MONITORING THE SEVERITY AND VARIABILITY OF BROWN RUST (Puccinia melanocephala) IN SUGARCANE VARIETIES IN THE CAUCA VALLEY, COLOMBIA

THE VARIETIES CC 85-92 and CC 84-75 are grown on more than 80% of the area planted with cane in the Colombian sugar industry; these varieties were initially resistant to brown rust disease. Brown rust has been present in Colombia since 1979. Genetic variability in Puccinia melanocephala is thought to have affected resistance in some varieties through the appearance of what are thought to be new races of the pathogen. This situation has been observed in some resistant varieties (e.g. CC 85-92, CC 84-75, CC 93-3895), where recently the disease has occurred at low severity. Therefore, an evaluation was made of the severity and possible variability of brown rust in the varieties selected by CENICANA in the Cauca River Valley. Samples were taken from plants from 1–14 months of age in the varieties CC 84-75, CC 85-92, CC 93-3895, CC 92-2804 and MZC 74-275 on 91 estates (10 sugar mills). On each plantation 20 stalks were selected at random, and the third leaf from the top visible dewlap leaf was taken from each stalk. Both disease reaction and severity were evaluated. Morphological and microscopic analyses of the structures found in the rust pustules were undertaken in leaf samples taken from each variety. Simultaneously, pathogen samples were collected and molecular techniques used (focusing on initiators of the ribosomal DNA (rDNA)) to detect possible genetic variation of P. melanocephala. The results showed that the disease reaction type in the varieties evaluated was 5 or less, with severities ranging from 0–12% leaf area affected. Variety MZC 74-275 showed susceptibility, with a reaction of 6 and a severity of 20% on the estates where it was evaluated. No differences were found among the morphological structures in the samples evaluated, all of which corresponded to P. melanocephala. The results obtained from the amplification of the ITS1 and ITS2 regions of the rDNA and from the PCRRFLP did not show differences among the samples evaluated. These results could indicate that the variation of the pathogen is not reflected in its rDNA or that the molecular technique used was not sufficiently sensitive to detect small variations in the genome. Initial results about the presence or absence of gene Bru1 and susceptibility to brown rust are discussed.