SCREENING FOR DOWNY MILDEW RESISTANCE AT RAMU AGRI-INDUSTRIES, GUSAP, PNG 1986–2008

DOWNY MILDEW is the most important disease affecting the commercial cropping of sugarcane on the Estate of Ramu Agri-Industries, Gusap, Morobe Province, Papua New Guinea. An endemic disease of commercial crops, and of other Saccharum species growing in village gardens or along road-sides and stream banks, there is a sustained downy mildew (DM) infection pressure on commercial crops. Significant crop yield effects are experienced in susceptible commercial hybrid canes at Gusap. There is a critical need, therefore, to incorporate sufficient resistance into commercial varieties to minimise DM-associated yield losses. This paper reports on the analysis of 12 resistance screening trials conducted in the 1986–2008 period; a set of standard varieties with known field reaction to downy mildew was planted in each trial. Analyses examined the consistency of disease severity in each of these standard varieties. To do this, average disease levels using all trial data were used to refine ratings applied to each standard variety, and these in turn were regressed with the actual disease level recorded in each trial. Reaction of the standard varieties was reasonably consistent between trials, with a few notable exceptions. Regression analyses suggested that higher infection pressure (disease levels) led to more reliable resistance data. Analysis of individual variety and trial data revealed a few instances where individual varieties, or results in individual replicates for a few varieties, were either unusually low or high; wrong variety identity could have explained some of this variation. Rainfall at Gusap was remarkably consistent during the 1986-2008 period and is unlikely to have influenced variation in trial results.