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.
The rate of breeding progress of 10 medium and 11 late maturing promiscuous soybean varieties developed by IITA for the savanna zones of Nigeria from 1980 to 1996 was assessed at two locations in 2001 and 2002. The two maturity groups were evaluated in separate experiments using randomized complete block design with four replications. There were highly significant effects of variety on grain yield, fodder yield, and another set of eight traits both in the medium and late maturity groups. In the medium varieties, grain yield increased linearly from 1184 kg ha -1 for an old variety TGx 1019-2EN to 1764 kg ha -1 for TGx 1910-7F released in 1996. The annual rate of progress against year of release was 23.61 kg ha -1 (1.99%). Modern varieties of the medium maturity group had high number of pods per plant, high harvest index, and reduced pod shattering. For late varieties, grain yield ranged from 1569 kg ha -1 for an old variety Samsoy-2 to 2303 kg ha -1 for a recently developed variety TGx 1910-14F. The annual rate of breeding progress during the improvement period of 16 years for late varieties was 22.23 kg ha -1 or 1.42%. The overall yield of the late maturing varieties was 19% higher than the medium maturing varieties.
Freely nodulating soybean genotypes vary in their phosphorus (P) uptake and P-use efficiency (PUE) in low-P soils. Understanding the genetic basis of these genotypes' performance is essential for effective breeding. To study the inheritance of PUE, we conducted crosses using two high-PUE genotypes, two moderate-PUE genotypes and two inefficient-PUE genotypes, and obtained F 1 , F 2, BC 1 and BC 2 populations. The inheritance of PUE was evaluated using a randomized complete block design. A generation mean analysis of phenotypic data showed that PUE was heritable, with complex inheritance patterns and the presence of additive, dominance and epistatic gene effects. Seed P, shoot P, root P, P-incorporation efficiency and PUE were largely quantitatively inherited traits. There were dominance, additive 9 additive and dominance 9 dominance gene effects on PUE, grain yield, shoot dry weight, 100-seed weight, root dry weight and shoot dry matter per unit P for populations grown under low-P conditions. Dominance effects were generally greater than additive effects on PUE-related indices. These PUE indices can be used to select P-efficient soybean genotypes from segregating populations.
Nigeria is the only country in West Africa where soybean rust, caused by Phakopsora pachyrhizi, has been officially reported (1). During a disease survey in Ghana during October 2006, soybean (Glycine max) leaves with rust symptoms (tan, angular lesions with erumpent sori exuding urediniospores) were observed in 11 fields in the following districts: Kassena Nankana in the Upper East Region; East Gonja, Central Gonja, and Tolon-Kumbungu in the Northern Region; and Ejisu-Juabeng in the Ashanti Region. Disease incidence in these fields ranged from 50 to 100% and disease severity ranged between 3 and 40% of the leaf area on infected plants. Urediniospores were hyaline, minutely echinulate, and 23 to 31 × 14 to 18 μm. Within a week of collection, leaf samples were sent to the USDA-ARS Foreign Disease-Weed Science Research Unit for verification of pathogen identity. DNA was extracted from leaf pieces containing sori with the Qiagen DNeasy Plant Mini kit (Valencia, CA), and all 11 field samples amplified in a real-time fluorescent PCR with the P. pachyrhizi-specific primers Ppm1 and Ppa2 (2). Sequence alignment of the internal transcribed spacer (ITS) region 2 further confirmed the identification as P. pachyrhizi (2). Infected leaves from three fields were separately washed in sterile water to collect urediniospores that were used to separately inoculate three detached leaves (for each isolate) of susceptible cultivar TGx 1485-1D (3). The abaxial surface of detached leaves was sprayed with 400 μl of spore suspension (1 × 106 spores per ml). A single leaf piece was placed in a 9-cm-diameter petri dish with adaxial side appressed on 1% technical agar amended with 10 μg/ml of kinetin. Lactic acid (1.5 ml/liter) and benomyl (12.5 mg/liter) were added to the agar medium to inhibit growth of saprophytic fungi and bacteria. Petri dishes were incubated at 20°C with a 12-h light/12-h dark cycle. Lesions on inoculated leaves developed 5 to 6 days after inoculation (DAI), and pustules (105 to 120 μm) formed 7 to 8 DAI and erupted 3 days later exuding columns of urediniospores similar in size to the initially collected isolates. Inoculating another set of detached leaves with a spore suspension (1 × 106 spores per ml) from the first set of detached leaves resulted in typical rust symptoms. The PCR assay, alignment of ITS region 2, morphological characters of the isolates, and pathogenicity tests demonstrate that P. pachyrhizi occurs in Ghana. To our knowledge, this is the first report of P. pachyrhizi in Ghana. References: (1) O. A. Akinsanmi et al. Plant Dis. 85:97, 2001. (2) R. D. Frederick et al. Phytopathology 92:217, 2002. (3) M. Twizeyimana et al. Online publication. http://www.plantmanagementnetwork.org/ infocenter/topic/soybeanrust/2006/posters/41.asp. Plant Management Network, 2006.
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