Good General Combining Ability Research Articles

Evaluation of combining ablity in isabgol through line X tester analysis

The present study was undertaken in three distinct environments (E1, E2, and E3) during the Rabi season of 2022-2023 in Rajasthan. The experimental set comprised of 70 genotypes, including 19 parents, 48 crosses, and 3 checks (Niharika, VI-3, GI-2), which were assessed using a randomized block design with three replications. The analysis of variance for combining ability revealed significant effects due to lines, testers, and line x tester interactions across all three environments, except for specific traits in certain environments. The parental lines L5, L10, and L7 demonstrated good general combining ability (GCA) effects across various traits. The specific combining ability (SCA) effects were diverse, with several crosses displaying consistently high effects, particularly for seed yield per plant, where crosses such as L11 x T3, L16 x T1, L6 x T2, L10 x T2, and L7 x T1 exhibited the highest estimates. Overall, the study provides valuable insights into the combining ability of Isabgol genotypes and their performance across different environments, contributing to the understanding of traits and potential breeding strategies.

Genetic analysis for morphological, quality and biofuel related traits in sweet sorghum [Sorghum bicolor (L.) Moench

The present investigation was aimed to study the relationship between different methods of diallel data analysis of ethanol yield traits in sweet sorghum to select the relevant superior genotypes. Half-diallel hybrid analysis revealed that both general and specific combining abilities were significant for all the traits studied but the specific combining ability (SCA) was higher than the general combining ability (GCA), suggesting that the non-additive gene action is more important than additive one in controlling the studied traits. Graphically, traits of juice extraction percent, juice yield, and stripped stalk yield were most correlated with ethanol yield. Griffing and GT-biplot analyses produced equivalent results for GCA and SCA and were useful in estimating gene action by identifying the superior crosses across all the correlated traits and comparing the genotypes based on multiple traits. For the ethanol yield trait, genotype, Rio showed good GCA and was considered the best combiner parent, while crosses, Sugar Drib× Ramada and Sugar Drib × Honey may be used for the selection of the relevant traits in sorghum breeding programs to improve ethanol yield trait.

Combining ability studies to develop superior hybrids in bell pepper (Capsicum annuum var. grossum L.)

General combining ability (GCA) among parents and specific combining ability (SCA) of crosses were studied to identify best crosses for growth and yield traits in bell pepper. Experimental material included 21 F1 hybrids developed by crossing seven parents in half di-allel mating design. Parents and crosses were planted in randomized block design during Rabi 2019 for estimating their combining ability. Among parents, Arka Mohini showed good GCA effects for most of the traits (number of secondary branches, early flowering and harvesting, fruit weight and yield). Among crosses, Arka Mohini × CW308, Arka Mohini × California Wonder and Yolo Wonder × California Wonder were identified as potential one for yield attributing traits based on SCA effects.

Genetic Parameters for Determining Useful Parents in Mungbean (Vigna radiata (L.) Wilczek) Breeding for Early Maturity, Small Seed Size, and High Seed Yield.

Early maturity, small seed size, and high seed yield are important characters of mungbean in Indonesia. The objective of the study was to determine the useful parents in mungbean crosses for early maturity, small seed size, and high seed yield varieties by estimating the genetic parameters and their inheritance. The study was conducted at the ILETRI, Malang, East Java, Indonesia, during the dry season of 2014. 20 F1 and 5 parents were evaluated using a randomized block design, repeated three times. Results of the study showed that all observed traits showed the importance of both additive and dominance gene effects. The relative value of general combining ability (GCA) was greater than specific combining ability (SCA) for number of pod clusters per plant, number of branches per plant, plant height, days to maturity, and 100-seed weight which indicated the importance of additive gene effect. The dominance gene effect occurred on number of pods and seed yield per plant. Among five parents, G3 was the best combiner for all the observed characters except pod length; therefore, G3 could be exploited for late maturity, small seed size, high number of branches and pod cluster, and high seed yield. G5 has a high GCA for 100-seed weight. G1 and G2 have good GCA for early maturity. G3 and G5 genotypes are useful as parents in mungbean breeding for small and large seed size varieties, respectively. The best combination for seed yield was G2 × G3 and G3 × G1 crosses and could be proceeded with selection for early maturity, small seed size, and high seed yield varieties.

