Evaluation of greenhouse inoculation techniques to screen sorghum for resistance to downy mildew

Summary Six inoculation techniques were compared for the artificial promotion of downy mildew (Peronosclerospora sorghi) in sorghum. These were (1) sprouted seeds incubated between sporulating infected leaves, (2) sprouted seeds depped in conidial suspension, (3) sprouted seeds sprayed with conidial suspension, (4) seedlings at plumule stage inoculated with drops of a conidial suspension, (5) seedlings at plumule stage sprayed with a conidial suspension, and (6) seedling showered with conidia falling from infected leaves. Seedlings at the one-leaf stage sprayed with a conidial suspension (6 × 10⁵ ml^-1) showed the highest systemic infection (100%) in the susceptible lines IS 643 and IS 18433. This technique is effective, repeatable, and allows the deposition of a conidial suspension as a fine mist on the entire seedling surface. In the greenhouse, the technique was used to test the downy mildew reaction of genotypes previously reported as resistant (< 5% incidence) in 3–4 years of field screenings. Of the 61 genotypes tested, 21 were free from downy mildew, 14 had less than 5% incidence, and the rest showed variable susceptible reactions. Therefore, the technique can be reliably and effectively used in the greenhouse to detect disease escapes and to indentify resistance.     

Genetic analysis of biotype I greenbug resistance in sorghum

The greenbug has been a major insect pest of sorghum since 1968. Although sources of genetic resistance have been identified to combat this pest, new and virulent biotypes have successfully overcome these resistance genes. KS 97 was developed and released by the Kansas Agricultural Experiment Station as a new germplasm source of biotype I greenbug resistance in sorghum. The objectives of this study were to evaluate combining ability effects for greenbug resistance in KS 97 and to determine the number of genes responsible for this trait. Six inbred lines, including KS 97 and greenbug resistant and susceptible checks, were intercrossed using a Design-II mating scheme to produce nine F1 hybrids. Responses of seedlings of parent lines and hybrids to biotype I greenbug were evaluated in replicated growth chamber experiments. The results of these studies indicated significant effects of general combining ability (GCA) and specific combining ability (SCA). Greenbug resistance derived from KS 97 was found to be incompletely dominant, and the GCA effect for resistance associated with KS 97 was superior to that associated with PI550610, the resistant check. Segregation studies to determine the number of genes responsible for greenbug resistance in KS 97 were conducted in BC1F1 populations. KS 97 was introgressed into three greenbug-susceptible genetic backgrounds. Segregation analysis indicated a consistent 1:3 (resistant:susceptible) segregation ratio for greenbug resistance across populations. The simplest explanation for these results is that two dominant genes requiring complementary gene action control resistance. This revised version was published online in August 2006 with corrections to the Cover Date.     

Pattern analysis of genotype × environment interaction for striga resistance and grain yield in African sorghum trials

The parasitic weed Striga hermonthica (Del.) Benth. seriously limits sorghum [Sorghum bicolor (L.) Moench] production in Sub-Saharan Africa. As an outbreeder, S. hermonthica is highly variable with an extraordinary capacity to adapt to different hosts and environments, thereby complicating resistance breeding. To study genotype x environment (G x E) interaction for striga resistance and grain yield, nine sorghum lines, 36 F₂ populations and five local checks were grown under striga infestation at two locations in both Mali and Kenya. Mean squares due to genotypes and G x E interaction were highly significant for both sorghum grain yield and area under striga severity progress curve(ASVPC, a measure of striga emergence and vigor throughout the season). For grain yield, the entry x location-within-country interaction explained most of the total G x E while for ASVPC, entry x country and entry x location-within-country interactions were equally important. Pattern analysis (classification and ordination techniques) was applied to the environment-standardized matrix of entry x environment means. The classification clearly distinguished Malian from Kenyan locations for ASVPC, but not for grain yield. Performance plots for different entry groups showed differing patterns of adaptation. The ordination biplot underlined the importance of entry x country interaction for ASVPC. The F₂ derived from the cross of the striga-resistant line Framida with the striga-tolerant cultivar Seredo was the superior entry for both grain yield and ASVPC, underlining the importance of combining resistance with tolerance in striga resistance breeding. The observed entry x country interaction for ASVPC may be due to the entries' different reactions to climatic conditions and putative differences in striga virulence in Mali and Kenya. This revised version was published online in August 2006 with corrections to the Cover Date.     

