Resistance of sunflower inbred lines to various forms of attack by Sclerotinia sclerotiorum and relations with some morphological characters

Summary Twenty sunflower inbred lines were studied for their reactions to 7 Sclerotinia sclerotiorum tests on different plant parts. A principal component analysis (P.C.A.) and t-tests on the means of resistant and susceptible groups indicated that reactions of inbred lines to infection of capitula by ascospores are independent from those to ascospore infections of terminal buds. They are even more contrasted with the results of any test measuring mycelial extension. However, for the last, there is a close association between the reactions of roots, leaves and capitula. Of the inbred lines, some showed good levels of resistance to most forms of attack, others were generally susceptible and there were some with good resistance to one particular form of attack. A P.C.A. of 18 morphological and field characters showed no general association between these characters and Sclerotinia sclerotiorum test results, although it was found that lines resistant to mycelial extension on capitula were generally earlier than those that were more susceptible. It is proposed that breeding programmes for general resistance to Sclerotinia sclerotiorum should include a combination of two or three tests.     

Optimal allocation of resources in evaluating current sunflower inbred lines for resistance to Sclerotinia

Sclerotinia sclerotiorum (Lib.) de Bary is a major limiting factor for sunflower (Helianthus annuus) production. This study was conducted to (1) evaluate a large number of current sunflower lines for their reaction to Sclerotinia, and (2) determine the optimum allocation of resources for their selection. A set of 85 inbred lines was screened under artificial leaf infection in three environments. The test revealed ample variation for all resistance traits in the current germplasm. A line selected from NDBLOS, an oilseed sunflower germplasm poo, and its derivatives were most resistant to Sclerotinia. Lines derived from interspecific crosses with the wild species Helianthus tuberosus and Helianthus argophyllus also showed satisfactory Sclerotinia resistance. The evaluation of five plants per plot in two replications and four environments appears to be a reasonable allocation of resources in order to optimize the selection response.     

The genetics of resistance in sunflower capitula to Sclerotinia sclerotiorum measured by mycelium infections combined with ascospore tests

To enhance efficiency of breeding programmes for Sclerotinia sclerotiorum resistance in sunflower capitula, two separate resistance tests have been recommended. However, the time necessary to develop genotypes with two types of resistance makes this impractical. A strategy to combine the two tests was consequently proposed to reduce the number of seasons per selection cycle, but genetic studies were necessary to determine if it could be applied in breeding programmes. This was the objective of this work. Data from two genetically different sunflower inbred lines and their F1, F2 and backcross generations were analysed in two years to determine the genetics of resistance to S. sclerotiorum in capitula measured by application on the same plants of a mycelium test combined with an ascospore test. Effects of maternal origin were detected when the reciprocal generations were evaluated for the mycelium test. This suggests the importance of choice of the inbred line used as female in population formation by hybridisation. Progeny subjected to the mycelium test did not show any change in relative reactions to two S. sclerotiorum isolates suggesting that ranking genotypes according to the results of combined S. sclerotiorum tests is repeatable across Sclerotinia isolates and experimental seasons. Moderate narrow sense heritability indicated that selection of the best F2 plants should be effective. Genetic gain from selection is possible because a reduction of lesion areas produced by the mycelium test can be expected. Additive gene effects contribute significantly to reduction in lesion area. Consequences of results in population improvement for S. sclerotiorum resistance in capitula are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Interspecific hybrids of sunflower as a source of Sclerotinia resistance

Sclerotinia sclerotiorum is considered the most devastating pathogen of sunflower grown in humid environments. In this study, progenies of partial hybrids between Helianthus maximiliani, a wild species that has been shown to be resistant to S. sclerotiorum, and H. annuus were characterized by amplified fragment length polymorphism (AFLP) analysis to identify whether there are introgressions from H. maximiliani into the cultivated sunflower at the molecular level. Wild species‐specific fragments as well as fragments not found in either parent were detected. Progenies tended to cluster together according to the original partial hybrids in the dendrogram by the use of bootstrap procedures. The progenies were studied for their reaction to S. sclerotiorum using artificial inoculation of sunflower heads. Some of the progenies showed a higher level of resistance compared with resistant inbred lines. It was possible to identify two AFLP‐fragments which seem to be linked to Sclerotinia resistance.     