Combining Ability and Heterosis for Agronomic Traits, Husk and Cob Pigment Concentration of Maize

The objective of this study was to identify the maize inbred lines with good general combining ability (GCA), good specific combining ability (SCA), high heterosis for yield and phytochemicals, and the crosses with high yield of yellow kernels and high anthocyanin content in cobs and husk, which was probably related to the high antioxidant activity. The parental lines including five unpigmented females and five pigmented males were crossed in North Carolina design II. The parents, the resulting 25 hybrids, and 5 controls were evaluated at two locations in the dry season of 2016/2017. Additive and non-additive gene effects controlled the inheritance of grain yield, agronomic traits, and phytochemicals. KKU–PFC2 and KKU–PFC4 had the highest GCA effects for phytochemical traits in husk and cob, whereas Takfa1 and Takfa3 were good combiners for grain yield. F1 hybrids had significantly higher total anthocyanin content (TAC), total phenolic content (TPC), (2,2-diphenyl-1-picrylhydrazyl) (DPPH), and trolox equivalent antioxidant capacity (TEAC) in husk and cob than pigmented control cultivars. The hybrids superior for individual traits were identified, but the experiment was not able to identify superior hybrids for multiple traits. The Takfa3 × KKU–PFC5 and NakhonSuwan2 × KKU-PFC4 had the highest anthocyanin in husk and cobs, respectively. The breeding strategies to develop maize varieties with high anthocyanins and normal yellow kernels and utilization of the hybrids are discussed.

Combining ability analysis of yield and resistance to late blight disease caused by Phytophthora infestans in Uganda

Breeding for resistance to late blight disease caused by Phytophthora infestans (Mont.) de Bary in potato is the most economically, effective and ecologically sustainable method to control the disease and boost productivity. The study estimated combining ability effects of yield, yield related traits, late blight resistance and gene action controlling resistance to Phytophthora infestans. Eighteen F1 families generated from two sets of 12 parents, using a North Carolina Design II, were evaluated for relative area under disease progress curve (rAUDPC), yield and yield related traits in two late blight hotspot locations of Kachwekano and Kalengyere research stations. Trials were established using an alpha lattice design (6 x 5) with two replications. Both additive and non-additive genetic effects were important in controlling yield and late blight resistance. However, additive gene action was dominant. The general combining ability (GCA) to specific combining ability ratio (SCA); (GCA/SCA) for total tuber weight and late blight resistance was 0.53 and 0.62 respectively. Broad-sense heritability estimates were 0.78 for total tuber weight and 0.68 for relative area under disease progress curve (rAUDPC). This study showed some evidence of maternal effects for rAUDPC (1.45) and average tuber weight (ATW) (1.56), although these were not significant at P<0.05. Parents Kinigi, 392657.8, 396034.103, 396038.107, 395011.2, NKRK19.17, NKRN59.58 and 395017.14 had good GCA effects for both late blight resistance and yield related traits. Crosses of 392657.8 x 395017.14 and 396038.107 x NKRN59.58 had the highest SCA effects for yield related traits, while families Kinigi x NKRK19.17and 392657.8 x NKRN59.41 had the lowest SCA effects for rAUDPC. The relatively high heritability estimates and predominant additive genetic effects imply that genetic advances in resistance to Phytophthora infestans and tuber yield among these genotypes can be realized by selecting superior clones.

Combining ability studies on terminal heat tolerance in wheat (Triticum aestivum L.) under late sown irrigated condition using Line X Tester analysis

