INTRODUCTION

BACKGROUND.  Aflatoxin accumulation in corn (Zea mays L.) grain is a concern within U.S. grain markets every year.  Ideally, aflatoxin accumulation in grain should be managed with inbred sources that contribute genetic resistance to disease and insects, tolerance to environmental stress, and superior agronomics (i.e., standability and yield) to hybrid performance.  Although superior resistance to aflatoxin accumulation in grain, severity of Aspergillus ear rot, and/ or kernel infection by Aspergillus flavus has been identified in publicly available sources, agronomic performance from the majority of these sources as inbred lines per se or in hybrid combination is generally poor.

OBJECTIVE.  Evaluate testcrosses developed with GEM and Holdens’ lines for yield and agronomic quality in Mississippi.  Evaluate a subset of the same testcrosses for resistance to Aspergillus ear rot and aflatoxin in inoculated trials at MS State, MS.  The authors also had hoped to develop testcrosses of GEM lines and 7 Holdens’ lines for future evaluations, however, severe weather and herbicide damage terminated this portion of the project.

MATERIALS AND METHODS

Testcross yield and agronomic evaluation.  The experiment was a randomized complete block with 3 replicates.  Experimental units consisted of one row of each genotype.  Rows were approximately 4 meters in length (±0.5 meter fallow alley), spaced 1 meter apart, and included a maximum of 27 plants.  F₁ seed were developed in 2004 with Holden’s lines LH195 or LH210 and 145 GEM lines (designated by pedigree), 9 public inbred lines (Mp420, Mp494, Mp715, Mp80:04, Mp717, NC258, NC298, NC320, and Tx601), or 4 experimental lines from USDA-ARS at, MS State (designated by source).  Three commercial checks (Pioneer Brand P3394, Terral Seeds TV2100, and Dekalb DK697) and the cross LH195/LH210 were included for comparison.  Seed were planted in two locations at MS State, MS, however, one location was lost due to hurricane damage.  Genotypes were evaluated for emergence (number of plants plot^-1 two weeks after planting), maturity (modified growing degree units (MGDU) to anthesis and silking), root and stalk lodging (% of plants plot^-1), staygreen (% green tissue on plants 65 days after silking), yield (bu A^-1), and grain moisture (%).  Grain was harvested from all plots with an Almaco research plot combine.

Testcross ear rot and aflatoxin evaluation.  The experiment was a randomized complete block with 4 replicates.  Experimental units consisted of one row of each genotype.  Rows were approximately 4 meters in length (±0.5 meter fallow alley), spaced 1 meter apart, and included a maximum of 21 plants.  F₁ seed was developed in 2004 with Holden’s lines LH195 or LH210 and 16 GEM lines identified as resistant or susceptible in previous work, 5 public inbred lines (Mp494, Mp715, Mp80:04, NC258, NC320, and Tx601), or 10 experimental lines from USDA-ARS at MS State (designated by pedigree).  One commercial check (Terral Seeds TV2100), one susceptible check (GA209/SC212M), one resistant check (Mp313E/Mo18W), and the cross LH195/LH210 were included for comparison.  Seed were planted in one location at MS State, MS.  Primary ears on all plants were inoculated with a propagule suspension of A. flavus Link:Fr.  Three-point-four milliliters of the propagule suspension (1 x 10⁶ propagules ml^-1) were injected through husk leaves into the side of the primary ear on all plants at the R2 (blister) growth stage.  Primary ears from all plants were harvested, shucked, dried to approximately 15% grain moisture, and visually rated for severity of Aspergillus ear rot as percentage of the ear with symptoms of disease development.  Grain was shelled from cobs, bulked by experimental unit, and ground.  Concentration of aflatoxin in grain currently is being determined with VICAM AflaTest.

Statistical analyses.  Data were analyzed with analysis of variance (ANOVA) using the general linear models procedure of Statistical Analysis System software (SAS Institute, NC).  Replicates were considered random terms in analyses, while genotypes were considered fixed.  Differences between genotypes were compared with Fisher’s protected least significant difference test (LSD).

