Bc. Flett et Nw. Mclaren, OPTIMUM DISEASE POTENTIAL FOR EVALUATING RESISTANCE TO STENOCARPELLA-MAYDIS EAR ROT IN CORN HYBRIDS, Plant disease, 78(6), 1994, pp. 587-589
Field trials were carried out at four localities in South Africa to ev
aluate 35 corn cultivars for resistance to ear rot caused by Stenocarp
ella maydis. Trials at each locality were split to include inoculated
plants and plants infected by natural inoculum. Differences in hybrid
disease resistance reactions were recorded, but ranking of genotypes o
ver trial sites was poorly correlated. Regression analyses (Y = AX(b))
were used to determine the relationship between disease potential of
a trial site (X) and observed disease incidence (Y) within a genotype.
Disease potential was quantified as the mean disease incidence over a
ll hybrids in a trial. Genotypes could be divided into three categorie
s: 1) linearly related to disease potential, 2) high susceptibility de
spite a low disease potential, and 3) various degrees of resistance de
spite increasing disease potentials. This served to explain the absenc
e of constant rankings of hybrids and the often conflicting results wh
en genotypes screened at different localities were compared for diseas
e resistance. Confidence limits fitted to regression lines showed that
screening of hybrids within a disease potential of +/-0.6% to +/-50.6
% was acceptable for determining differences between highly resistant
and susceptible hybrids, but the range for distinguishing moderately r
esistant (intermediate) hybrids from resistant and susceptible hybrids
was limited to 4.4 and 26.3% and 1.9 and 40.9%, respectively. Beyond
these points, disease reactions converged, suggesting that at very hig
h or very low potentials, the use of cultivars in disease control is l
imited.