Sclerotinia resistance in sunflower: I. Genotypic variations of hybrids in three environments of Argentina

Sclerotinia sclerotiorum infections on sunflower capitula produce white rot, one of the most dangerous diseases sunflower bears in all humid areas. Therefore, disease quantification on developed genetic materials is a very important action in sunflower breeding. Given the horizontal type of resistance these evaluations should be made in different environments. Several hybrids obtained after crossing seven female × seven male sunflower inbred lines in a factorial mating design were evaluated for resistance to white rot in two locations in the southeast of the province of Buenos Aires, Argentina. Disease incidence and the symptom appearance index indicated significant genotypic effects and genotype × environment (G × E) interactions. Genotypic effects were a greater source of variance than the G × E interaction effects. The G × E interactions only represented changes in magnitude. These results were used to identify the best genotypes for both resistance factors. Four hybrids showed low disease incidence values under both artificial and natural infections, of which only two had high values of the symptom appearance index at both locations. These hybrids are considered to have adequate resistance attributes for the ascosporic penetration and the mycelium extension in the capitulum tissue and could therefore be sown in the province of Buenos Aires without increasing risks of S. sclerotiorum attacks.     

Breeding for Sclerotinia resistance in sunflower: A modified screening test and assessment of genetic variation in current germplasm

Sclerotinia sclerotiorum (Lib.) de Bary is one of the most devastating pathogens of sunflower (Helianthus annuus), causing yield losses up to 100%. This study was conducted to (1) screen sunflower lines for their reaction to Sclerotinia using artificial infection by a modified leaf test, and (2) identify the most suitable trait for screening against Sclerotinia resistance. A set of 85 inbred lines was screened in three environments by infecting the leaves with mycelium and covering them with plastic bags containing water. Susceptibility was assessed by leaf lesion, petiole score, stem rating, stem lesion and diseased stems. The test proved to be effective and practicable, with an infection rate of up to 99%. All scoring traits were strongly correlated with each other and effective in differentiating among susceptible and resistant lines. Although petiole score had the highest heritability, its association with leaf length hampers its utilization as a scoring trait. Stem lesion was a good predictor of mid-stalk rot and can be recommended as a reliable trait for screening sunflower lines against Sclerotinia based on its high heritability estimate (h²= 0.59) and close correlations with other traits measured after artificial and natural infection.     

Sclerotinia resistance in sunflower. II. Combining ability and midparent heterosis for reaction to ascospore infections at flowering

In Argentina, susceptibility of the sunflower crop to Sclerotinia sclerotiorum capitulum attacks can cause significant yield losses. We reported earlier two hybrids among a series obtained from crosses between inbred lines following a factorial mating design that had a sufficient level of resistance to be adapted to the main Argentina sunflower growing area. Results concerning the per se behavior of the parental lines involved in these crosses and the combining ability and heterosis effects of such parental genotypes are presented in this article. The lines showed different responses to white rot incidence and relative incubation period. There were significant effects for general and specific combining ability and also for midparent heterosis, for the two variables evaluated. Preponderance of additive gene effects on genetic control white rot resistance level was shown. The development of sunflower inbred lines to be used as parents in the production of hybrids for areas requiring moderately resistant hybrids is an important phase in breeding programs. To optimize the use of resources and time, our results suggest that testing general combining ability at early generations of selfing followed by testing of hybrids may be considered as an adequate strategy in breeding for white rot resistance.     

