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The soybean aphid, Aphis glycines Matsumura, has become a serious pest of soybean [Glycine max (L.) Merr.] in North America, and host-plant resistance is one potential management tool. In the current study, various F2-derived soybean selections with the Rag1 gene for resistance to soybean aphid were evaluated among F2-derived soybean selections without Rag1 and among contemporary soybean lines in a two-year field test. Overall, aphid levels per plant were over tenfold greater in 2006 than in 2005, but lines generally performed similarly relative to one another between years with regard to aphid-infestation levels. In both years, the Rag1 selections ILL4, ILL27, ILL35, ILL37, ILL64RR, ILL76RR, and ILL77RR had the lowest mean number of soybean aphids per plant. In 2005, three putative Rag1 selections—ILL26, ILL67RR, and ILL87—had intermediate aphid infestation levels greater than those of other Rag1 selections, and in 2006 ILL26 and ILL67RR also had intermediate aphid levels that did not differ from all other lines. Irrespective of the Rag1 gene, all soybean lines tested in 2006 had potentially injurious infestations ( 799 soybean aphids per plant) that exceeded action thresholds for this pest. These results show varying levels of resistance among lines homozygous for the Rag1 resistance allele and that protection may be equivocal in years of heavy infestation by soybean aphid. Implications for testing putatively aphid-resistant soybean selections in the field and the potential for field deployment of aphid-resistant lines are discussed. [ABSTRACT FROM PUBLISHER]

Thirteen early-maturing and promiscuously nodulating soybean [Glycine max (L.) Merr.] varieties that belonged to maturity group I developed at the International Institute of Tropical Agriculture (IITA) from 1980 to 1996 were evaluated at two locations in the Guinea savanna of Nigeria in 2001 and 2002 to determine genetic gain in grain yield and associated agronomic traits. The varieties were grown in a randomized complete block design with four replications. Data on grain yield and seven other traits were recorded. The study showed that grain yield potential of the soybean varieties increased from 1117 to 1710 kg ha-1 during two decades of breeding in IITA. The average rate of increase per year per release period (1980-1996) was 24.2 kg ha-1, which is 2.2%. Fodder yield also showed an annual increase of 22.81 kg ha-1 yr-1. Recent varieties had better lodging resistance scores of 1.25 to 1.37 in a 1 to 5 scoring scale (1 being resistant and 5 susceptible), shattering resistance scores of 1.6 to 2.7, and improved natural nodulation scores of 3.1 to 3.9. Old varieties had 50% or more pod shattering while the corresponding figure for new varieties was around 25%. Old varieties produced few and small nodules, while new varieties had many big nodules although size and number of nodules alone may not necessarily indicate nodule efficiency in N2 fixation. Grain yield showed a positive and highly significant (P ≤ 0.01) correlation coefficient (r = 0.75) with fodder yield, implying that high biomass could serve as an indicator of good yielding ability. [ABSTRACT FROM AUTHOR]

Current methods of screening maize ( Zea mays L.) germplasm for susceptibility or resistance to corn rootworms (Coleoptera: Chrysomelidae) rely primarily on information from large-scale field experiments. Due to labour and cost constraints associated with field trials, alternative evaluation methods are desirable. We used a previously developed behavioural bioassay to: (1) investigate the host search behaviour of rootworm larvae after contact with 14 maize genotypes, (2) compare the behaviour of non-diapausing Diabrotica virgifera virgifera LeConte, diapausing D. v. virgifera, and diapausing D. barberi Smith & Lawrence and (3) determine if this technique can be used to separate susceptible vs. resistant maize genotypes. The majority of rootworm larvae engaged in intensive (local search) behaviour after exposure to maize roots, whereas larvae continued to exhibit extensive (ranging) behaviour after contact with negative controls. Even though a transgenic hybrid with resistance to D. v. virgifera was included in analyses, quantitative path measurements were similar among genotypes and only differed between specific maize lines and controls. Notably, there were differences in host search behaviour among rootworm groups, with non-diapausing D. v. virgifera having more convoluted paths and engaging in intensive search more frequently than diapausing rootworms. Correlations between larval path measurements and historic root damage ratings were not significant, although there were weak positive correlations between historic adult emergence densities and measures of path linearity. However, due to the lack of significant behavioural differences among maize lines with a range of susceptibility levels, we concluded that this bioassay is not useful in screening maize germplasm for rootworm resistance. [ABSTRACT FROM AUTHOR]

