Based on the pedigree of the inbred lines, presumed (or putative) resistance genes were used in this experiment. Four lesion types were recorded 15-20 days after silking: 1) resistant lesion (R) = stripe or narrow elliptical green-yellowish lesion; 2) moderately resistant lesion (MR) = narrow, long, elliptical gray lesion with green-yellowish border; 3) moderately susceptible lesion (MS) = long, elliptical, gray lesion with green-yellowish border; and 4) susceptible lesion (S) = long, elliptical, gray or tan lesion. Table 2 summarizes the lesion types and putative alleles affecting northern leaf blight expression recorded for each genotype in the 8 x 8 diallel cross. Several conclusions can be drawn from this data:
1. Resistant gene(s) appeared to exist in both 73353 and 73405. From the pedigrees (Table 1), the putative resistant gene of 73353 is Htm1, which came from Mayorbela (Robbins and Warren, Maydica 38: 209-213, 1993. Maize DB, www.agron.missouri.edu/cgi-bin/sybgw_mbd/mdb3/Locus/ 65746); and the putative resistant gene of 73405 is Ht2, which came from Oh43Ht2.
2. No reciprocal differences exist in the expression of these genes.
3. The presumed resistance gene, Htm1, of 73353, did not always express dominance. Its expression was affected by different genotypes and environmental factors. In this diallel cross, Htm1 expressed dominance in crosses of 73353 x 73405, 73353 x CHN2, 73353 x CHN1, 73353 x CO387 and their reciprocal crosses. Htm1 expressed partial dominance in 73353 x CM174 and its reciprocal cross in 1999 but complete dominance in 2000. 1999 was much warmer than 2000. Htm1 only expressed partial dominance in 73353 x CO354 and its reciprocal cross in both years. Htm1 did not express dominance in the cross 73353 x CO325 and its reciprocal; CO325 may have a gene(s) that inhibits the expression of Htm1. This inhibition also was affected by environmental factors, with complete inhibition in 1999, the warmer year, but only partial inhibition in 2000.
4. The expression of the putative resistance gene Ht2 of 73405 was difficult to explain. Inbred 73405 itself expressed moderate resistance, but when crossed with all susceptible inbred lines (CHN2, CHN1, CM174, CO325, CO354, and CO387) all hybrids were susceptible. This raises several questions. (A) What race (Leonard et al, Plant Dis. 73:776-777, 1989) was used for the inoculations? If it were race 0 (Ht1, Ht2, Ht3, HtN/0) or race 1 (Ht2, Ht3, HtN/Ht1), why were all hybrids susceptible? If it was race 12 (Ht3, HtN/Ht1, Ht2), race 23 (Ht1, HtN/Ht2, Ht3), or race 23N (Ht1/Ht2, Ht3, HtN), why is 73405 moderately resistant? (B) Because 73405 came from B59 x Oh43Ht2 and B59 includes a dominant inhibitor gene to Ht2 called Sht1 (Ceballos and Gracen, Plant breeding 102: 35-44, 1989) (Maize DB, Variation Sht1, www.agron.missouri.edu/cgi-bin/sybgw_mdb/mdb3/Variation/ 99275), then if 73405 had genes like Sht1Sht1/Ht2Ht2, it should be susceptible. If 73405 had no Sht1 gene, then why were all of its F1 hybrids susceptible and not moderately resistant? All of these questions indicate that the relationships among Ht2, Sht1, and races are much more complicated than we expected.
5. Both 73405 and CM174 possibly have Sht1, which comes from B14 (Ceballos and Gracen, Plant Breeding 102:35-44, 1989. Maize DB, Variation Sht1, www.agron.missouri.edu/cgi-bin/sybgw_mdb/mdb3/Variation/99275). Crosses of 73353 to 73405 and CM174 showed resistance, indicating that Htm1 in 73353 might not be affected by Sht1.
Table 1. Pedigree and putative alleles affecting northern corn leaf
blight reaction in eight inbred lines used in an 8 x 8 diallel study.
|73353||USA||FR23 x H111. FR23 = Mo17 x A653,
H111 = Mayorbela x B37(2)
|Htm1 from Mayorbela (Robbins and Warren, Maydica 38:209-213, 1993)|
|73405||USA||B59 x Oh43Ht2,
B59 = N32 x B14(2)
|Ht2 from Oh43Ht2,
Possibly Sht1 from B14 (Ceballos and Gracen,Plant Breeding 102:35-44, 1993)
|CHN2||China||Chinese inbred x B73(2)||None|
|CHN1||China||Chinese inbred Yellow early 4||None|
|CM174||Canada||V3 x B14(2)||Possibly Sht1 from B14 Ceballos and Gracen, Plant Breeding 102:35-44, 1993)|
|CO325||Canada||CO256 x CO264 (3).
CO256 = BSSS,
CO264 = CO109 x 190(2)
|CO387||Canada||CO272 x CO266.
CO272 = BSTE x [(CO109 x CO106)CO109(2)](2),
BSTE = Iowa Two Ear Synthetic
CO266 = Pioneer 3990
Table 2. Lesion types expressed for resistance to northern leaf blight
in an 8 x 8 diallel cross in 1999 and 2000.
aDiagonal are lesion types expressed for eight inbred lines. Above diagonal are F1 hybrid crosses and below diagonal are their reciprocal crosses.
bR (resistant lesion) = stripe or narrow elliptical green-yellowish lesion; MR (moderately resistant lesion) = narrow, long, elliptical gray lesion with green-yellowish border; MS (moderately susceptible lesion) = long, elliptical, gray lesion with green-yellowish border; and S (susceptible lesion) = long, elliptical, gray or tan lesion.
cLesion type expressed differently in 1999 and 2000 (types
noted are for 1999/2000).
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