GENERATING AND ASSESSING CHLOROTIC STREAK DISEASE (CSD) RATINGS FOR TWENTY SUGARCANE VARIETIES GROWN AT HARWOOD

EXTREMELY HIGH LEVELS of chlorotic streak disease (CSD) were observed by Harwood field staff in both commercial and approved seed plot (ASP) plantings following three successive years of above average annual rainfall and three floods. In the absence of ratings for all but two varieties under ASP propagation, there was a critical need to generate ratings for the new varieties so growers could be aware of their relative susceptibility. Two methods were trialled. The first involved counting the number of infected stools over a given length of row and determining the number of infections per metre, while the second assessed the visual level of streak and the general ‘up-anddown’ appearance of varieties. The correlation between leaf symptom and ‘up-anddown’ indices ranged from 88% in the plant ASP to 78% in the first ratoon. The data from the two methods were combined and consolidated ratings for twenty varieties were generated on a scale of 1 to 9. These ratings were then compared with field observations undertaken during routine RSD seedbed inspections, where the correlation broke down, possibly due to the presence of RSD or differences in crop management practices. Ratings were presented to NSW growers, as well as provided to BSES Limited, which incorporated them into SPIDNET.