The Effect of Ratoon Stunting Disease on the Yield of Some South African Sugarcane Varieties under Irrigated and Rainfed Conditions

The results of two field trials to determine the effect of ratoon stunting disease on the growth and yield of some widely grown and newly released sugarcane varieties are described. In the plant crop of a trial grown under full irrigation, the greatest reductions in yields of cane and sugar among six varieties, approximately 15%, occurred in N 14 and NC0376. In the first ratoon, the greatest reductions in cane yield occurred in Nl4 (25%),Nl7 (19%),NC0376 (17%) and CP66/1043 (16%) and there were similar reductions in yields of sugar. The responses to infection recorded in the ratoon crop of the irrigated trial were probably under-estimated because of widespread infection detected in the control plots. In the plant crop of a trial with eight varieties grown under rainfed conditions and which received 80% of long term mean rainfall, the greatest reductions in cane yield occurred in varieties Nl7 (50%), Nl4 (32%) and NC0376 (22%). Significant losses were also recorded in N 19 in both trials and in N21 in the rainfed trial. Variety N22 was tolerant ofinfection in both the irrigated and rainfed trials, and varieties N 12 and N23 were tolerant in the rainfed trial. Yield reductions in both trials were mainly due to reduced stalk mass. There was little effectofinfection on cane quality.