The efficacy of multiple isolate resistance evaluation for stenocarpella maydis in breeding for ear rot resistance in tropical maize(Zea mays L.)

Abstract

Maize ear rots caused by Stenocarpella maydis are among the most important impediments to increased maize production in Zambia. They cause reduction in yield and quality of the maize. The mycotoxins produced by the pathogen have health implications on livestock and humans. Breeding for resistance is the most feasible option in managing ear rots and it is affordable to small scale farmers. However, to obtain stable resistance to S. maydis has been a challenge partly due to effect of the environment. The effect due to isolates of S. maydis is not yet understood. The objectives of this study were therefore: (i) Determine the general combining ability (GCA) and specific combining ability (SCA) effects of the lines and crosses respectively (ii) the effect of multiple isolate inoculations in breeding for resistance to S. maydis; (iii) to determine the appropriateness of indirect trait selection for resistance to S. maydis in tropical maize. Seven inbred lines of varying resistance to S. maydis were mated in a half diallel to generate 21 F1 single crosses. The resultant crosses together with their parents were planted at two sites, Lusaka and Mpongwe for evaluation during the 2015/16 cropping season. The experiment was laid out as a randomized complete block design with 3 replications. The treatments were: (1) single inoculation of each isolate A, (2) single inoculation of each isolate B and (3) a multiple inoculation of two isolates AB and (4) control with no inoculation at all. The mean genotypic scores were found to be 5.52, 4.96, 5.50 and 1 for treatment 1, 2, 3 and 4 respectively. The t-test analysis revealed that treatment 1 had a higher mean disease severity score (5.52) as compared to treatment 2 (4.96) (P < 0.001). Equally mean for treatment 2 (4.96) and 3 (5.50) were significantly different (P < 0.001). However, there were no significant differences between mean disease severity score between treatment 1 and 3. This indicated that multiple inoculations cannot be used as a screening resistance breeding technique as it could give rise to inappropriate genetic information due to probable antagonistic effect between isolates (A could have suppressed isolate B when multiple inoculated). Specific combining ability (SCA) effects across all treatments were significant (P < 0.05) in specific locations. The variance components using Baker’s ratio were found to be 0.012 and 0.03 in Lusaka and Mpongwe respectively. The results indicate that nonadditive gene effects were important in this study. Six and two crosses in location 1 and 2 respectively showed negative significant SCA effects to S. maydis reaction. Two crosses, (P2 x P4) and (P3 x P6) showed consistent significant negative SCA effects in both locations implying that they possess a trait resistant to S. maydis. Negative correlations of grain texture (r = -0.58, in both locations) and husk cover (r = -0.63 and -0.70 in Lusaka and Mpongwe respectively) to mean disease severity score of S. maydis across treatments were significant (P < 0.01). However, the r2 values i.e. grain texture ( 0.34 for both locations) and husk cover (0.40 and 0.49 for Lusaka and Mpongwe respectively) in relation to mean disease severity indicate that indirect selection for resistance to S. maydis cannot be utilized as a substitute for direct selection but can be used to supplement it. Keywords: Maize, ear rot, Stenocarpella maydis, resistance, mycotoxin