Screening for Ramu stunt resistance at Ramu Agri-Industries, Gusap, PNG 1986-2008.

RAMU STUNT is one of the major diseases affecting the commercial production of sugarcane at Gusap, Morobe Province PNG. The disease was unknown prior to 1985 and was clearly recognised as a disease only in 1986, when it almost bankrupted Ramu Sugar. A severe epidemic in the widely grown susceptible variety Ragnar very significantly reduced commercial yields and profits. The host range for Ramu stunt is largely un-investigated; the disease is thought to have a viral etiology with an insect vector (Eumetopina flavipes). Resistance screening trials have been conducted at Gusap since the late 1980s and this paper reports on the results from 10 trials conducted since that time. Nineteen standard varieties were included in each trial and the results provide foundational data on their disease reaction. Average disease levels using data from all trials were used to apply refined ratings to each standard variety. Analyses assessed the reliability of individual trial data by correlating disease severity in each standard in each trial with long-term data for those same varieties. It appears that trial reliability was reasonable but there were cases where there was a low coefficient of determination. This suggested that, in these trials, the results differed sufficiently from the expected results to prevent the application of reliable resistance ratings to test canes. Potential options for improving trial reliability are considered. As for downy mildew, there is a critical need to ensure sufficient resistance is present in commercial varieties to minimise diseaseassociated losses.