Pathology THERMIC CONTROL OF RATOON STUNTING DISEASE OF SUGARCANE IN COLOMBIA Jorge I. Victoria, Oscar Ochoa and Climaco Cassalett Centro de Investigacion de la Cana de Azucar de Colombia, CENICANA. Apartado Aereo 91 38. Cali-Colombia. ABSTRACT Ratoon stunting disease (RSD) is one of the main diseases affecting sugarcane in Colombia, hence attempts were made to control it by using different thermic treatments. Germination was increased significantly by pre-treating seedcane setts at 50 OC for 10 minutes one day before the usual treatment in hot water at 50 OC for 2 h. Treatment with hot water at 50 OC for 2 h gave better control of RSD than aerated steam or hot air. The degree of control nevertheless, depends on the level of susceptibility of different varieties to RSD. Hot water at 51 O for 1 h gave similar results of RSD control to 50 OC for 2 h, and much better control than was obtained at other temperatures. The best control of RSD was achieved when the shortest setts were used. INTRODUCTION In Colombia, sugarcane is important not only in the Cauca Valley where there are 135000 ha sugarcane under cultivation, but also in other parts of the country where there are 317587 ha of sugarcane for production. It makes an important contribution to the economy of the country through its direct and indirect effects on employment. Of the many diseases which affect the production of sugarcane, ratoon stunting disease (RSD) is among the most important. It is one of the most widely distributed throughout the world, and is caused by Clavibacter xyli subsp, xyli (Davis4). The effects of the disease depend on its incidence and the susceptibility of a variety to However, although a variety may be very susceptible to RSD, different systems r~f heat treatment of seedcane setts provide a substantial degree of control of the disease (Benda and ~icaudl). Immersion of the cane in hot water at 50 OC for 2 h is one of the most widely used treatments but this has the disadvantage of causing germination to be poor in some instances. Recently Dr B. Schlub (University of Louisiana) has developed a pre-treatment of seedcane to improve germination (personal communication), and this was evaluated under Cauca Valley conditions. The various sugar mills in the Cauca Valley use different heat treatments for their seedcane setts; these include hot water, aerated steam, and hot air. It was there- Keywords: Thermic control, RSD control 325
PATHOLOGY 1 fore considered to be very important to evaluate these treatments in terms of their efficiency in controlling RSD in three cane varieties having different levels of susceptibility to the disease. MATERIALS AND METHODS Pre-treatment with hot water The following treatments were compared: a) Pre-treatment alone by dipping seedcane setts in hot water at 50 "C for 10 min; b) Pre-treatment and then treatment in hot water at 50 "C for 2 h one day (24 h) later; c) Treatment of seedcane setts in hot water at 50 "C for 2 h without any pre-treatment; d) Check. The variety CP57-603 was used. A random block design with 4 replications was used for the, experiment and each replication comprised single rows 10 m long, planted with 30 seedcane setts having 3 buds per sett, giving a total of 90 buds per plot. Germination was determined 30, 45 and 60 days after planting. Comparison of thermic treatments The efficiency of RSD control by hot water, aerated steam and hot air was determined by comparing their effects on three varieties that were known to have different levels of susceptibility. The following treatments were wed: a) aerated steam (54 "C for 4 h); b) hot air (54 "C for 8 h); c) hot water (50 "C for 2 h); and d) check. The following varieties were used: CP57-603, PR1152 and EPC73-356. The experiment design was a random block with 4 replications. Each replication consisted of rows 10 m long each planted with 30 setts, each with 3 buds, giving a total of 90 buds per plot. Germination was determined 30, 45 and 60 days after planting. When the cane was 8-9 months old, the possible presence of the RSD bacterium was observed in 20 stalks taken at random from each plot. Assessments were made by phase contrast microscope (Gillaspie6), isolation on an artificial medium (~avis~) and by flourescent microscope (~amann~). Effect of hot water at different temperatures The effect of hot water at different temperatures was evaluated as follows: a) pre-treatment (50 "C for 10 min); b) pre-treatment followed one day later by treatment at 50 "C for 2 h; c) pre-treatment followed one day later by treatment at 51 "C for 1 h; d) pre-treatment followed one day later by treatment at 52 "C for 30 min; e) pre-treatment followed one day later by treatment at 53 "C for 15 min and f) check. Temperature was controlled in the water tank by an electronic transductor model Digitemp 11-100-TE (Sincron), with 0.1 "C precision. The following varieties were used: CP57-603, PR1152 and EPC73356. The experiment consisted of a random blocks design with 4 replications. Each replication was made up of rows 10 m long, each planted with 30 setts containing 3 buds per sett, giving a total of 90 buds per plot. Germination was assessed 30, 45 and 60 days after planting. When the cane was 8-9 months old the possible presence of the RSD bacterium was observed by phase contrast and by fluorescent microscope. Effect of length of sett The effect of the length of sett on the efficiency of RSD control by hot water was established using the following treatments: a) setts with one bud; b) setts with