The present investigation entitled “Combining ability studies on terminal heat tolerance in wheat (Triticum aestivum L.) under late sown irrigated condition using Line X Tester design” was carried out during rabi 2017-18 and 2018-19 at the All India Co-ordinated Wheat and Barley Improvement Project, B.T.C. College of Agriculture and Research Station (IGKV), Bilaspur (C.G.). The experiment was conducted in RBD involving six lines, three testers and 18 F1s hybrids of wheat with two replications for study of character associated with yield and yield contributing traits. The analysis indicated that variance due to treatments was highly significant for all the characters under studied. All the characters except spike weight were highly significant. GCA, SCA and additive effect was found highest for character of harvest index. Among the lines, CG 1015 (Chhattisgarh Gehu)-04 was found good general combining ability (GCA) effect for the character of peduncle length, spike length, number of seed per plant and biological yield per plant. MP 3336 was found good GCA effect for chlorophyll reflection index, harvest index and number of tillers per plant. HD 2285 was found good GCA effect for days to maturity, 1000 seed weight and canopy temperature depreciation. Among the testers, Halna was good GCA effect for the character of days to 50% flowering, days to maturity and spike length. PHSL 10 was good GCA effect for number of tillers per plant, spike weight, seed yield per plant and harvest index. Among the crosses, CG 1015 (Chhattisgarh Gehu-04) X HUW 661 was good specific combining ability (SCA) effect for seed yield per plant. MP 3336 X PHSL 10 was good SCA effect for number of spikelet per spike and number of seed per plant. HD 2932 X Halna was good SCA effect for spike weight and 1000 seed weight and crosses HD 2864 X Halna was good SCA effect for the character of canopy temperature depreciation and biological yield per plant.

Genetics analysis of nitrogen use efficiency component traits under nitrogen-limiting environment

To address the environmental concerns and increases the total factor productivity to the farmers simultaneously, forces the breeders to develop the genotypes adapted to low input agriculture especially, low nitrogen (N) management practices. However, inadequate knowledge about the genetics of plant response to the low nutrient regime is complicating the breeding procedure. Therefore, the present study was designed to work out the genetics of nitrogen use efficiency (NUE) and its component traits under N-limiting (LN) environment in Indian wheat germplasm. Analysis of variance showed the presence of a large amount of genetic variability for NUE as well as its component traits. Combining ability analysis for N efficiency under LN indicated the importance of both additive and nonadditive gene action in their inheritance. Additive gene action was more important for shoot dry weight, NUE, and maximum root length. Improvement in these traits can be achieved by targeting the selection in early fixed lines (F4–F5) in the breeding population created by crossing good general combining ability (GCA) lines such as DPW 621-50 and HD2967/HD3024. Whereas, root dry weight (RDW), gram N in shoot (gN), nitrogen uptake efficiency (NUpE) are influenced by nonadditive gene effects which can be improved either through population improvement programme or hybrid breeding approach. Recurrent selection can be quite useful in developing the desirable recombinants for RDW, gN, NUpE, and nitrogen utilization efficiency from crosses with high specific combining ability (SCA) effects and involving high × low GCA parents.

Combining capacity and heterosis in eggplant hybrids under high temperatures

ABSTRACT The objective of this work was to estimate the combinatorial capacity and heterosis of eggplant hybrids under high temperature conditions. Seven genitors, twelve hybrid combinations, originated from a partial diallel, and the Ciça F1 hybrid, as control, were evaluated. The experiment was conducted under greenhouse conditions in randomized block design with four replications, from April to December 2017. The assessed traits related to high temperatures were pollen viability (PV) and fruit fixation index (FFI); the morphoagronomic traits were number of fruits per plant (NFP), fruit weight (FWe), production per plant (PP), fruit length (FL), fruit width (FWi), fruit length/width ratio (FLWR) and plant height (PH). The variance analysis showed greater participation of the additive gene effects in relation to the non-additive gene effects in most traits, except for PV. The genitors CNPH 141, CNPH 135, CNPH 109 and CNPH 51 stood out with favorable gene effects to obtain genotypes tolerant to high temperatures, since they present good general combining ability (GCA) for the traits FFI, NFP and PP. The 1x4 and 3x4 hybrids presented positive estimates for both GCA and specific combining ability (SCA), demonstrating a greater potential to be used in breeding to increase the FFI, NFP and PP, under high temperatures. The 1x4, 1x5 and 1x6 hybrids expressed positive heterosis for most analyzed traits. The 1x4 hybrid stood out for the highest averages for PV, FFI, NFP and PP. For FWe, FL, FWi and FLWR, both positive and negative heterosis were observed, as consequence of the phenotypic variability of the genitors for these traits and suggests the possibility of selection for different sizes and formats.

