DISTRIBUTION AND INCIDENCE OF RATOON STUNTING DISEASE IN LOUISIANA SUGARCANE

Different incidences of ratoon stunting disease (RSD) were found by alkaline-induced metaxylem autofluorescence (AIMA) in stalks of sugarcane cultivar CP 76-331 released to Louisiana growers in the fall of 1984. The canes released in the eastern and western sugarcane growing areas were the culmination of 14 yr of selection and increase from a common source. The frequencies of diseased stalks assayed in the plant cane crop (1984) and first ratoon crop (1985) were 0 and 3% in the eastern area and 15 and 33% in the western area, respectively. The RSD increases from plant cane to first ratoon crops resulted from mechanical transmission of the disease. The asymmetric distribution between areas reflects the different levels of infection among the original seed sources. Frequencies of diseased stalks and infested fields in commonly grown cultivars assayed in 1986 were: CP 65-357, 33 and 75%; CP 74-383, 33 and 75%; CP 76-331, 22 and 55%; CP 70-321, 18 and 54%; CP 74-356, 16 and 55%; and CP 72-370, 3 and 34%. Disease incidence in CP 76-331 stalk samples was 2% in the eastern area and 36% in the western area, reflecting the asymmetric distribution found in the release plots in 1984 and 1985. Disease incidence across all cultivars was 22% of 3,607 stalks and 59% of 184 fields. Incidence of RSD for all cultivars increased approximately 10% annually after heat treatment of seed through the fifth year. The 22% incidence in standing cane across all cultivars, plus a 10% increase presumably attributable to mechanical spread by the harvester before replanting, suggested that one-third of the seed planted in Louisiana in 1986 may have had RSD.