ENVIRONMENTAL PARAMETERS AFFECTING THE RELIABILITY OF RESISTANCE SCREENING TRIALS FOR ORANGE RUST AND YELLOW SPOT

RESISTANCE SCREENING for orange rust and yellow spot has been undertaken in parent varieties at BSES Meringa since the year 2000. The data for a group of standard varieties were analysed and the coefficient of determination for the standards relating resistance rating and disease severity in any one year calculated. These data showed significant variation with year, especially for yellow spot. Data for several weather parameters were accessed and related to the reliability of the reaction of the standards for both yellow spot and orange rust. Analyses showed that for yellow spot, rainfall in the December– March period just prior to the disease assessments was highly related to the reliability of resistance assessment. Modelling of this relationship suggested that more reliable assessments for yellow spot could be obtained in higher rainfall areas, such as Tully and Feluga. Variation in reliability of orange rust assessments was associated with minimum temperatures in the January–March period. Because orange rust resistance ratings, as judged by the standard varieties, were more reliable than for yellow spot, there is less need to change the current methods for assessing orange rust resistance. Orange rust resistance assessment at BSES Meringa is therefore generally satisfactory; however, it is recommended that higher rainfall sites be selected for yellow spot screening.