Daya Gabung Inbrida Jagung Toleran Cekaman Kekeringan dan Nitrogen Rendah pada Pembentukan Varietas Hibrida

<p class="Abstrak">Low maize grain yield due to drought and low N stresses maybe overcome by growing tolerance hybrid maize varieties.<strong> </strong>The objectives of this research were to analyze the combining ability of eight inbred lines possessing those traits and indentify suitable parental inbred lines which can be used to develop single cross hybrid tolerant to drought and low nitrogen. Eight inbred lines were crossed using complete diallel cross design of Griffing’s- I on Februry to June, 2014. Those inbred lines were 1044-30, DTPYC9-F46-3-9-1-1-B (for drought tolerance), MR 14 (medium drought tolerance), G20133077 (tolerant to low N), CY 11, CLRCY039 (medium tolerant to low N), and CY12 and G2013631 (for drought and low N sensitive). Fifty-six experimental hybrids derived from diallel crosses, eight inbred lines and four commercial hybrid varieties as checks were evaluated under managed drought, low dosage of N fertilization and under optimum condition, during the dry season (July to October) of 2014. Randomized complete block design three replications was used in each different stress condition. The results showed that CY 11, G2013631, and MR 14 inbred lines possesed good general combining ability (GCA) for grain yield under drought, low N, and optimum conditions. These inbred lines also showed good GCA for the secondary traits such as large stem, delayed leaf rolling and late leaf senescence to support for high grain yield under drought stress and low N condition. Inbred lines of drought and low N sensitive such as G2013631, but had high GCA, could be used as parent to develop drought and low N tolerance hybrids if crossed with inbred lines having medium-tolerant to drought or tolerant to low N, such as G20133077, which had low DGU, or CY11 which had high GCA. Hybrid maize tolerant to drought and tolerant to low N, such as CY11 x G20133077, G20133077 x G2013631, and CY 11 x G2013631, produced significantly higher grain yield, over 8 t/ha, under drought and under low N, as compared with hybrid varieties Bima 3 and Bisi 2, which yielded 5.91 and 6.59 t/ha under drought, and 6.82 and 7.01 t/ha under low N condition. Grain yields under optimum condition were positively correlated with those under both drought and low N conditions. The result suggests that it is possible for improving grain yield of hybrid maize, simultaneously for optimum, drought stress and low N conditions.</p>

Breeding potential of raspberry primocane selections based on their combining abilities

Information on the genetic potential of raspberry (Rubus idaeus L.) genotypes for their use in breeding programs is currently limited. We used a diallel mating design to study the breeding values of raspberry primocane fruiting cultivars in terms of their combining ability. The objectives of this study were to identify raspberry genotypes with high general combining ability (GCA) for use in cultivar development, detect the best crosses in terms of their specific combining ability (SCA), and determine the gene action type and heritability of yield and eight of its components. The obtained results showed that the parent cultivar MRSL exhibited the highest GCA effects for the total yield per plant, fruit weight, number of fruit per plant, fruit length, fruit diameter, and number of drupelets per fruit. Genotype C47 had good GCA for the number of canes per plant and plant height and TD-865 had high GCA for high soluble solids content. The narrow-sense heritability estimates were low to moderate (0.00–0.62) fo...

DIALLEL ANALYSIS OF SOME IMPORTANT GRAIN YIELD TRAITS IN BREAD WHEAT CROSSES

One of the most important tasks in wheat breeding program is development of high yielding wheat cultivars by crossing the lines with good general combining ability (GCA) and selecting desirable genotypes within its segregating population for grain yield traits. The objective of this study was to estimate the combining ability and gene action involved in the expression of the traits grain weight per spike and grain weight per plant of wheat in F1 generations, in 5 × 5 half diallel cross of bread wheat varieties (Triticum aestivum L.). The results of applying combining ability analysis indicated that among the parents, genotypes Sara, Pobeda and Renesansa were found to be the best general combiners for the both investigated traits. The best specific crosses for both traits were Sara/Partizanka, Pobeda/Sara, Renesansa/Partizanka, Partizanka/Pesma and Pobeda/Pesma. Most of the specific crosses involved high × high, high × low and low × low general combiners. The results of regression analysis, as well as the analyses of components of genetic variation indicated over-dominance in the inheritance of examined traits suggested that selection in later segregating generations may lead to fairly good improvement in these characters

Combining ability studies exploiting gynoecy in cucumber (Cucumis sativus L.)

