Centre International en Recherche Agronomique pour le Développement, CIRAD-CA, Programme Canne à Sucre, BP5035, 34032 Montpellier Cedex 1, France

Background and objectives
Sugarcane productivity can be maintained through steady efforts to enhance the varietal status (i.e. all varieties grown in an area at a given time) by introducing new varieties. However, sugarcane crops are especially vulnerable to diseases, because of various factors: propagation from cuttings facilitates the spread of pathogens, monocropping over large areas is favourable to the development of epidemics, and the pluriannual aspect of this crop (4-10 years on average) lengthens and complicates breeding. Sugarcane stalks can be infected by various pathogens without exhibiting any symptoms, and therefore exchange and transport of sugarcane cuttings present a serious risk that should be fully controlled. CIRAD thus set up a sugarcane quarantine unit in Montpellier, France in the 1970s. The objective of this unit is to supply disease-free cuttings, mainly to sugar-producing areas of West and Central Africa, sugarcane breeding stations in the West Indies (Guadeloupe), the Mascareignes (Reunion) and others.

Materials and methods
Varieties to be quarantined are sent directly by the source countries. After germination of cuttings in a climate-controlled room at 30°C, the young shoots are planted in pots on artificial medium, drip watered and placed in quarantine greenhouse no. 1. The canes are sprayed with fungicides and insecticides at planting and several times during their growth cycle [1]. They are visually inspected regularly, and various methods such as isolation on selective media, serological techniques (ELISA, irnmunofluorescence) and molecular techniques are applied to detect sugarcane pathogens [1]. The sugarcane stalks are heat-treated (cuttings soaked in water at 25°C for 2 days, and then at 50°C for 3 h) at the end of the 9-12-month growth cycle in the greenhouse. Thereafter, canes are recultured, in quarantine greenhouse no. 2, from the hot water-treated cuttings. The plants are once more inspected and treated with phytosanitary products [1]. After 9-12 months' growth, stalks are harvested, and cuttings undergo a short heat treatment (soaking in hot water for 1 h at 50°C) and are dipped into a fungicide. These cuttings are then dispatched from the quarantine unit.

Results and conclusions
Approximately 100 varieties are released each year via quarantine: international varieties with various origins worldwide (10%), clones from early stages of selection from CIRAD's research station in Guadeloupe (45%) and the WICSCBS in Barbados (45%). All this material was found to be disease-free according to the available diagnostic techniques. Most quarantine cycles occur without detecting any pathogens; however, since the set-up of the sugarcane quarantine unit of CIRAD in Montpellier, various pathogens were detected in the varieties received: Clavibacter xyli subsp. xyli, Xanthomonas albilineans, Ustilago scitaminea, sugarcane mosaic virus, etc. Plants found to be infected by these pathogens were removed from the quarantine greenhouses. Since 1996, all varieties in quarantine were also tested for the presence of the luteovirus causing yellow leaf syndrome [2]. The PCR protocol used to detect this virus was kindly provided by Mike Irey (US Sugar Corporation, unpublished information). The virus was diagnosed in cane originating from Brazil, Florida, Guadeloupe, Mauritius, Puerto Rico, Reunion, and Taiwan. Forty commercial varieties out of the 74 (44%) tested were found to be infected. The presence of the virus in numerous varieties was a major concern because diseased material was usually eliminated in quarantine. As a consequence, only a few varieties would be able to be released. Because of this high percentage of infected valuable material, a tissue culture programme was started to produce healthy plants. ln vitro sugarcane cultures were established by various methods, but meristem culture proved to be the most efficient in producing virus-free plantlets (no virus was found in more than 90% of plants issued from meristem culture). Thanks to this result it will be possible to continue the distribution of sugarcane with high phytosanitary standards.

1. Rott P, Bousquet JFB, Muller M, Chatenet M, 1997. Agriculture et Developpement 13, 22-28.
2. Vega J, Scagliusi SMM, Ulian EC, 1997. Plant Disease 8, 21-26.