Head smut (also known as tassel smut) of corn, which is caused by the fungus (Sphacelotheca reiliana (Kühn) Clint), has been occasionally a serious disease in North America, Mexico, Australia, New Zealand, S. Africa and Europe. One hundred recombinant inbred lines (RILs), derived from the cross Hi34 (Hawaii inbred) and TZi17 (Nigeria inbred), were used for mapping quantitative trait loci conferring resistance to head smut. Field evaluation for resistance to head smut was carried out by Dr. Nowell in Greytown, South Africa, under natural infection during the 1994 crop cycle. The percentage of smutted plants of each plot was rated on a 1-9 scale (1 = no symptom, 9 = severe symptom). The experimental design was an 11 x 11 double lattice design with two replications. The 121 entries include ten sub-lines each from the resistant parent TZi17 (mean 2.4, range 1 to 4.5) and the susceptible parent Hi34 (mean 5.8, range 1.4 to 8.4), one F1 hybrid (mean 5.5, range 4.5 to 6.5), and one hundred RILs (mean 3.9, range 1 to 9).
One hundred and sixteen RFLP and four SSR markers, well distributed throughout the 10 chromosomes of maize, were used for analysis of RILs under Dr. M. Khairallah at CIMMYT. The constructed linkage map had a total length of 2060 cm and an average spacing of 18.7 cm between markers. Identification of marker loci linked to S. reiliana resistance was conducted using single factor analysis of variance. Marker loci on chromosomes 1, 2, 9, 10 carried genes influencing resistance (Table 1). The most influential region for resistance to S. reiliana was located in the proximal portion of chromosome 1, where RFLP marker asg30 showed the highest F value (F=10.24). Two linked markers, umc167 and asg75, were also associated highly with resistance to S. reiliana. The data suggest a major QTL for resistance to S. reiliana on the short arm of chromosome 1 closely linked to marker asg30.
A LOD score of 4.2 was set as the genome-wise threshold value at P<0.05 for identifying putative QTL based on the 1000 permutation test from QTL Cartographer. A scan of all ten chromosomes using composite interval mapping by QTL Cartographer revealed only one peak. This was on the short arm of chromosome 1 close to the marker asg30, with LOD score of 5.3 that accounted for 10.6% of phenotypic variation for head smut. The map position was in bin1.04, about 4 cM from marker asg30 and 16 cM from umc157. The data all support the contention that a single major gene, here designated as spr1, confers resistance to S. reiliana..
Seven pairwise interactions (P<0.01) for resistance to S. reiliana were detected. All of the three most significant pairwise interactions (P<0.001) involved one marker in the long arm of chromosome 7. The mixture model, including the resistance allele spr1 and all the three interactions with markers of chromosome 7, could explain up to 60% phenotypic variation (SAS GLM).
Table 1. Loci significantly associated with resistance to corn head
smut from single-factor analysis of variance
| Locus | Chromosome bina | F (1, n-2) | Probability (F) |
| asg75 | 1.03 | 5.327 | 0.023 |
| asg30 | 1.04 | 10.168 | 0.002 |
| umc167 | 1.05 | 4.591 | 0.035 |
| umc50 | 3.04 | 4.349 | 0.040 |
| phi022 | 9.03 | 5.115 | 0.026 |
| csu25b | 10.00 | 7.408 | 0.008 |
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