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.