Maize ( Zea mays L.) is a valuable commodity throughout the world, but corn rootworms (Chrysomelidae: Diabrotica spp.) often cause economic damage and increase production costs. Current rootworm management strategies have limitations, and in order to create viable management alternatives, researchers have been developing novel maize lines using Eastern gamagrass ( Tripsacum dactyloides L.) germplasm, a wild relative of maize that is resistant to rootworms. Ten maize Tripsacum-introgressed inbred lines derived from recurrent selection of crosses with gamagrass and teosinte ( Zea diploperennis Iltis) recombinants and two public inbred lines were assessed for susceptibility to western corn rootworm ( Diabrotica virgifera virgifera LeConte) and yield in a two-year field study. Two experimental maize inbred lines, SDG11 and SDG20, had mean root damage ratings that were significantly lower than the susceptible public line B73. Two other experimental maize inbred lines, SDG12 and SDG6, appeared tolerant to rootworm damage because they exhibited yield increases after rootworm infestation in both years. In the majority of cases, mean yield per plant of experimental maize lines used in yield analyses was equal to or exceeded that of the public inbred lines B73 and W64A. Our study indicates that there is potential to use Tripsacum-introgressed maize germplasm in breeding programs to enhance plant resistance and/or tolerance to corn rootworms, although further research on insect resistance and agronomic potential of this germplasm needs to be conducted in F1 hybrids. [ABSTRACT FROM AUTHOR]

In the hopes of lessening the current reliance on soil insecticides, developing a viable alternative for transgenic maize hybrids, and providing sustainable options for Europe, researchers recently have been developing novel maize lines that exhibit resistance and/or tolerance to corn rootworm larvae. Here we report the results of a 2-year field experiment in a northern growing region assessing the resistance and tolerance of 10 experimental synthetic maize populations selected for varying levels of damage from western corn rootworm larvae, Diabrotica virgifera virgifera LeConte (Col.: Chrysomelidae) and four maize hybrids. Maize non-preference, antibiosis and tolerance to rootworms was evaluated using previously established methods, including: the Iowa 1–6 root damage rating scale, root fresh weight, compensatory root growth ratings and adult rootworm emergence. Among the experimental synthetic maize populations, BS29-11-01 was the most susceptible, and had a mean root damage rating that was greater than the highly susceptible maize hybrid B37 × H84. This line also had the lowest mean root fresh weight and one of the lowest mean compensatory root growth ratings. In contrast, CRW8-3 appeared to be tolerant to western corn rootworms, and had the lowest mean root damage rating, which was comparable with that of the non-transgenic hybrid DeKalb® 46-26. [ABSTRACT FROM AUTHOR]

Examines the inheritance of nonnodulation in peanut using the nonnodulating peanut line, M4-2, selected from the original nonnodulating germplasm described by D. W. Gorbet and J. C. Burton in 1979. Details on nonnodulation mutants derived either spontaneously or through mutagenesis; Comparison of foliage color with nodule characteristics; Description of three nodule characteristics observed.

Soybean cultivars capable of nodulating with indigenous Bradyrhizobium spp. have been developed by the International Institute of Tropical Agriculture (IITA) and national programs in Africa in order to avoid artificial inoculation by resource-poor farmers in Africa. The current selection procedure for enhanced N2 fixation is based on an assessment of nodule formation which does not directly quantify the proportions of crop N derived from the atmosphere. We have monitored N accumulation patterns and N2 fixation in nine promiscuous soybean cultivars with different maturity periods, using the 15N dilution technique. Nodule development generally peaked at the early podfill stage for all cultivars except Tgx 1519-1D and Tgx 1447-2D in which it continued to increase. The proportion of crop N derived from fixation (%NDFA) ranged between 51% and 67%, 77% and 84%, and 66% and 73% at full bloom, early podfill, and physiological maturity stages, respectively. Total N accumulation increased in all soybean genotypes with increasing plant age. Significant correlations ( P<0.001) were established between nodule weight and %NDFA, even though this did not explain the relationship between nodule development and N2 fixation in cultivars such as Tgx 1519-1D. Promiscuous soybean cultivars retained between 10% and 19% of total N accumulated at the final harvest, in belowground biomass. Our results indicated that these soybean cultivars can derive substantial proportions of plant N from N2 fixation in soils where compatible indigenous bradyrhizobia populations are adequate and effective. Also, we have substantiated the claims that qualitative nodulation parameters currently used to select varieties with a high N2 fixation capacity need to be validated with other measurements of N2 fixation. [ABSTRACT FROM AUTHOR]

With limited access to P fertilizers, soybean (Glycine max (L.) Merr.) varieties cultivated in the moist savanna zone of West Africa should use P as efficiently as possible. Field trials were conducted on the effect of P application on grain yield per unit P accumulated (physiological efficiency or PPE), amount of P accumulated in plant biomass per unit P applied in fertilizer (recovery efficiency or PRE), and the amount of grain produced per unit of applied P (yield efficiency or PYE) in soybean. There was greater proportional root proliferation at lower P rates, and at sites where soil test P was low. Physiological efficiency was higher under these conditions. Although a greater proportion of total dry weight was accumulated in the roots of the early and medium (27-28%) compared with the late varieties (22-23%), root dry weight and PPE in the latter were higher. At maturity, PPE was significantly reduced from 245 kg kg-1 with no P applied to an average of 173 kg-1 (or by about 29%) with P application. Phosphorus yield efficiency was higher at sites (Kasuwan Magani and Gidan Waya) and in the late varieties where response to P was highest. PYE with 60 kg ha-1 P applied rate was almost half the value observed with 30 kg P ha-1. [ABSTRACT FROM AUTHOR]