Twelve F1 hybrids of cucumber derived from a top cross involving twelve monoecious parents and a stable gynoecious inbred (EC 709119) as female parent were evaluated in randomized block design (RBD) with three replications to assess general combining ability (GCA). Significant GCA effects were observed for all the characters except days to first male flower anthesis. The estimates of GCA effects revealed that none of the parents exhibited good GCA for all the characters together. Among 12 parents, CS123 was observed as the good general combiner for fruit yield per vine, length of main vine, branches per plant, number of harvests, duration of the crop, fruits per plant and number of seeds per fruit. The parents, CS128 and IC 538186 were general good combiners for earliness. The parent CS127 was found to be superior for fruit length, weight and girth.

Combining ability analysis in linseed (Linum usitatissimum L.) for improvement of seed yield and its component traits

Twenty eight hybrids developed from eight genotypes of linseed through diallel mating design (excluding reciprocal) along with one checks viz., Shekhar were evaluated in randomized block design with three replications for eleven distinct morphological traits during rabi season of 2012-13, to estimate the general combining ability (GCA) of the parents and specific combining ability (SCA) of hybrids considered for the development of high yielding varieties. The analysis of variance for combining ability revealed significant mean sum of squares due to GCA and SCA for all the traits under study. Genotype EC-1392 (0.50) and TL-11 (0.45) has been found good general combiner with highest magnitude of GCA effects for seed yield per plant. In addition to above, EC-1392 and TL-11 also showed good GCA effects for plant height, secondary branches per plant, capsules per plant and biological yield per plant. Hybrids A.95.B x Shekhar (2.43), TL-11 x EC-1392 (1.71) and NDL-2004-05 x GS-234 (1.53) showed higher positive significant SCA effects for seed yield per plant. These hybrids also exhibited positive significant SCA effects for plant height, secondary brances per plant, capsules per plant, indicating potential for exploiting hybrid vigour in breeding programme.

English

Verticillium wilt disease causes significant losses to cotton yield and varietal development for resistance is critical in combating this threat. A study was carried out to determine the underlying genetic pattern controlling disease resistance and identify suitable parental lines to use in varietal tolerance development. Five cotton varieties were crossed in a half diallel mating system to produce ten crosses and five selfed parental lines which were screened against Verticillium wilt by artificial inoculation. There were significant variations on the severity scores, vascular brown index scores, morphological and agronomical traits under Verticillium wilt pressure. The Verticillium wilt severity scores ranged from 1.22 to 3.07 and Vascular Brown Index scores from 0.78 to 2.77. The mean squares of general combining ability (GCA) for the various characteristics of the parents were significant and also for the specific combining ability (SCA) of the vascular brown index score which was 0.27 was also significant. Three parental lines were identified as breeding material with good GCA, morphological and agronomical performance and these were CRI-MS-1, SZ9314 and BC853. This study implies that varietal tolerance can be developed by incorporating genotypes with resistance genes in breeding programmes. Future work should focus on developing resistant varieties suitable for production in Verticillium wilt prone areas.   Key words: Verticillium wilt, general combining ability, specific combing ability, cotton.&nbsp

Combining ability of maize (Zea mays) inbred lines resistant to Striga hermonthica (Del.) Benth evaluated under artificial Striga infestation

