454), were moderately to highly susceptible to infection by C. ;xyli subsp. xyli. The loss of sugar among these cultivars ranged from 8& to 46% across the three-crop planting cycle. Cultivar CP 79-318 was resistant to yield loss from RSD; however, the bacterium was readily detected in the xylem of infected plants.

In plant-cane, increasing the planting rate from two to four stalks led to higher yields in RSD-free and in diseased plants. The increased yield appeared to result from the increased planting rate and not as compensation of yield loss due to RSD. The advantage of an increased planting rate in the diseased plots was lost in the first- and second-ratoon crops; the yield reduction caused by RSD was similar for cane planted at 2- and 4-stalk planting rates. In contrast to the advantage demonstrated with sugarcane mosaic [2], there does not appear to be an advantage of using a higher planting rate as a means of compensating for the yield loss caused by RSD.

1. Grisham MP, 1991. Phytopathology 81, 337-340.
2. Steib RJ, Chilton SJP, 1974. Proceedings International Society Sugar Cane Technologists 15, 300-304.