Resistance in the leaf and stem of sunflower after infection with two isolates of Phomopsis

Phomopsis helianthi (sexual stage Diaporthe helianthi) is a major disease in most sunflower growing countries. The objectives of this study were (1) to investigate whether resistance to Phomopsis differs between the leaf and stem tissue and (2) to examine the genetic variation for resistance to Phomopsis in sunflower inbreds. A set of 16 inbred lines was tested using artificial leaf infection with mycelial explants. The lines were screened for their reaction on leaf and stem in four environments to a French Phomopsis isolate and in two environments to a Yugoslavian isolate. Significant genetic variance and mostly high genetic ratios (analogous to heritability) were found for all resistance traits. While fungal growth reflected resistance to Phomopsis in the leaf, petiole score was associated with leaf length. Stem lesion proved to be the most suitable trait for evaluating resistance in the petiole-stem passage. No correlation existed between resistance in the leaf and stem, suggesting that both resistance factors are inherited independently. Both isolates resulted in similar resistance rankings of the inbreds and no difference in their aggressiveness was detected with regard to leaf resistance. Screening for Phomopsis consisting of several steps is recommended, beginning with petiole score or fungal growth to select the most susceptible plants in the early testing, and measuring stem lesion in further generations of breeding.     

The inheritance and biochemistry of resistance to Sclerotinia sclerotiorum leaf infections in sunflower (Helianthus annuus L.)

Summary Resistance of sunflower leaves to attack by Sclerotinia sclerotiorum was studied by infecting them, in the field, with agar disks containing Sclerotinia mycelium. Resistance levels were determined by the length of lesions after a given period. There were significant differences in reaction between both sunflower hybrids and inbred lines. Different Sclerotinia isolates gave the same classification of sunflower genotypes. The results of trials repeated in one year or different years were significantly correlated. The general combining ability variance/specific combining ability variance ratio was 1.35. Strict sense heritability was 0.61±0.03. The midparent-offspring correlation coefficient was significant, but the relation between per se values of the male parents used with the values of their hybrids was much closer than that for the female parents. Mean heterosis for resistance was 28.9%, compared with the midparent. The results of the leaf resistance test are frequently correlated with levels of resistance to root attack by Sclerotinia. Possible use of this test in breeding both directly for leaf resistance and indirectly for root resistance are discussed. H.P.L. Chromatography studies of the phenols present in healthy and infected leaves distinguished 19 compounds, all of the inhibitin type. There was a large increase in phenol content in leaves infected by Sclerotinia, for all genotypes. However, more especially in uninfected leaves, the contents of 3 chromatogram peaks, numbered 4, 6 and 9, showed a close relation with levels of Sclerotinia resistance. It is proposed that these compounds could be used as markers of certain types of resistance.     

Gamete selection for improving physiological resistance to white mold in common bean

White mold (WM), caused by Sclerotinia sclerotiorum (Lib.) de Bary, is a widespread disease of dry and green bean (Phaseolus vulgaris L.) in North America. Gamete selection (GS) was effective to combine and pyramide resistant genes and quantitative trait loci (QTL) for common bacterial blight. Our objective was to determine the effectiveness of GS to introgress physiological resistance to white mold. Two inter-gene-pool double-cross populations were developed. Selection for WM resistance was practiced from F₁ to F₄. Thirteen selected F_1:5 breeding lines of each population and their four parents were evaluated. Two separate inoculations were made on each plant 1 week apart using a cut-stem method. The WM reaction was scored at 16, 23, and 33 days post inoculation (DPI) using a scale from 1 (no disease) to 9 (severely diseased or dead). In F₁, 52% of Pop I (USPT-WM-1/CORN 601//USPT-CBB-1/92BG-7) and 67% of Pop II (Chase/I9365-25//ABL 15/A 195) susceptible plants were discarded. In F_4, only 1.2% of families from Pop I, and 0.9% for Pop II, survived the selection process. An average of 20.5% gain in WM resistance was obtained for both populations in F₄. Four breeding lines of Pop I had significantly (P = 0.05) lower WM score (4.1–4.6) and four were equal (4.7–4.9) to the best WM-resistant parent 92BG-7 (4.9), while ten breeding lines of Pop II were equal (4.5–4.8) to the best WM-resistant parent A 195 (4.6). Thus, GS was effective for improving WM resistance in common bean.     