Genetic analysis of grain mould resistance in coloured sorghum genotypes

Grain moulds are a major constraint to sorghum production and to adoption of improved cultivars in many tropical areas. Information on the inheritance of grain mould reaction is required to facilitate breeding of resistant cultivars. The genetic control of grain mould reaction was studied in 7 crosses of 2 resistant sorghum genotypes. P₁, P₂, F₁, F₂, BC₁ and BC₂ families of each cross were evaluated under sprinkler irrigation for field grade and threshed grade scores and subjected to generation mean analysis. Frequency distributions for grain mould reaction were derived and F₂ and BC₁ segregation ratios were calculated. Grain mould reaction in crosses of coloured grain sorghum was generally controlled by two or three major genes. Resistance to grain moulds was dominant. Significant additive gene effects were also found in all cross/season combinations. Significant dominance effects of similar magnitude to additive effects were also observed in five out of ten cross/season combinations. Gene interactions varied according to the parents with both resistant and susceptible parents contributing major genes. Choice of parents with complementary resistance genes and mechanisms of resistance will be critical to the success of resistance breeding. This revised version was published online in July 2006 with corrections to the Cover Date.     

Mechanisms of resistance to shoot fly, <Emphasis Type="Italic">Atherigona soccata</Emphasis> in sorghum

Summary Sorghum shoot fly, Atherigona soccata (Rondani) is an important pest of sorghum in Asia, Africa, and Mediterranean Europe, and host plant resistance is an important component for the management of this pest. The levels of resistance in the cultivated germplasm are low to moderate, and therefore, it is important to identify genotypes with different mechanisms of resistance to pyramid the resistance genes. We studied the antixenosis for oviposition, antibiosis, and tolerance components of resistance in a diverse array of shoot fly-resistant and -susceptible genotypes. The main plants and tillers of SFCR 151, ICSV 705, SFCR 125, and, IS 18551 experienced lower shoot fly deadhearts at 28 days after seedling emergence, produced more number of productive tillers. The insects fed on these genotypes also exhibited longer larval period (10.1–11.0 days compared to 9.3 days on Swarna), lower larval survival and adult emergence (54.7–67.8 and 46.7–52.2% compared to 73.3 and 60.6% on Swarna, respectively), and lower growth and adult emergence indices as compared to the susceptible check, Swarna. Physico-chemical traits such as leaf glossiness, trichome density, and plumule and leaf sheath pigmentation were found to be associated with resistance, and chlorophyll content, leaf surface wetness, seedling vigor, and waxy bloom with susceptibility to shoot fly and explained 88.5% of the total variation in deadhearts. Step-wise regression indicated that 90.4% of the total variation in deadhearts was due to leaf glossiness and trichome density. The direct and indirect effects, correlation coefficients, multiple and step-wise regression analysis suggested that deadhearts, plants with eggs, leaf glossiness, trichomes on the abaxial surface of the leaf, and leaf sheath pigmentation can be used as marker traits to select for resistance to shoot fly, A. soccata in sorghum.     

Compensation in grain weight and volume in sorghum is associated with expression of resistance to sorghum midge, Stenodiplosis sorghicola

Sorghum midge, Stenodiplosis sorghicola (Coquillett) is one of the most important pests of grain sorghum worldwide. We studied the inheritance of resistance to sorghum midge and compensation in grain weight and volume in panicles of sorghum hybrids and their parents under uniform infestation (40 midges per panicle for two consecutive days). Sorghum midge damage ranged from 8.2 to 82.4% in the maintainer lines (B-lines) of the females parents (A-lines), and 9.0 to 67% in the male parents (restorer lines). Hybrids involving resistant × resistant parents were highly resistant, while those involving resistant ×susceptible and susceptible × resistant parents showed moderate susceptibility. Susceptible × susceptible hybrids were susceptible. Compensation in (percentage increase) grain weight and volume in midge-infested panicles of midge-resistant parents and their F₁ hybrids was greater than in midge-susceptible parents and hybrids. General combining ability effects for midge damage, and grain weight and volume were significant and negative for the midge-resistant females (ICSA 88019 and ICSA 88020), whereas those for the midge-susceptible females (ICSA 42 and 296A) were significant and positive. However, the reverse was true in case of compensation in grain weight and volume. Inheritance of compensation in grain weight and volume and resistance to sorghum midge is controlled by quantitative gene action with some cytoplasmic effects. Resistance is needed in both parents to realize full potential of midge-resistant hybrids. This revised version was published online in July 2006 with corrections to the Cover Date.     