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RESULTS AND DISCUSSION

Testcross yield and agronomic evaluation.  Emergence, MGDU to silking, MGDU to anthesis, and root lodging were affected significantly (P < 0.05) by genotype within trials in which LH195 or LH210 was the common tester.  Emergence varied from 7 to 26 plants row^-1 (mean 20 plants row^-1) and from 7 to 25 plants row^-1 (mean 19 plants row^-1) in trials within which LH195 (Table 1) or LH210 (Table 2) was the common tester, respectively.  Greatest emergence was observed from hybrids developed with 2250-01_XL370A_S11_F2S4_9214-Blk21/00-sib-B in trials within which LH195 or LH210 was the common tester.  Greatest emergence also was observed from hybrids developed with FS8A(S):S09-362-1-B-B and LH195 and 2132-03_DK888_S11_F2S4_

9187-Blk22/00-sib-B and LH210.  MGDU to silking varied from 1244 to 1528 °D (mean 1412 °D) and from 1214 to 1537 °D (mean 1389 °D) in trials within which LH195 (Table 1) or LH210 (Table 2) was the common tester, respectively.  Earliest and latest GEM lines were DKXL370:N11a20-2-1-B-B-B and 2127-01_DK888_S11_F2S4_9181-Blk21/00-sib-B in the trial within which LH195 was the common tester, and CH05015:N15-184-1-B-B-B and 2146-01_DK888_S11_F2S4_9196-Blk29/00-sib-B in the trial within which LH210 was the common tester.  MGDU to anthesis varied from 1206 to 1566 °D (mean 1389 °D) and from 1206 to 1517 °D (mean 1354 °D) in trials within which LH195 (Table 1) or LH210 (Table 2) was the common tester, respectively.  Earliest and latest GEM lines were DKXL370:N11a20-2-1-B-B-B and UR11003:S0302-937-1-B-B in the trial within which LH195 was the common tester, and CH05015:N12-50-1-B-B-B and 7451-22DK888N11 F2S3 691-Blk/02-B in the trial within which LH210 was the common tester.  Root lodging varied from 0 to 46% of plants (mean 5%) and from 0 to 61% of plants (mean 11%) in trials within which LH195 (Table 1) or LH210 (Table 2) was the common tester, respectively.  Fifty-one and nineteen GEM lines had root lodging scores of 0% in trials within which LH195 or LH210 was the common tester, respectively.

Grain moisture was affected significantly by genotype (P < 0.05) only in trials within which LH195 was the common tester (Table 1).  Grain moisture varied from 11.3 to 14.1% (mean 12.9%).  Of the GEM materials, line CH1S775:S1911b-16-1-B-B-B had the least grain moisture, while line 2142-01_DK888_S11_F2S4_9190-Blk19/00-sib-B had the greatest grain moisture at harvest.

Stalk lodging, staygreen, and yield were not affected significantly by genotype (P > 0.05) in trials within which LH195 or LH210 was the common tester.  Stalk lodging averaged 4 or 10%, Staygreen averaged 10 or 3%, and yield averaged 138 or 144 bu A^-1 in trials within which LH195 or LH210 was the common tester, respectively (Tables 1 and 2).

Testcross ear rot and aflatoxin evaluation.  Severity of Aspergillus ear rot was affected significantly (P<0.05) by genotype in trials within which LH195 or LH210 was the common tester.  Severity of Aspergillus ear rot ranged from 2 to 22% (mean 9%) or from 2 to 23% (mean 11%) of the ear rotted in trials within which LH195 or LH210 was the common tester, respectively (Table 3).  Least Aspergillus ear rot was observed on the resistant check, Mo18W/Mp313E.  Six GEM lines had severity of Aspergillus ear rot that did not differ significantly from ear rot on Mo18W/Mp313E in the LH195 or LH210 trial.  GEM lines with severity of Aspergillus ear rot that did not differ significantly from least ear rot in both trials were 2250-01_XL370A_S11_F2S4_9214-Blk21/00-sib-B and 2258-03_XL380_S11_F2S4_71/97_

Bulk/98-sib-B.  Data for aflatoxin concentration in grain from this experiment is not yet available.

CURRENT DIRECTION 

Despite severe weather in 2005, ear to row and recurrent selection programs that incorporate several of the most promising GEM lines and breeding crosses continue through efforts of the Corn Host Plant Resistance Research Unit at Mississippi State.

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