Comparative genotype reactions to Sclerotinia sclerotiorum within breeding populations of Brassica napus and B. juncea from India and China

Twenty Brassica breeding populations derived from mass selection or inter-specific hybridization were field screened for resistance to three separate isolates of Sclerotinia sclerotiorum, the cause of Sclerotinia stem rot (SSR). Variation due to S. sclerotiorum isolates (P ≤ 0.001) and host populations (P ≤ 0.001) were highly significant. Populations × isolate interactions were also significant. S. sclerotiorum isolates, MBRS1 and MBRS5 were the most pathogenic and almost similar in terms of population reactions, with WW3 clearly being distinct and having a much smaller range in lesion length across the populations. There were wide ranging and variable responses in terms of resistance against S. sclerotiorum in Brassica napus and B. juncea, with or without B. carinata introgression, among these breeding populations. In B napus, ZY006 (resistant check) and Line6 (HZAU) were the most resistant, closely followed by Line1 (HZAU), OCRI-3 and Line5 (HZAU). Line6 (HZAU) showed excellent resistance against the highly virulent isolates MBRS1 and MBRS5; while OCRI-1 appeared most resistant against isolate WW3. The B. juncea × B. carinata hybrid JC134 (PAU) was the most resistant against isolate MBRS5 and B. juncea RH9902 × JN026 the most resistant against isolate MBRS1. B. napus lines Line2 (HZAU), Line4 (HZAU), OCRI-3; and OCRI-4, and the B. napus × B. carinata hybrid Surpass4000 NCB4 (PAU), showed a significant degree of isolate-dependency in their reactions. In contrast, some other genotypes such as B. napus lines Line1 (HZAU), OCRI-5; Ding 110× Oscar and, particularly, Line5 (HZAU), were largely isolate-independent, making them ideal sources of resistance to target and exploit in developing new commercial cultivars with more effective resistance to SSR across multiple pathotypes of this pathogen. Cluster analysis allowed categorization of the test populations into five groups, based on their resistant responses. B. napus ZY006 was the sole genotype in the most resistant group. B. napus lines Line6 (HZAU), Ding 110 × Oscar (HAU) and Line4 (HZAU) clustered in another genetically distinct resistant group. That lines could be grouped into those with similar responses across the three different isolates of S. sclerotiorum will save breeders much time and expense by eliminating duplication of breeding efforts that occurs from using genotypes that are essentially similar in terms of host resistance against this serious pathogen. Further, that populations of similar levels of resistance but narrow variation in the resistance range could be identified is significant, as these are most likely to reliably provide breeders with advanced populations that not only consistently display the level of resistance expected but also reflect genetic diversity of resistance sources needed to successfully develop new more-resistant commercial varieties.     

Accumulation of soluble phenolic compounds in sunflower capitula correlates with resistance to Sclerotinia sclerotiorum

Disease symptoms and total soluble phenolics content have been analysed in four sunflower (Helianthus annuus L.)lines with different resistance levels(from highly susceptible to resistant) to head rot caused by Sclerotinia sclerotiorum (Lib.) de Bary. At the beginning of the flowering stage, capitula were inoculated by spraying with a water suspension of ascospores, and disease symptoms were evaluated from day 6 to day14 after inoculation. The most susceptible genotypes showed all their ovaries to be necrosed and abundant lesions in corollas, bracts and receptacle. In the resistant line, the ovary and corolla were only partially necrosed with no symptoms in the bracts or the receptacle. Total soluble phenolics were extracted and quantified from different parts of the capitulum in both inoculated and non-inoculated plants. The amount of phenolic compounds depended on the sunflower line, the time after inoculation, and the tissue. Higher constitutive and induced phenolic content as well as phenylalanine ammonia-lyase activity were present in the most resistant line, these differences correlated with the absence/presence of disease symptoms. This revised version was published online in July 2006 with corrections to the Cover Date.     

