Mycotoxigenic fungi associated with ear-rots in Zimbabwe: identification and inheritance of resistance in southern and West African maize inbred lines

[摘要] English: Fumonisin, a mycotoxin produced by Fusarium verticillioides, is an intrinsic constraint in maize (Zea mays L.) that has received a low level of attention in Zimbabwe, despite existence of laws nationally and globally setting acceptable limits. Breeding for resistance to the causal fungus is important for the poor farmers that depend on this crop. A survey was conducted that highlighted the presence of F. verticillioides in 23 national storage facilities in the major maize growing areas of Zimbabwe. The morphological analysis identified 33 fungi in storage with F. verticillioides having the highest incidence among field fungi while Eurotium repens was the highest in storage. Aspergillus flavus was observed at significant levels in Bindura in the maize delivered in the previous year. The Fusarium species identified were confirmed by gene sequencing that clustered the derived isolates among the F. verticillioides sequences in the databases. Such high incidences motivated the study of inheritance of resistance where 12 mid-altitude lines from the Seed Co southern African breeding programme and 12 from IITA central and West Africa were mated in a NCDII. The lines had variable levels of resistance to the causal fungi and aflatoxins. F1 hybrids SC 2/IITA 7, SC 3/IITA 1, SC 3/IITA 10, SC 5/IITA 10 and SC 10/IITA 2 had the lowest F. verticillioides ear rot incidences per se and the lowest fumonisin contamination. Within the best yielding hybrids, tester line IITA 4 contributed most in terms of grain yield as it appeared more frequently in the best hybrids. In the parent trial planted alongside the F1 hybrid trials, tester IITA 4 had the highest yield besides also being one of the four lines classified as resistant to aflatoxins. The GCA effects for grain yield, days to mid pollen, days to mid silk and anthesis to silking interval were significant across all sites. Both GCA and SCA were significant for F. verticillioides ear rots and fumonisins contamination, it therefore can be concluded that additive and non-additive gene effects had a role in conferring resistance to these two traits, with additive gene effects playing a major role in the fumonisins, particularly for the southern African inbred lines. Seed Co inbred lines SC 2, SC 3, SC 4, SC 9, SC 11 and SC 12 had desirable GCA for F. verticillioides ear rot with lines SC 2, SC3 and SC12 also having negative GCA for fumonisins. The IITA tester lines with negative GCA for ear rots and fumonisins were IITA 4, IITA 8, IITA 3, IITA 5, IITA 7, IITA 6 and IITA 1. The inbred lines with the highest GCA for yield were SC 10, SC 5, SC 8 (Seed Co), and testers IITA 4 and IITA 2 (IITA) were identified as lines that can play a significant role in the southern African maize breeding programmes. Besides these lines, several more lines and F1 hybrids with per se low infection levels for ear rot were identified and these lines and hybrids can be used in southern Africa to improve the local gene pool in terms of both reaction to ER causing fungi, fumonisins and agronomic performance. To study genetic diversity 1144 SNP markers were used on the 24 inbred lines. Rogers' dissimilarity coefficients successfully distinguished the Seed Co and the IITA lines which formed the main groups besides an additional group comprising of a single line. The lines IITA 12 and SC 11 had the highest distance of 0.38. There were some sub-groups with the Seed Co materials forming clusters that were consistent with the pedigree data except for one line (SC 3). The IITA material formed two subgroups that could possibly represent two opposite heterotic groups. High heterosis for grain yield averaging 295% and 225% for the MPH and HPH, respectively, was obtained across sites in two years. Negative MPH and HPH for flowering related traits were observed. The ear rot and fumonisin accumulation had negative MPH and HPH. The correlations between the GD and MPH and HPH were significant, low and negative. There was no significant correlation between the SCA for grain yield and GD while there was significant correlation between GD and mean of grain yield per se. Grain yield mean and grain yield SCA were significantly correlated. SCA for F. verticillioides ear rot and fumonisin was significantly positively correlated with MPH and HPH of the same traits. It can therefore be concluded that good genetic gain can be obtained from the use of exotic germplasm when targeting traits such as resistance to ear rot causing fungi and the fumonisins than yield with exceptional few cases.

[发布机构] University of the Free State