Breeding investigations and validation of molecular markers linked with spot blotch disease resistance in wheat (Triticum aestivum L.) germplasm for the rain-fed conditions of Zambia.

Abstract

Wheat (Triticum aestivum L.) is an important cereal crop, second after maize in Zambia. Its production during summer rainy season is hampered by several abiotic and biotic constraints. Among the biotic constraints, spot blotch disease caused by Bipolaris sorokiniana (Sacc.) Shoem is the most devastating causing large wheat yield losses and grain quality deterioration. Under severe cases complete crop losses result. This is because resistance to spot blotch disease in most of the genotypes in Zambia is inadequate. Breeding for high yielding disease resistant genotypes is the most cost effective and sustainable way of increasing summer rain-fed wheat yields and achieving sustainable wheat production. The study was, therefore, undertaken to: a) determine farmers` preferences for rain-fed wheat cultivars and identify production constraints, b) assess genetic diversity using agro-morphological traits, c) screen germplasm for resistance to spot blotch, d) determine gene action controlling the inheritance of resistance to spot blotch disease, and e) validate simple sequence repeat (SSR) molecular markers previously reported to be linked with resistance to spot blotch disease. A participatory rural appraisal established that wheat was an important crop among the small-scale farmers as it was a dual purpose crop used for home consumption and income generation. Coucal was the only wheat variety grown by the farmers under rain-fed conditions. The major constraints affecting summer wheat production in order of importance were; lack of good wheat seed, bird damage, weeds, termite damage, diseases (spot blotch being the most important), lack of markets and drought. High yielding cultivars with white coloured grain, combined with resistance to spot blotch disease, resistance to bird damage, termite damage and drought were the traits most preferred by the farmers. The genetic diversity study revealed the existence of genetic variability amongst the genotypes. Principal component analysis identified plant height, tillers/m2, peduncle length, days to heading, days to maturity and grain yield as the main traits that described the variability among the genotypes. The 150 genotypes tested were clustered into five groups based on Ward’s method, indicating that they were from different genetic backgrounds. This suggests that superior genotype combinations could be obtained by crossing genotypes in the opposing groups. The study also established that hectolitre weight, tillers/plant, thousand grain weight (TGW), grains/spike, peduncle length, and tillers/m2 could be effective selection criteria for high yield as they exhibited positive direct effects on yield and also significant and positive association with yield. One hundred and fifty wheat genotypes from Zambia and CIMMYT-Mexico were screened for resistance to spot blotch disease. The study revealed significant variability among the genotypes in their reaction to spot blotch disease. Genotypes were classified as resistant, moderately resistant, moderately susceptible, and susceptible. Genotypes 19HRWSN6 (Kenya Heroe), 19HRWSN7 (Prontia federal) were amongst the genotypes that were resistant across seasons. Most of the genotypes obtained from Zambia were moderately susceptible to susceptible across seasons. Nonetheless, eight genotypes with varying resistant reactions were selected for genetic analysis studies. A genetic analysis using Hayman diallel approach of 8 × 8 mating design and generation mean (GMA) analysis of six generations (P1, P2, F1, F2, BCP1 and BCP2) of two cross combinations was conducted. The two biometrical methods revealed the importance of additive gene effects in controlling resistance to spot blotch disease. The absence of maternal and non-maternal reciprocal effects indicated that choice of female parent was not important in breeding for resistance to this disease. Epistatic gene effects were absent in the inheritance of resistance suggesting that selection would be effective in early generation. Resistance exhibited partial dominance. Both diallel and GMA revealed moderately narrow sense heritability of 56.0% and 55.5%, respectively, an indication that the trait could be improved through selection. The Wr/Vr graph showed that parents 30SAWSN10 (P1), 30SAWSN5 (P3) and Coucal (P4) displayed the maximum number of dominant genes hence can be used in breeding for resistance to spot blotch. The molecular markers Xgwm570, Xgwm544 and Xgwm437 previously reported to be linked with resistance to Bipolaris sorokiniana were validated and their association with resistance confirmed. The markers amplified fragments in resistant parental genotypes that were similar to the F2 resistant and moderately resistant lines but not in susceptible ones. The significant relationship between the marker and resistance to Bipolaris sorokiniana was also established considering the significance of regression analysis (Xgwm570, P=0.003; Xgwm544, P=0.03 and Xgwm437, P=0.03). The adjusted R2 values observed (Xgwm570 =11.0%; Xgwm544=10.0% and Xgwm437=7.0%) further revealed the association between the marker and resistance. The study, therefore, shows that the markers can be useful in Zambia as they would increase the efficiency for the identification of resistant genotypes. This implies that screening of the genotypes could be done even in the absence of the disease epidemic. Overall, the results from this study indicate that the opportunity of improving resistance to spot blotch disease exists by utilizing the information generated. This information could be important during planning and implementation of breeding for resistance.