USE OF WEATHER VARIABLES TO QUANTIFY SORGHUM ERGOT POTENTIAL IN SOUTH-AFRICA

Quantification of resistance to ergot requires that the observed ergot severity within a sorghum line be compared with expected ergot severi ty (ergot potential) to compensate for differences in environmental fa vorability for the disease among flowering dates and seasons. The ergo t potential required to induce the onset of disease is referred to as the ergot breakdown point of that line. In earlier studies, the ergot potential of a specific flowering date was defined as the mean ergot s everity in all sorghum heads over all lines in the nursery which comme nced flowering on that date in a genetically broad-based sorghum nurse ry. In this study, results of field trials enabled accurate prediction of ergot potential by using a multiple regression analysis which incl uded three weather variables-namely pre-flowering minimum temperature (mean of days 23 to 27 pre-flowering), mean daily maximum temperature, and mean daily maximum relative humidity (mean of days 1 to 5 post-fl owering; R-2 = 0.90; P = 0.91E-5). Evaluation of predicted and observe d ergot severity in an independent data set gave an index of agreement of d = 0.94 and R-2 = 0.84 (P = 0.106E-4), showing that ergot severit y, assuming the presence of viablez inocolum, can be accurately predic ted. Low pre-flowering minimum temperature was associated with reduced pollen viability, which appeared to be the primary factor predisposin g lines to ergot.