Genetic analysis of grain mold resistance in white seed sorghum genotypes

Grain molds in rainy season sorghums can cause poor grain quality resulting in economic losses. Grain molds are a major constraint to the sorghum production and for adoption of the improved cultivars. A complex of fungi causes grain mold. Information on genetics of grain mold resistance and mechanisms is required to facilitate the breeding of durable resistant cultivars. A genetic study was conducted using one white susceptible, three white resistant/tolerant sources, and one colored resistant source in the crossing programme to obtain four crosses. P₁, P₂, F₁, BC₁, and BC₂, and F₂ families of each cross were evaluated for the field grade and threshed grade scores, under sprinkler irrigation. Generation mean analyses and frequency distribution studies were carried out. The frequency distribution studies showed that grain mold resistance in the white-grained resistance sources was polygenic. The additive gene action and additive × additive gene interaction were significant in all the crosses. Simple recurrent selection or backcrossing should accumulate the genes for resistance. Epistasis gene interactions were observed in colored resistance × white resistance cross. Gene interaction was influenced by pronounced G × E. Pooled analysis showed that environment × additive gene interaction and environment × dominant gene interaction were significant. The complex genetics of mold resistance is due to the presence of different mechanisms of inheritance from various sources. Evaluation of segregating population for resistance and selection for stable derivatives in advanced generations in different environments will be effective.     

Genotypic resistance in sorghum to head bug,Calocoris angustatus Lethiery

Summary Sorghum head bug, Calocoris angustatus Leth., is an important pest of grain sorghum. We screened nearly 15000 germplasm accessions for resistance to this pest between 1980 and 1990 under natural and headcage conditions. Data were recorded on bug numbers, grain damage (1 = highly resistant the 5 = highly susceptible), and seed germination. Under natural conditions, 34 genotypes suffered moderate levels of grain damage (damage rating (DR) 1.7 to 2.9) compared with a DR of 4.0 to 4.6 in the susceptible controls CSH 1, CSH 5 and CSH 9. IS 17610, IS 17645, IS 21444, IS 19948, IS 25069 and IS 19949 suffered a DR of less than three, and harbored less than 150 bugs/panicle compared with a DR of 4.3 to 4.7, and 248 to 353 bugs/panicle in the susceptible controls CSH 1, CSH 5 and CSH 9 when infested under headcage with 5 pairs of bugs/panicle. IS 18274, IS 20664, IS 20059, IS 25069, and IS 19951 had 150 to 300 bugs/panicle but suffered moderate levels of grain damage (DR less than 3), while the reverse was true in case of IS 8064, IS 19455, IS 19955, IS 20024, IS 20740, IS 23627, IS 2761, and IS 9692. During the 1989 rainy season, IS 14108, IS 17610, IS 17618, IS 17645, IS 19949, IS 19950, IS 19957, IS 20068, IS 25760, IS 27452, IS 27477 and IS 27329 suffered moderate levels of grain damage when infested with 5 and 10 pairs of bugs/panicle, and recorded more than 80% seed germination compared with a DR of 3.9 to 5.0, and seed germination of 15–18% in the susceptible controls CSH 1, CSH 5 and CSH 9. There is a considerable diversity in the genotypes resistant to head bugs, and attempts should be made to transfer the resistance into agronomically acceptable cultivars.     

Analysis of resistance mechanism to Atherigona soccata in crosses of sorghum

In sorghum, shoot fly resistance is important for grain yield and fodder value. An experiment was conducted to estimate genetic parameters of sorghum for resistance to shoot fly in 50 hybrids, by crossing 5 × 10 genotypes in line × tester manner. Plant height, number of leaves per plant, number of eggs per plant, trichomes on upper and lower surface per unit area of lamina and dead heart per cent were measured on 14 and 21 days after emergence (DAE) and glossiness of leaves was graded on 14 DAE. The correlation between midparent and hybrid performance, GCA : SCA ratio revealed predominance of non-additive gene effects for the traits studied, which could be exploited through hybrid breeding. Of the parents, SPSFPR 94004A and IS 4777 were the best general combiners for shoot fly resistance. Correlation and path analysis revealed the importance of resistance traits and phenol estimation confirms the resistances against shoot fly.     

