Possibilities for the Control of Sugarcane Smut

Most investigations of the use of fungicides for the control Results are described of field trials in which attempts were of Smut have been carried out with setts that were inoculated made to control sugarcane smut with fungicides. Smut was by dipping in spore suspensions, either before or after treatsuccessfully controlled in the plant cane crop when seedcane ment with f~ngicides.~~ s> 'Partial control has been reported was treated in hot water containing the fungicide triadimefon. for the fungicides Aretan '* pyracarbolid2 and several coded Control occurred when treated setts were subsequently either ~ o m ~ u n d s . ' " h d e r such ~~ndi t ions of testing, with the inoculated with smut or were planted into soil infested with fungicide in close contact with the smut spores on the setts, smut spores. A triadimefonlhot water treatment for the fungicides are most likely to appear effective. However, cane is suggested as a means of controlling both smut and complete control has not been obtained in tests with inoratoon stunting disease (RSD) . In areas where smut is severe culated setts. Little or no work has been reported on the this treatment may allow hot water treatment (HWT) to be testing of fungicides applied to healthy or diseased setts that used to eliminate RSD and smut from sugarcane also have been planted in the field under severely smut-contamiprotecting the treated seedcane from subsequent infection by nated smut. Materials and Me4hods Introduction The six experiments reported in this paper were all conSugarcane smut (Ustilago scif~minea S~dow) is becoming ducted under irrigated conditions in an arid environment. a more i m ~ f i a n t problem in many cane growing areas of Experiments 1 to 5 were conducted at Big Bend, Swaziland. the world. In recent Years smut has spread to the Chribbean In this area smut is endemic and is developing into a major and it first appeared in the continental United States in 1978. production problem in variety NCo 376, The experimental The disease is COrr~mon in all cane growing ~ountries in site was on a Somerling series soil, a shallow, gravelly clay southern and central Africa, where production is largely based loam. The incidence of smut at this site was high, ~ ~ ~ r i on the susceptible varieties NCO 310 and NCO 376. The merits 1 and 2 were conducted from ~~~~~t 1976 to ~~~~t incidence of smut is increasing in Swaziland and in the more 1978 and expriments 3, 4 and 5 from November, 1977 to northerly parts of the South African sugar industry and in November, 1978. Experiment 6 was carried out at Pongola some areas has reached an economically serious l e ~ e l . ~ ~ during 1978179 on a deep, Makatini series clay loam. Smut Experience from Rhodesia and some parts of Swaziland has is also endemic in this area but the incidence of infection at shown that smut can be contained by the intensive application the trial site was lower than at the Big Bend site. of field control measures, provided that the varieties grown The variety used in the experiments was NCo 376, which are not extremely susceptible and that smut is not allowed is highly susceptible to smut when inoculated or when exposed to reach a very serious level of development. These masures to a high inoculum concentration of the disease. NCo 376 include the production and planting of healthy seedcane* in has a resistance rating of 9 (on a scale of 0, immune to 9, very addition to the roguing of diseased plants and the ploughing highly susceptible) from its reaction after the inoculation of out of severely ~mutted fields to reduce the inoa~lum conseedcane setts, when 50% or more plants, usually develop centration of the disease. smut. With the exception of most of the treatments in experiThe smut pathogen usually infects the cane plant through ment 4, all the NCo 376 seedcane used was obtained from the buds1* and the infection of the buds of seedcane at or nurseries grown in a smut-free area. This seedcane was free shortly after planting is likely to be an important factor in from all systemic diseases before planting or before inocuthe development of epidemics. lation with smut. A successful fungicide treatment that aided in the producExpeiments I md tion of healthy seedcane and that protected seedcane from infection at planting could make a useful contribution to the The first two experiments were intended to investigate control of smut. Application to seedcane at planting is also whether the addition of fungicides to a 10% I?WT (2 h/5O0C) likely to be the simplest and &apest method by which (-hemicould Overcome the deleterious effects of HWT on the recals could be used for disease control in sugarcane. action of seedcane to smut. The five fungicides tested were Hot water treatment is well known to eliminate smut from benomyl 50% wp (Benlate), carboxin 75% wp (Vitavax), infected seedcane. However, there is much evidence from fenfuram 80% wp, (Panoram), pyracarbolid 40% ec (Sicarol) and triadimefon 25% wp (Bayleton). Carboxin was also South Africa that HWT increases the susceptibility of seedapplied to seedcane that had not been heat treated and as cane to subsequent infection4. Thus treated seedcane can be rapidly infected if it is planted into soil that is severely confO1iar spray On Young cane The treatments were taminated with smut spores and the resulting crop can develop HWT (2 h/50°C), no fungicide. I more smut than if the seedcane were untreated. It is for this HWT + benomyl in HWT tank (500 pg/ml ai) . reason that HWT has not been used in the northern, irrigated HWT + pyracarbdid in HWT tank (500 pg/ml ai). production areas of South Africa and in Swaziland since HWT + fenfuram in HWT tank (500 pg/ml 1976. In these areas smut is widespread in the dominant variety, NCo 376, but RSD is not a production problem under HWT + triadimefon in HWT tank (500 pglml ai). fully irrigated growing conditions. A fungicide treatment for HWT in HWT tank (500 pgfml seedcane that afforded protection against smut after HWT HWT + carboxin foliar sprays, 1,6 kglha at 4, 6 and 8 would permit the re-introduction of HWT into these areas. weeks after planting. ir poceedings of The South African Sugar Technologists' Association June 1979 HViT + carboxin in HWT tank + carboxin foliar sprays. No HWT, no fungicide. No HWT + carboxin, 2 h cold soak (500 pglml ai). Plots consisted of single rows of 7,5 m at a spacing of 1,5 m. Each plot contained 20, 4-budded setts and the treatments were replicated four times. In experiment 1, after the hot water and fungicide treatments had been carried out, the setts were inoculated with smut by soaking for 15 minutes in a fresh suspension of smut spores (approximately 5 x 106/ml) immediately before planting. In experiment 2 the treated setts were planted in furrows into which smut spores had been incorporated (one fresh smut whip per row of 7,5 m). Considerable natural contamination of the soil of the trial site firom infected surrounding cane was likely. Thae first experiments were conducted over the plant and first ratoon crops on a growing cycle of 12 months. Smut whips developing in the plots were counted at monthly intervals. In the plant crop the whips were removed at each inspection and a cumulative total of the whips in each plot was determined. In the first ratoon crop the smut whips were not removed and the maxinlum number of whips in each plot was recorded.