THE INFLUENCE OF TWO PACHYMETRA ROOT ROT RESISTANCE SCREENING METHODS ON THE RATINGS OF STANDARD VARIETIES

PACHYMETRA ROOT ROT is a major soil-borne disease in the Australian sugarcane industry. The main management strategy applied since the early 1990s has been the cropping of resistant varieties. Screening for disease resistance has been an essential component of the plant improvement program and is undertaken by BSES Tully staff. Ratings applied to test clones and varieties depend on the reliability of the reaction of standard varieties of known resistance status. A selection of varieties was chosen for this purpose in the late 1980s, based on field observations and results from initial screening trials. Since that time, modifications have been made to the resistance screening method, with a subjective assessment rather than an objective measure of root injury; a very large amount of trial data has now been accumulated using both methods. All data obtained using both objective and subjective resistance screens were carefully analysed and the resistance of the standard varieties refined. It was found that the rapid subjective method for disease resistance assessment resulted in some variation in the ratings for the standard varieties. Analyses also showed these changes would affect the retention of clones of borderline resistance in the plant selection program. Application of refined ratings to standard varieties will lead to slight changes to the ratings applied to test canes in the plant improvement program.