Quantitative-genetic parameters of sorghum growth under striga infestation in Mali and Kenya

To estimate quantitative‐genetic parameters of sorghum for resistance to the hemi‐parasitic weed striga [Striga hermonthica (Del.) Benth.] and for agronomic traits, 36 diallel F₂ populations and their nine parental lines were evaluated under severe striga infestation at two locations each in Mali and Kenya. Location means for grain yield ranged from 132 to 254 g/m². F₂ populations outyielded lines on average by 18%. For striga emergence traits, F₂ heterosis values ranged from ‐36% to 232% among populations. Genetic and genotype x environment interaction variances of lines and F₂s were highly significant for all traits. Broad‐sense heritabilities for areas under striga severity progress curves and grain yield were 0.83 and 0.90 in lines, and 0.81 and 0.89 in F₂s, respectively. General and specific combining ability, and their interaction effects with locations were significant for most traits. F₂ superiority for grain yield under striga infestation demonstrates the potential merit of heterozygous cultivars in the target areas. Significant genotype x environment interaction entails multilocational testing to identify stable resistance. A combination of resistance with striga tolerance is recommended to breeders.     

A single recessive gene for stem sweetness in sorghum

Summary The inheritance of stemsweetness in sorghum has followed a qualitative pattern, non-sweet being monogenically dominant over sweet. Hence sweetness was controlled by a single recessive gene. A maternal effect was not involved for the expression of sweetness.     

Diallel analysis of sweetpotatoes for resistance to sweetpotato virus disease

Sweetpotato virus disease (SPVD) is due to the dual infection and synergistic interaction of Sweetpotato feathery mottle potyvirus (SPFMV) and Sweetpotato chlorotic stunt crinivirus(SPCSV), and causes up to 98% yield loss in sweetpotato in East Africa. This study was conducted to determine the inheritance of resistance to SPVD in sweetpotato and to estimate the nature of genetic variance. Ten parental clones varying in reaction to SPVD were crossed in a half diallel mating design to generate 45 full-sib families. The families were graft-inoculated with SPCSV and SPFMV to induce SPVD and evaluated for resistance in a randomized complete block design at two sites in Namulonge, Uganda during 1998–2000. In serological assays for SPFMV and SPCSV,resistance to symptom development and recovery from initial systemic SPVD symptoms, characterised resistant genotypes. Genetic component analysis showed significant effects for both general combining ability (GCA) and specific combining ability (SCA) for resistance to SPVD. GCA to SCA variance component ratios were large (0.51–0.87), hence GCA effects were more important than SCA effects. Resistant parents exhibited high GCA indicating that additive gene effects were predominant in the inheritance of resistance to SPVD and recovery. Narrow-sense heritability (31–41%) and broad-sense heritability (73–98%) were moderate to high, indicating that rapid genetic gains for SPVD resistance could be accomplished by mass selection breeding techniques. Two genotypes, New Kawogo and Sowola, had high negative GCA effects and had several families in specific crosses,which exhibited rapid recovery from SPVD,and are promising parents for enhancement of SPVD resistance and recovery. This revised version was published online in August 2006 with corrections to the Cover Date.     

Environmental factors influence the expression of resistance to sorghum midge, Stenodiplosis sorghicola

Host plant resistance is an effective means of controlling sorghum midge (Stenodiplosis sorghicola). We studied the influence of environmental factors on expression of resistance to sorghum midge in three midge-resistant and two midge-susceptible genotypes. Midge-resistant lines AF 28, ICSV 197, and TAM 2566 suffered 8.8 to 17.3% damage across seven so wings compared to 25.6%damage in ICSV 112, and 69.4% damage in CSH 5. Susceptibility of the midge-resistant lines (AF 28, ICSV 197, and TAM 2566) decreased with an increase in open pan evaporation, maximum and minimum temperatures, and solar radiation; while the midge-susceptible lines (ICSV 112 and CSH 5) showed a poor interaction with these factors. Midge damage in ICSV 197 showed a negative correlation with minimum temperature and relative humidity and positive correlation with sunshine hours,while the reverse was true for CSH 5. Grain growth rate between 0 and 3 days after anthesis was lower in crops sown on 1st October, when AF 28 and ICSV 197 suffered maximum midge damage. Maximum and minimum temperatures and maximum relative humidity influenced the moisture content of the grain, grain growth rate, and sorghum midge damage. There was considerable variation in genotype × environment interaction for expression of resistance to sorghum midge,and the implications of these results have been discussed in relation to development of sorghum cultivars with resistance to this insect. This revised version was published online in August 2006 with corrections to the Cover Date.     