Mapping of QTLs detected in a <Emphasis Type="Italic">Brassica napus</Emphasis> DH population for resistance to <Emphasis Type="Italic">Sclerotinia sclerotiorum</Emphasis> in multiple environments

Sclerotinia stem rot, caused by the fungus Sclerotinia sclerotiorum, is one of the most devastating diseases of rapeseed (Brassica napus L.) in China. The two major factors limiting the development of disease resistance are (1) the absence of accessions with complete resistance and (2) the lack of a single method that can be widely applied to assess tolerance—even though accessions with differential tolerance to S. sclerotiorum have been identified in China. In the study reported here, we have used one doubled haploid (DH) population consisting of 72 lines, which was derived from the F₁ generation of a cross between a partially resistant line (DH821) and a susceptible line (DHBao604), to identify quantitative trait loci (QTLs) involved in the resistance to S. sclerotiorum. Three inoculation methods, namely, mycelial toothpick inoculation (MTI), mycelial plug inoculation (MPI), and infected petal inoculation (IPI), were used to assess resistance at the adult plant stage. A genetic linkage map with 20 linkage groups covering 1746.5 cM, with an average space of 6.93 cM, was constructed using a total of 252 molecular markers, including 91 simple sequence repeats, 72 randomly amplified polymorphic DNA, 86 sequence-related amplified polymorphisms, two restriction fragment length polymorphisms, and one expressed sequence tag. Composite interval mapping identified ten, one and ten QTLs using MTI, MPI and IPI methods, respectively, at a LOD > 2.5. One QTL was detected in linkage group N12 by MTI in 2004 and 2005 and by IPI in 2005. Another QTL was detected in linkage group N3 and N4 by MPI in 2006 and 2007. There was one common QTL detected by MTI in 2005 and by MPI in 2006. These results provide information on the genetic control of resistance to S. sclerotiorum in oilseed rape.     

Identification of resistant sources against <Emphasis Type="Italic">Sclerotinia sclerotiorum</Emphasis> in <Emphasis Type="Italic">Brassica</Emphasis> species with emphasis on <Emphasis Type="Italic">B. oleracea</Emphasis>

Stem rot caused by Sclerotinia sclerotiorum is one of the most devastating diseases of rapeseed (Brassica napus L.) which causes huge loss in rapeseed production. Genetic sources with high level of resistance has not been found in rapeseed. In this study, 68 accessions in six Brassica species, including 47 accessions of B. oleracea, were evaluated for leaf and stem resistance to S. sclerotiorum. Large variation of resistance was found in Brassica, with maximum differences of 5- and 57-folds in leaf and stem resistance respectively. B. oleracea, especially its wild types such as B. rupestris, B. incana, B. insularis, and B. villosa showed high level of resistance. Our data suggest that wild types of B. oleracea possess tremendous potential for improving S. sclerotiorum resistance of rapeseed.     

甘蓝型油菜抗菌核病研究进展

油菜是主要油料作物之一,由核盘菌(Sclerotiniasclerotiorum(Lib.)deBary)引起的菌核病(sclerotiniastemrot)是油菜的主要病害。针对当前与菌核病相关研究的新进展,本研究从4个方面对其进行了概括：（1）核盘菌的侵染方式以及在侵染过程中核盘菌分泌的草酸与寄主中钙离子的动态关系;（2）油菜通过合成植保素、酚类化合物、木质素、几丁质酶和β-1,3-葡聚糖酶等来抵抗核盘菌入侵的抗病机理;（3）现有综合防治技术中抗性种质筛选、无花瓣育种和生物防治;（4）利用与草酸代谢相关基因、抗病相关基因、防御相关转录因子基因和抗菌肽基因开展的油菜基因工程研究成果。并且进一步提出了未来油菜抗菌核病研究的可能方向,这些总结与建议为今后油菜抗菌核病研究提供了有益参考。     

Cluster analysis of RFLP data from related maize inbred lines of the BSSS and LSC heterotic groups and comparison with pedigree data