7 JORGE I. VICTORIA, OSCAR OCHOA AND CLIMACO CASSALETT 327 two buds; c) setts with three buds; d) whole stalks chopped into setts after hot water treatment; and e) check, with three buds per set. The following varieties were used: Cp57-603, PR1152 and EPC 73356 pre-treated at 50 "C for 10 min and treated one day later with hot water at 51 "C for 1 h. The experiment had a random block design with 4 replications. Each treatment consisted of one row 10 m long, planted with a total of 90 buds per row. Germination was assessed 30, 45 and 60 days after planting. The possible presence of the RSD bacterium was determined in samples made up of 20 stalks taken at random per plot when the cane was 8-9 months old. Observations were made by phase contrast and by fluorescent microscope. RESULTS AND DISCUSSIONS Effect of pre-treatment with hot water The germinations in the different treatments, expressed as percentages of check plot results, are shown in Table I. In comparison with hot water treatment at 50 "C for 2 h alone, there were significant increases in germination when pre-treatment was carried out, with or without the 2 h treatment one day later. TABLEI. Germination of setts of variety CP 57-603 treated with hot water, (expressed as a percentage of check plot results) Percent germination at : Pretyatment (P) 152.8I1c2/ 118.2~ 120.2 c (50 C, 10 min) Hot w~ter (HW) (50 C, 2 h.) k i Check 100.0 b 100.0 b 100.0 b 11 Mean of four replications 21 Numbers followed by the same letter are not statistically different at the 5% level. Comparison of thermic treatments When seedcane was treated by means of aerated steam (54 "C for 4 h), hot air (54 "C for 8 h) and hot water (50 "C for 10 min 50 "C for 2 h), no detrimental effects on germination 60 days after planting of the three different varieties used were observed (Table 11). However, there were significant differences in the incidence of RSD following the different treatments. The best control of RSD was observed with hot water, followed by aerated steam and finally by hot air in PR1152. The hot water treatment also gave the least control of RSD in EPC73356. For CP57-603 the three treatments were equally
7 JORGE I. VICTORIA, OSCAR OCHOA AND CLIMACO CASSALETT 329 presumabiy because this variety is less susceptible to RSD than the other two varieties. According to Victoria (unpublished data), PR1152 and EPC73356 are highly susceptible varieties, whereas CP57-603 is considered to be susceptible. Similar differences were detected additionally when the approximate number of RSD bacteria present in juice, obtained by centrifugation from infected stalks, was determined for each of the varieties. The quantities were 0.2 x lo9, 2.2 x lo9 and 117 x lo9 cells/ml for CP57-603, PR1152 and EPC73356, respectively, each result being the average for 10 internodes per stalk and 4 stalks per variety. The determination was made by dilution series and a Petroff Hausser counting chamber. Effect of different temperatures Some differences in germination 60 days after planting were observed due to different temperature treatments, but they were small and not statistically significant. Good control of RSD was obtained at 50 OC and 51 OC and in CP57-603 some control was gained from the pre-treatment alone (Table 111). I TABLE Ill. Treatment Effect of hot water treatment at different temperatures CP 57603 PR 1152 EPC 73356 Germination RSD Germination RSD Germination RSD %Check % %Check % %Check % Check Pre-treatment (P) 100.0~/c~/ 95.0~ 100.0 b 100.0~ 100.0~ 100.0~ I/ Percent of germination related to check, mean of four replications. 21 Numbers followed by same letter are not statistically different at the 5% level. Apparently the treatments with hot water at 52 "C and 53 "C were not long enough to give good RSD control, but prolonging the duration of treatment at temperatures above 51 "C would almost certainly cause germination to be affected significantly. Effect of seedcane sett length The length of setts pre-treated and treated with hot water at 51 OC for 1 h did not appear to have any major effects on germination 60 days after planting (Table IV). However, there were significant differences in RSD control between treatments.