Apomictic frequency in sorghum R473

Summary Apomixis has been reported in a few lines of sorghum, among them R473 which was originally reported to be an obligate apomict. Although this line has multiple embryo sacs, the frequency of apomictic seed formation has not been determined because a progeny test has not been possible. R473 does not cross as a female with other lines except when its own pollen is present. In the present study mutations were induced in R473 by hydrazine and irradiation. Crosses were made between male-sterile mutants as females and normal R473 as males. Plants of R473 produced F₁ hybrids sexually, thus indicating that they were not obligate apomicts. These F₁'s also reproduced sexually, as indicated by segregation for male sterility and male fertility in F₂ progenies.     

Diallel analysis of barley for resistance to leaf stripe and impact of the disease on genetic variability for yield components

Barley breeders in Syria attempting to develop barley (Hordeum vulgare L.) cultivars resistant to barley leaf stripe (BLS) disease caused by Pyrenophora graminea Ito & Kuribayashi [anamorph Drechslera graminea (Rabenh. Ex. Schlech. Shoem.)]. Information on the combining ability for BLS resistance in Syria is not available. This study was conducted to evaluate, in 10 genetically diverse barley parents, general combining ability (GCA) and specific combining ability (SCA) effects towards the determination of the genetic basis of disease resistance and to estimate genetic variability for yield components and its modification by BLS. Ten parental genotypes varying in their reactions to BLS were crossed in a half-diallel mating design to generate 45 full-sib families. The families and the parents were inoculated with P. graminea and evaluated for resistance in replicated field tests (three inoculated and three non-inoculated plots). The parents chosen showed wide variations for resistance to BLS. Genetic component analysis showed significant effects for both GCA and SCA for resistance to BLS, suggesting that additive as well as non-additive genetic mechanisms were involved in the expression of resistance in these parents. GCA effects were more important than SCA effects. Resistant parents exhibited high negative GCA indicating that additive gene effects were more predominant, and suggesting their prime suitability for use in barley breeding programs to improve resistance to BLS. Narrow-sense heritability was 58% and broad-sense heritability was 99% indicating that selection for BLS resistance should be effective in these crosses. A high genetic variability for the agronomic traits studied was observed. Yield components decreased significantly in inoculated plants and more pronounced in diseased plants. Significant GCA was observed for all traits. Values for GCA were, in some cases, significantly modified by BLS. This indicates that attention must be paid to the danger of drawing conclusion in quantitative genetics studies dealing with both diseased and healthy plants. Two genotypes, Banteng and Igri, had high negative GCA effects and are promising parents for enhancement of BLS resistance.     

Inheritance of resistance to spotted stem borer, Chilo partellus, in sorghum, Sorghum bicolor

The spotted stem borer, Chilo partellus, is one of the most important pests of sorghum, and host plant resistance is an important component for the management of this pest. Most of the sorghum hybrids currently under cultivation are based on cytoplasmic male-sterility (CMS). In order to develop a strategy for resistance to stem borer, we studied the traits associated with resistance, and their nature of gene action in F₁ hybrids derived from resistant, moderately resistant, and susceptible CMS and restorer lines. The hybrids based on stem borer-resistant, moderately resistant, or susceptible CMS and restorer lines were equally resistant or susceptible as the parents for leaf feeding [Damage rating (DR) 5.8 to 6.6 vs. 5.9 to 6.6], and had significant and decreasing trend in deadheart formation (resistant CMS × resistant restorer lines < moderately resistant CMS × moderately resistant restorer lines < susceptible CMS × susceptible restorer lines), respectively. Proportional contributions of restorer lines were greater than those of the CMS lines for leaf feeding, deadhearts, recovery and overall resistance, stalk length, nodes per plant, stem borer holes per plant, and peduncle tunneling. The general (GCA) and specific combining ability (SCA) estimates suggested that leaf feeding score, number of nodes, overall resistance score, panicle initiation, recovery score, and stalk length (dominance type of gene action) have been found to be associated with resistance to spotted stem borer, governed by additive type of gene action, their correlation and direct effects in the same direction, and explained 65.3% of the variation in deadhearts, and thus could be used as marker traits to select and breed for resistance to C. partellus in sorghum. The parents having significant SCA effects for two or more resistance traits for either or more parents have also been discussed for their use in the stem borer resistance breeding.     