Summary In this study, 31 elite inbred lines of maize (Zea mays L.) were analyzed with 149 clone-enzyme combinations for their respective RFLP profiles. Objectives were (1) to determine the utility of RFLPs for estimation of genetic similarties among 16 inbred lines from the Iowa Stiff Stalk Synthetic (BSSS) and among 15 inbred lines from the Lancaster Sure Crop (LSC) heterotic groups and (2) to compare genetic similarities based on molecular markers with those based on pedigree information. Coefficients of genetic similarity (GS) and coancestry (f) between pairs of lines from the same heterotic group were calculated from RFLP and pedigree data, respectively. For lines from the BSSS heterotic group, cluster analyses based on RFLP and pedigree data revealed similar associations. GS and f values were closely correlated (r=0.70) for related BSSS lines. For lines from the LSC heterotic group, considerable discrepancies existed between the GS and f values, especially for those pairs involving inbreds Va22 and Lo924. Effect of selection and/or erroneous pedigree records are discussed as possible explanations for the low correlation of GS and f values (r=0.07) for related LSC lines. RFLPs seem useful for investigation of relationships among maize inbreds, verification of pedigree records, and quantification of the degree of relatedness.     

Inheritance of stalk rot resistance in cauliflower (Brassica oleracea var. Botrytis L.)

Summary The inheritance of resistance in cauliflower to stalk rot (Sclerotinia sclerotiorum (Lib.) de Bary) was investigated in population from six generations of six crosses. Disease incidence was recorded on 4 parents, 6 F^s ₁, 6 F^s ₂ and 12 back-crosses in a screenhouse under artificially created epiphytotic conditions. Resistance to stalk rot in this set of parents was found to be polygenic and under the control of recessive genes and due primarily to additive gene action. A breeding strategy emphasizing recurrent selection should lead to improvement in resistance.     

Genetic control of kernel modification found in South African quality protein maize inbred lines

Summary The genetic control of endosperm modification in 12 opaque-2 maize (Zea mays L.) inbred lines was investigated by means of a diallel cross experiment conducted across two environments. Kernel vitreousness and kernel hardness were determined by partially dominant genes. Additive gene action was largely responsible for kernel modification. A favourable general combining ability for kernel vitreousness and kernel hardness was positively correlated with an accumulation of dominant kernel modifying genes. South African sources of endosperm modifiers have been found to be similar to those used in other quality protein maize breeding programmes. Certain inbred lines displayed sufficient genetic potential for use in a quality protein maize hybrid breeding programme.     

Development of drought‐tolerant breeding lines derived from Helianthus annuus × H. argophyllus interspecific crosses

Helianthus argophyllus is a wild species known as “silver sunflower”, which possesses several traits, including morphological traits that increase drought tolerance. Therefore, introgressions between chosen cultivated lines and two H. argophyllus accessions were made, and segregating generations were established. Important agronomic traits including single heading, high pollen fertility, silver canopy (indicating more cuticular wax), and content of cuticular waxes and oil were selected over six segregating generations. The resulting F₆ lines showed introgression of water saving traits, as they had lower excise leaf water loss, with comparable yield to standard checks. However, these F₆ lines were late maturing and showed poor flowering synchronization between the cultivated and introgressed lines. Introgressed line “D‐22” was particularly promising as a breeding line, with superior agronomic and drought resistance traits. This line had the potential to be used as an inbred parental line for introgression of drought resistance traits into elite sunflower germplasm. Combining ability analysis of the introgressed lines further showed their potential for heterosis breeding or to be used as parental lines in breeding programme.     

Genetic control of resistance to the aphid Brevicoryne brassicae in the wild species Brassica fruticulosa

Initial studies have shown variable resistance in Brassica fruticulosa to the aphid Brevicoryne brassicae The aim of this work was to fix high levels of resistance to B. brassicae in true breeding lines of B. fruticulosa and obtain data on the genetic control of resistance. Plants from initially variable B. fruticulosa accessions were selfed to produce inbred resistant and susceptible lines that were studied in three separate experiments to determine the extent to which resistance to B. brassicae had been fixed. Results from three experiments using successive generations of resistant and susceptible inbred lines showed that continued selection resulted in resistant inbred lines that supported an average of three aphids per plant compared with an average of 96 aphids per plant for susceptible inbred lines. Data collected from an experiment determining the resistant phenotype of lines including the selfed progenies and the F₁ and F₂ progeny of a cross between resistant and susceptible individual plants indicated that the resistance was not controlled by a single gene.