6.19
DISEASE-FREE NURSERIES IN SUGARCANE

JI VICTORIA1, ML GUZMAN1 and AD JARAMILLO2

1Colombian Sugarcane Research Center (CENICAÑA), PO Box 9138, Colombia; 2Mayagüez Sugar Mill, Cali, Colombia

Background and objectives
There are several diseases in sugarcane that may reduce production, depending upon the variety and the disease. Yield losses ranging from 5-60% have been reported for ratoon stunting disease (RSD), 10-30% for leaf scald (LSD), 65% for smut and 1-85% for sugarcane mosaic virus (SCMV) [1]. These diseases are readily transmitted by infected vegetative planting pieces, however, they could be controlled by use of clean planting material. Various methods for eliminating pathogens from infected seed cane were evaluated in order to develop a system to produce disease-free nurseries.

Materials and methods
Treatment with hot water: RSD-affected seed cane pieces of several lengths from commercial varieties cultivated in the zone were treated with hot water at different temperatures to eliminate bacteria.
Thermal treatment and meristem tip culture: cane buds affected by different pathogens were treated by a combined system in which single eyes are treated with hot water (51°C) for 1 h, germinated, grown at a constant 41°C for 20-25 days with 12-h cycles of fluorescent light, and then cultivated in vitro using a system developed by Moreno & Victoria [2].
Cleaning of variety BJ 68-08: the methods above were then applied to the variety BJ 68-08 which has been grown commercially at the Mayagüez Sugar Mill for 10 years.
Cane production decreased by 0.5 tons cane per hectare per month (TCHM) with each ratoon over a period of seven crop cycles. At the time of the seventh harvest there was an average of 100% incidence of RSD and production of 5.4 TCHM. Consequently, some seed-cane pieces (40-50 cm long) from this field were treated with hot water to establish a new field. Another part of the planting material was treated as follows: single eyes were treated with hot water, then germinated plants were grown at 41°C for 20 days, with 12-h cycles of fluorescent light and then meristem tip culture and in vitro multiplication to obtain sufficient material for establishing a commercial field.

Results and conclusions
Treatment with hot water: marked differences were noticed in the growth and development of healthy and RSD-affected plants, confirming the importance of planting disease-free seed-cane setts when establishing new commercial fields.
Thermal treatment and meristem tip culture: it was possible to produce disease-free plants, eliminating several pathogens from the cane tissue (e.g. chlorotic streak, SCMV, LSD and RSD).
Cleaning of variety BJ 68-08: the commercial field established with seed-cane pieces treated only with hot water had 20% incidence of RSD at 8 months of age and a production of 7.5 TCHM at harvest. The other field established with planting material that received the combined hot-water treatment of single eyes, growth at 41°C and meristem tip culture had 0% RSD incidence and produced 10.7 TCHM, almost twice that of the original field.

These results demonstrate that the combined system eliminated not only RSD but also other cane pathogens, resulting in the significant increase in production obtained in the experiments.

References
1. Ricaud C, Egan BT, Gillaspie AG Jr, Hughes CG, eds, 1989. Diseases of Sugarcane: Major Diseases. Elsevier, Amsterdam, pp. 59-80.
2. Moreno B, Victoria JI, 1991. Proceedings of the 12th Congress of the Asociación Colombiana de Fitopatología - ASCOLFI, Manizales, Caldas, Colombia, p. 76.