Inheritance of Resistance to Helminthosporium carbonum Race 3 in Maize
Since the early 1970s, several reports of a new helminthosporium leafspot disease of maize (Zea mays L.) caused by Helminthosporium carbonum Ullstrup (syn. Bipolaris zeicola (Stout) Shoemaker) (teliom. Cochliobolus carbonum Nelson) Race 3 indicate that it has become widespread in the eastern USA and can cause heavy yield losses. This pathogen inflicts a range of symptoms on maize, depending on pathogen isolate and host genotype, confounding the development of screening techniques to identify plant genotypes resistant to this pathogen. The objective of our study was to determine if adequate progress from selection for resistance could be accomplished against a range of polymorphic symptoms by a field breeding program. An eight-parent diallel analysis and a generation mean analysis were used to evaluate the potential for genetic improvement. Inbred maize parents (resistant A239, Ay499, Mo17, and R181; susceptible B8, Co109, W64A, and W182BN) and their F1's, F2's and backcrosses were grown in the field during two seasons and artificially inoculated. Lesion area was subjectively classified into a numerical disease rating scale. The results of the diallel analysis indicated that only general combining ability (GCA) effects, or additive genetic effects, were highly significant. The generation mean analysis confirmed that the additive component was the most important factor, accounting for 52 to 75% of the variation in the models. Dominance and digenic interaction components were occasionally significant. Broad-sense and narrow-sense heritabilities ranged from 58 to 77% and 57 to 67%, respectively, for two seasons. Field screening techniques to select inbred parents and to evaluate their progeny were found to be satisfactory, but growth chamber evaluations were not always an accurate or reliable predictor of field responses, particularly regarding intermediate disease reaction.
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