Further Evidence of the Effects of Ratoon Stunting Disease on Production under Irrigated and Rainfed Conditions

In a field experiment under rainfed conditions, reductions in cane yield over three crops due to infection by ratoon stunting disease (RSD) ranged from 41% to 19 % respectively in varieties N17, N14, N21, NCo376 and N19. Losses in Nl2 were 10%, and in N23 and N22 were 7% and 1%. In a similar trial conducted under irrigation, the greatest reductions in cane yield over two crops were 32% in variety N25, 29% in NCo376, 22% in N24 and 21% in N19. The effects of RSD on yield of recoverable sucrose were similar to those on cane yields. Reductions in yield were mainly due to reductions in stalk mass. Based on field experiments conducted since 1980 to determine the effect of RSD on growth and yield, currently popular and new South African released varieties are ranked as follows in terms of the effect ofRSD on production: intolerant, N17, N14, NCo376, N24, N25; intermediate, N24, N19, N12, N23; relatively tolerant, N22. That substantial losses from RSD can occur in most varieties under irrigated as well as rainfed conditions demonstrates the importance of precautionary action by growers to prevent the introduction and spread of RSD in cane plantings. Introduction Ratoon stunting disease (RSD), caused by the bacterium Clavibacter xyli subsp xyli Davis et al.(Cxx), is well known to be an economically important disease of sugarcane in most countries where the crop is grown. It is also well known that varieties differ in response to infection by Cxx and that the effects of infection can vary with growing conditions. Generally, losses due to RSD are relatively greater when the crop suffers from stress. Until recently, there was little reliable information published on the effect of RSD on sugarcane grown under full irrigation in southern Africa. However, the results of an irrigated trial conducted at Pongola from 1991 to 1993 demonstrated yield reductions as great as 25% in N14 and 17% in NCo376 (Bailey and Bechet, 1995). These results confirmed that RSD can cause substantial yield reductions in irrigated cane, even under good growing conditions. The continually improving RSD situation in most parts of the South African industry has followed an increase in the application of control measures by growers, partly in response to the publicity given to the widespread occurrence of the disease Proc S Afr Sug Technol Ass (1997) 71 and to the magnitude of its effects on the production. An important aspect of the SASEX research programme therefore is to determine the effect of RSD infection on popular and new commercial varieties under different growing conditions. The results of two field trials are reported in this paper, one under rainfed and one under irrigated conditions, include some varieties from previous similar experiments and some varieties tested for reaction to RSD for the first time. The results from the plant crop of the rainfed trial were published in 1995 (Bailey and Bechet, 1995). This paper contains results from the first and second ratoon crops. The results from a further trial conducted under irrigated conditions are also presented. Experimental procedures The seedcane of all the varieties for both trials was obtained from special propagation plots at Mount Edgecombe. These had been established with seedcane obtained from healthy nurseries that was then either subjected to hot water treatment for 2 h at 50°C or inoculated with Cxx using the standard pressure cup method with juice from infected stalks immediately before planting. Stalks plucked from these propagation plots were used to establish the trials. When the trials were harvested, precautions were taken to prevent the spread of RSD to the healthy plots. Stalks were sampled from the guard rows of the trials in all crops and checked by phase contrast microscopy for the presence of Cxx. These tests confirmed that Cxx did not spread into the plots that had been planted with healthy seedcane and that the diseased plots were uniformly infected.