The parasitic weed Striga affects maize on an estimated 20 million ha in Africa, making it a major cause of maize yield reduction from near world average of 4.2 t/ha few decades ago to the present 1.5 t/ha. The objectives of this study were to examine the combining ability of 20 inbred lines and identify single crosses which can be used to develop other hybrids resistant to Striga hermonthica (Del.) Benth. Fourteen female lines were mated using North Carolina Design II with all six males. The resulting 84 F1s along with six commercial checks were evaluated in four separate trials for two rainy seasons during 2010. The trials were conducted on station under both artificial Striga infestation and Striga free environments using standard procedures at the Kibos and Alupe sites, both in the Kenya’s Lake Victoria Basin. Data were recorded on Striga counts, Striga damage rating (SDR), grain yield and other agronomic traits. General combining ability (GCA) and specific combining ability (SCA) effects were computed using SAS. The new F1 hybrids outperformed the commercial checks in grain yield and reaction to Striga. Single crosses JI-30-3/TESTR 151, JI-30-18/TESTR 151, CML206//56/44-6-3-7-1/TESTR 149 and JI-30-18/TESTR 156 gave the highest yield while single cross JI10-28-#/TESTR 136 gave the lowest yield. The ratio of GCA: SCA mean squares exhibited a predominance of additive gene effects in the inheritance of Striga resistance traits as opposed to dominance gene effects. Inbred lines with good GCA for yield and Striga resistance traits were identified as TESTR 151, TESTR 156 and OSU231//56/44-6-4-17-3. The high GCA inbred lines and the superior single crosses will provide a basis for future use perse and also development of three-way and double cross hybrids to be grown in Striga prone areas of the Lake Victoria Basin in eastern Africa. Key words: Maize, Striga hermonthica, general combining ability (GCA), specific combining ability (SCA), host plant resistance, sub-Saharan Africa.

DIALLEL CROSSES FOR ESTIMATION OF COMBINING ABILITY FOR YIELD AND AGRONOMIC ATTRIBUTES IN MAIZE

The present study was carried out to estimate combining ability of 36 hybrids generated by crossing 9 new maize inbred lines in a half diallel model in 2013 season at two locations of Sids and Gemmeiza Agric. Res. Stns. The experiment was laid out in a randomized complete block design with four replications. Number of days to 50 % silking, plant height, ear height, ear length, ear diameter and grain yield (Ard / Fed) were determined. The mean squares, due to general combining ability (GCA) and specific combining ability (SCA) were highly significant for all studied traits. There was preponderance of additive gene action for all studied traits. Estimates of GCA and SCA effects showed that parental lines P6 and P8 were generally good combiners for earliness, shortness and lower ear placement, while, P2, P3 and P9 were good combiner for grain yield and can be included in future improvement maize breeding programs. Two crosses exhibited desirable SCA effects for grain yield, four hybrids significantly out yielded the highest yielding check hybrids SC. 162, and twelve hybrids did not differ significantly than the check. These hybrids may be released as commercial hybrids after further testing and evaluation. It could be concluded that P2, P3 and P9 was found to be a good GCA can be used in synthetic variety and hybrids ( P1 x P2 ) and ( P7 x P8 ) was found to be a good combiner and can use as a potential single cross hybrid combination and tester further.

The Use of Combining Ability Analysis to Identify Elite Parents for Artemisia annua F1 Hybrid Production

Artemisia annua is an important medicinal crop used for the production of the anti-malarial compound artemisinin. In order to assist in the production of affordable high quality artemisinin we have carried out an A. annua breeding programme aimed at improving artemisinin concentration and biomass. Here we report on a combining ability analysis of a diallel cross to identify robust parental lines for hybrid breeding. The parental lines were selected based on a range of phenotypic traits to encourage heterosis. The general combining ability (GCA) values for the diallel parental lines correlated to the positive alleles of quantitative trait loci (QTL) in the same parents indicating the presence of beneficial alleles that contribute to parental performance. Hybrids generated from crossing specific parental lines with good GCA were identified as having an increase in both artemisinin concentration and biomass when grown either in glasshouse or experimental field trials and compared to controls. This study demonstrates that combining ability as determined by a diallel cross can be used to identify elite parents for the production of improved A. annua hybrids. Furthermore, the selection of material for breeding using this approach was found to be consistent with our QTL-based molecular breeding approach.