Effect of cytoplasmic male-sterility in sorghum on host plant interaction with sorghum midge,Contarinia sorghicola

Summary Sorghum midge, Contarinia sorghicola Coq. (Diptera: Cecidomyiidae) is one of the most important pests of grain sorghum worldwide. We studied the reaction of midge-resistant and midge-susceptible genic-cytoplasmic male-sterile (A-lines) and their maintainers (B-lines), and the effect of resistant and susceptible restorers on sorghum midge. Midge damage and adult emergence were significantly lower on the B-lines of midge-resistant genotypes (PM 7061 and PM 7068) than their corresponding A-lines, while the reverse was true for the midge-susceptible genotypes (296A and ICSA 42). Differences in midge damage and the number of midges emerged were not significant between the midge-resistant and midge-susceptible A-lines when infested without pollination (except midge emergence on PM 7061A). Pollination with a midge-resistant restorer (DJ6541) reduced midge emergence significantly in one of two seasons. Source of pollen did not influence midge emergence on the highly-resistant A-line, PM 7061A. The implications of these observations in the development of midge-resistant hybrids were discussed.     

A test for obligate apomixis in grain sorghum R473

Summary Segregation patterns in progeny arrays of selfed plants, heterozygous for the Mdh 1 isozyme marker locus, were used in an attempt to confirm the presence of apomixis in the grain sorghum line R473. No evidence for obligate apomictic reproduction was obtained. However, our studies did not rule out the possibility of a low level of facultative apomixis in R473.     

Cytoplasmic male-sterility and source of pollen influence the expression of resistance to sorghum midge, Stenodiplosis sorghicola

Sorghum midge, Stenodiplosis (Contarinia) sorghicola (Coquillett), is an important pest of grain sorghum, and host plant resistance is an important aspect of control of this pest. This research investigated how cytoplasmic male-sterility and source of pollen influence the expression of resistance to sorghum midge. Sorghum midge emergence was significantly lower in panicles of midge-resistant and midge-susceptible cytoplasmic male-sterile lines when pollinated with AF 28 - a midge-resistant restorer line, than those pollinated with Swarna - a midge susceptible restorer line, indicating the presence of xenia effects. Maintainer lines (B-lines) of midge-resistant parents had significantly lower numbers of eggs and larvae than the B-lines of midge-susceptible parents. Male-sterile lines of the both midge-resistant and midge-susceptible lines were equally susceptible, indicating that resistance to sorghum midge is influenced by factors in the cytoplasm of the B-line. These findings will have an important bearing on the production of hybrids with resistance to insects. This revised version was published online in August 2006 with corrections to the Cover Date.     

Genetic relationship between some cytoplasmic male sterility systems in sorghum

Summary Genetic analysis of cytoplasmic male sterility systems in sorghum was undertaken by evaluating a set of 25 A×B crosses and another set of 171 A×R crosses. Male steriles included diverse sources reported from U.S.A. and India. Fertility restoration in crosses was assessed by studying pollen sterility under microscope and seed setting under selfed earheads. Male fertility restorers are identified for diverse cytoplasms. Accordingly the diverse cytoplasms are grouped and listed in the increasing order of their sterility as A₁ & CK 60A A₂ A₄ M31-2A & M35-1A (A₄ tentative) A₃ & VZM2A G₁A (A₅ tentative) and consequently fertility restoration also becomes difficult in the same order for utilization in the breeding programmes. Among the non-milo cytoplasms, A₂ among exotics and maldandi (M31-2A and M35-1A) among Indian sources, can be utilised for practical exploitation in breeding programmes.