- JORGE I. VICTORIA, OSCAR OCHOA AND CLIMACO CASSALETT 331 Short setts gave the best control of RSD and whole stalks were the worst of all. Once again the degree of control of RSD was dependent on the level of susceptibility of the variety. Better RSD control was obtained with a susceptible variety (CP57-603) than was obtained with highly susceptible varieties (PR1152 and EPC73356). CONCLUSIONS The use of pre-treatment (50 OC for 10 min) prior to treatment with hot water at 50 OC for 2 h gave excellent increases in germination. Hot water treatment gave better RSD control than aerated steam or hot air, a result that is different from data reported by others (Damann and Benda3; Steib and Cifuentes8). Since hot water at 51 OC for 1 h gave similar RSD control to 50 OC for 2 h it can be recommended because it will increase the capacity of hot water treatment tanks considerably. It should be noted that RSD control is better in short setts than in long setts. REFERENCES 1. Benda, G.T.A. and Ricaud, C. (1978). The use of heat treatment for sugarcane disease control. Proc. ISSCT. 16:483-496. 2. Damann, K.E. Jr. (1983). Detection of ratoon stunting disease of sugarcane by metaxylem autofluorescence, Phytopathology 73500. 3. Damann, K.E., Jr. and Benda, G.T.A. (1983). Evaluation of commercial heat-treatment methods for control of ratoon stunting disease of sugarcane, Plant Dis. 67:966-967. 4. Davis, M. J., Gillaspie, A.G. Jr., Vidaver, A.K. and Harris, R. W. (1984). Cluvibacter : a new genus containing some phytopathogenic coryneform bacteria including Clavibacter xyli subsp. xyli sp nov, subsp nov and Clavibacter xyli subsp cynodontis subsp nov, pathogens that cause ratoon stunting disease of sugarcane and bermuda grass stunting disease, Int. J. Syst. Bacteriol. 34:107-117. 5. Davis, M. J., Gillaspie, A.G. Jr., Harris, R.W. and Lawson, R.H. (1980). Ratoon stunting disease of sugarcane: Isolation of the causal bacterium, Science 210:1365-1367. 6. Gillaspie, A.G., Jr., Davis, R.E. and Worley, J.F. (1973). Diagnosis of ratoon stunting disease based on the presence of a specific microorganism, Plant Dis. Rep. 57:987-990. 7. Gillaspie, A.G. Jr., Flax, G. and Koike, H. (1976). Relationship between numbers of diagnostic bacteria and injury by ratoon stunting disease in sugarcane, Plant Dis. Rep. 60:573-575. 8. Steib, R.J. and Cifuentes, O.M. (1977). Use of aerated steam as a possible method for the control of sugarcane mosaic and ratoon stunting disease, Sugarcane Pathol. Newsl. 18:24-27.