Quantifying Resistance of Sorghum Genotypes to the Sugary Disease Pathogen (Claviceps africana). N. W. McLaren, Principal Plant Pathologist, Grain Crops Research Institute, Private Bag X1251, Potchefstroom, 2520, Republic of South Africa. . Plant Dis. 76:986-988. Accepted for publication 25 February 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0986.

Sorghum lines were screened for resistance to the sugary disease pathogen (Claviceps africana) at two localities in South Africa. Of 71 lines that remained disease-free at Potchefstroom, only three remained disease-free at Bethlehem. Ranking of lines according to disease incidence was not correlated at the two localities, and the use of mean sugary disease incidence as a rating criterion was questioned. Nonlinear regression analysis was used to determine the relationship between disease potential associated with different inoculation dates and observed disease incidence within lines. Lines could be classified into three categories, i.e., those linearly related to disease potential, those highly susceptible even at low disease potentials, and those with various degrees of resistance despite increasing disease potentials. Lines in the latter group differed with respect to resistance breakdown points and the subsequent rate of resistance breakdown. These criteria proved to be useful for quantifying resistance as they are fixed values that are independent of fluctuations in flowering dates and variations in climatic conditions during flowering and infection.