GENETICS OF RESISTANCE TO TAR SPOT COMPLEX IN 18 TROPICAL MAIZE GENOTYPES

Abstract

Tar spot complex (TSC) of maize (Zea mays L.), caused by the fungi Phyllachora maydis Maubl, and Monographella maydis Müller & Samuels, is a disease of economic importance, as it causes severe losses in yield and forage quality. Genetic improvement of host resistance through the generation of resistant genotypes is the most efficient strategy for managing disease. Little is known about the genetic basis of maize resistance to TSC, therefore general combining ability (GCA) and specific combining ability (SCA) of 18 inbred lines S7 was determined by the Model I of fixed frequency defined by Griffing, under Method II which included 18 lines plus all its 153 possible single crosses. The experimental was laid out as a randomized complete block design and conducted in 4 different environments in the States of Veracruz, Guerrero, Puebla and Oaxaca. Both GCA and SCA were significant (P < 0.01), indicating that both additive and dominant gene effects are important in determining resistance to TSC. However, the GCA was 45 times larger than the SCA, indicating that additive gene effects are most important in TSC resistance and that different non-allelic genes may be involved in conditioning resistance to TSC in maize. It is therefore possible to pyramid or stack different resistance genes in the same background by breeding methods. The most resistant hybrids were derived from two lines with high negative GCA and SCA. The best lines to produce highly resistant hybrids were CML-329, CLRCW-105-B and [M37W/ZM607], which had the best negative GCA effects. Recurrent selection may be the most useful method of improving to accumulate and increase levels of TSC resistance in synthetic or composite populations. Dominance effects were the most important in some crosses.