COMBINING ABILITY AND HETEROSIS FOR MAIZE GRAIN YIELD AND SOME AGRONOMIC CHARACTERS

The present study was carried out at the Experimental Station Farm, Faculty of Agriculture, Mansoura University, Dakahlya Governorate, during the two successive seasons of 2011 and 2012 to determine general, specific combining abilities and heterosis for grain yield and yield associated traits by crossing 6 inbred lines of maize in a half diallel mating design. Fifteen F1 single crosses with their parents were planted in a randomized complete block design with three replicates. Results showed that mean squares of crosses were significant for all studied traits i.e. number of days to 50% tasseling and silking, plant height, number of rows/ear, number of grains/row, 100-grain weight, grain yield/plant and shelling percentage. The analysis of variance revealed highly significant mean squares of general combining ability (GCA) and specific combing ability (SCA) for all studied traits, indicating the importance of both additive and non additive genetic effects for these traits. GCA/SCA variances ratios were found to be greater than unity for number of days to 50% tasseling and silking, plant height and 100-grain weight, indicating that the additive and additive×additive types of gene action were greater importance in the inheritance of these 4 traits. Best GCA effects for earliness traits (number of days to 50% tasseling and silking) achieved inbred lines P4 (Inb.173) and P5 (Inb.174), for plant height were P1 (Sd63), P3 (Inb.19), P4 (Inb.173) and P5 (Inb.174), for number of rows/ear were P1 (Sd63), P3 (Inb.19) and P4 (Inb.173), for 100-grain weight was P1 (Sd63). Inbred lines P1 (Sd63) and P2 (Sd7) showed best GCA effects for grain yield. Also P2 (Sd7) and P6 (Inb.170) showed best GCA effects for shelling percentage. Hybrid combinations P2×P5 and P3×P6 showed largest SCA effects for number of days to 50% tasseling, for number of days to 50% silking were P1×P2, P1×P6, P2×P5 and P3×P4, for plant height were P1×P3, P1×P5, P2×P3, P2×P6, P4×P5 and P4×P6, for number of rows/ear were P1×P2, P1×P4, P2×P3, P3×P4, P4×P5 and P5×P6 and for shelling percentage were P1×P3, P1×P4, P2×P3, P2×P4, P3×P4 and P5×P6. Concerning number of grains/row, grain yield/plant and100-grain weight, most of crosses recorded positive significant or highly significant SCA effects. Results showed significant or highly significant heterosis for all studied traits. Cross P3×P6 was the best, with highly significant negative heterosis over mid and better parents for number of days to 50% tasseling. Cross P2×P3 recorded the highest negative heterosis over mid and better parents for number of days to 50% silking. Regarding to plant height, none of the cross combinations showed negative heterosis over mid and better parents. The best cross over mid and better parents for number of grains/row, grain yield and 100-grain weight was P1×P2. Crosses P1×P4 and P5×P6 recorded the highest positive heterosis over mid and better parents for number of rows/ear and shelling percentage, respectively. Therefore, it could be stated that parents with good positive GCA for grain yield (P1 and P2), negative GCA for tasseling, silking date (P4 and P5) and dwarf plant height (P1, P3, P4 and P5) may be extensively used in hybridization program as a donor. The better performing four crosses (P1×P2, P1×P4, P1×P6 and P2×P3) can be utilized for developing high yielding hybrid varieties as well as for exploiting hybrid vigor.

Diallel analysis of field resistance to brown streak disease in cassava (Manihot esculenta Crantz) landraces from Tanzania

Cassava brown streak disease (CBSD) is an economically important virus disease causing significant losses to cassava root yield and quality in east, central and southern Africa. Breeding for resistance in cassava requires an understanding of the underlying genetic control of CBSD resistance. Sources of CBSD resistance are available but little is known on the value of those varieties as parents for CBSD resistance breeding. Two resistant and two susceptible varieties were crossed in a half diallel design and 35 F1 progeny from each of the six families, plus parents, were screened at two locations in a randomised complete block design with four replications in warm sub-humid environments of coastal Tanzania in 2008. Screening for CBSD field resistance was done using disease severity scoring on a scale of 1–5. Significant variations in disease severity were observed for crosses, general combining ability (GCA) and specific combining ability (SCA) at both locations. The CBSD shoot symptom severity scores ranged from 1 to 4.4 while root necrosis ranged from 1.3 to 4.5. The contribution of GCA to the total sum of squares of crosses for disease scores ranged from 86.9 to 95.2 % compared to SCA that ranged from 4.8 to 14.2 %. Additive gene effects were more important than non-additive effects indicating that CBSD resistance is genetically determined and that selection should be successful to improve resistance. Selection of parents with good GCA effects will be important for success in CBSD resistance breeding.