Evaluation of a lucerne germplasm collection in relation to traits conferring grazing tolerance

Summary A collection of 90 accessions of different germplasm groups from the Medicago sativa complex was evaluated at Lodi, Italy, for dry-matter yield, time to anthesis, height of regrowth, plant diameter, stem density, vegetative recovery after winter, and presence of deep-set crown and ability to spread with creeping roots or rhizomes, these latter characters possibly contributing to grazing tolerance. The germplasm groups proved rather distinct for these traits, but all included types potentially suitable for grazing. Interesting types combining deep-set crown and high yield occurred mainly in subsp. x varia and materials originated from artificial crosses involving at various extent the subspecies falcata, sativa and x varia. Though belonging to subsp. sativa, mielga wild populations from Spain had aerial traits similar to those of the long-dormant, low-yielding subsp. falcata. Deep-crowned plants had greater plant diameter than shallow-crowned plants, and could be categorised into four distinct top-growth models according to their habit and vigour. One of these models combined spreading ability with high yield. The about 150 plants selected for their combination of deep crown, wide diameter, and good yield were characterised by a branch-rooted system. There was general correspondence between top-growth and root morphology of each model. Of the four deep-crowned models, three had rhizomes, with wide range of underground development and morphology, while the fourth had creeping roots. The latter model was the most rare among the examined accessions, never appearing in either cultivated or wild subsp. sativa. The implications of the results on the development of grazing-tolerant lucerne varieties are discussed.     

Histological components of <Emphasis Type="Italic">Trifolium</Emphasis> L. species related to digestive quality of forage

The microscopic analysis of leaflet, petiole, stem and peduncle of 26 populations of 17 Trifolium species was carried out in order to examine their structural characteristics related to digestibility, to compare them between the populations of forage species and their wild relatives and to evaluate, from histological aspect, the potential of wild-growing species to be used as forages. Forage species had low proportion of the tissues composed of thick-walled cells and high proportion of the tissues composed of thin-walled cells in all examined plant parts. The reduction of the proportion of lignified xylem, sclerenchyma and sclerenchymatous interfascicular parenchyma, were suggested as basic selection criteria in breeding Trifolium species for improved digestibility. T. angulatum, T. medium and T. pannonicum proved to be very similar to the forage species in their histological composition, proportion of thick-walled and thin-walled cells and type of variation of the examined parameters. We believe that these species have some potential for use as forages in their own right, but aditional biochemical, physiological and morphological investigations should be conducted.     

Traditional seed maintenance and origins of the French lucerne landraces

Summary As for other forage crops the notion of variety is quite recent in lucerne (Medicago sativa L.). Up to 1950 in France, farmers cultivated landraces whose origins were geographically defined. Seed production was a by-product of forage production. Usually, seeds were harvested on the second or third crop, in old lucerne fields. Natural selection created landraces adapted to local conditions. In some years, climatic conditions, especially in northern regions, were unfavourable to seed production. In the exchanges and trades of seeds between French regions, the genetic origin of the seeds was omitted, the geographical origin was only mentioned. Even if France usually exported lucerne seeds, imports occurred in bad years. Seeds were imported from European countries and from North and South America. Varieties from America were poorly adapted to the French conditions for forage production. These foreign varieties have probably intercrossed with the local landraces but no data is available to know to what extent. Among the various French landraces, five main types were defined using morphological characters: Flamande in the north, three types (Poitou, Marais de Luçon, Marais de Challans) in the west, and Provence in the south. These landraces have been widely used in breeding since 1950. Even though these five landraces are, up to now, maintained, the other landraces have probably been lost (no more cultivated and not in genebanks), replaced by registered varieties.     

The location and characteristics of novel sources of resistance to Bremia Lactucae Regel (downy mildew) in wild Lactuca L. Species

Summary Twenty-one accessions of 3 wild Lactuca species which could be hybridised with L. sativa, the cultivated lettuce, were inoculated at different stages of plant development with 3 multivirulent isolates of Bremia lactucae. Nineteen sources of resistance to B. lactucae, not attributable to the previously recognised resistance factors 1–11 were identified. Two lines of L. serriola showed similar resistance patterns as lines carrying R11. The resistance of some accession was incomplete particularly at the seedling stage and this phenomenon may be race specific.Tests on segregating F₂ populations of crosses between 2 different L. serriola accessions and L. sativa cultivars showed that the resistance in one line (LSE/18) appears to be inherited as a single dominant gene, which is sometimes incomplete in expression and allelic to either Dm6 or R7. The segregation patterns for resistance in PI 281876 did not give readily interpretable ratios.To assess the frequency of occurrence in B. lactucae populations of virulence factors to overcome this novel resistance, 11 of the novel sources of resistance were inoculated with numerous collections of the pathogen from the UK, Czechoslovakia and elsewhere and found to show a high level of resistance.     

Resistance to Verticillium albo-atrum in lucerne (Medicago sativa L.) to distinguish between varieties

France is responsible for DUS (Distinction–Uniformity–Stability) testing of new varieties of lucerne (Medicago sativa L.) for several European countries. Many varieties fail to be registered because of a lack of distinctness from the already registered varieties. The addition of the other traits such as disease or pest resistance could help in distinction. The aim of the present study was to optimize measurement of the resistance of lucerne to Verticillium albo-atrum. Although the resistance to V. albo-atrum is a qualitative trait for each plant, varietal resistance is a continuous variable because each variety contains a variable percentage of resistant plants. Fifty cultivars were tested for V. albo-atrum in two laboratories at random from among five private and public laboratories. Five check varieties were tested in the five laboratories in two tests. A score of resistance, corrected from the effect of the test, was given for each cultivar. A method to validate any new test is proposed. This data set can now be incremented by the resistance scores to V. albo-atrum of new varieties. It will help in distinguishing between lucerne varieties.     

Discovery and characterization of a molecular marker for <Emphasis Type="Italic">Sclerotinia minor</Emphasis> (Jagger) resistance in peanut

The production of cultivated peanut, an important agronomic crop throughout the United States and the world, is consistently threatened by various diseases and pests. Sclerotinia minor Jagger (S. minor), the causal agent of Sclerotinia blight, is a major threat to peanut production in the Southwestern US, Virginia and North Carolina. Although information on the variability of morphological traits associated with Sclerotinia blight resistance is plentiful, no molecular markers associated with resistance have been reported. The identification of markers would greatly assist peanut geneticists in selecting genotypes to be used in breeding programs. The main objective of this work was to use simple sequence repeat (SSR) primers previously reported for peanut to identify a molecular marker associated with resistance to S. minor. Out of 16 primer pairs used to examine peanut genomic DNA from 39 different genotypes, one pair produced bands at approximately 145 and 100 bp, consistent with either S. minor resistance or susceptibility, respectively. Cloning and sequencing of these bands revealed the region is well conserved among all genotypes tested with the exception of the length of the SSR region, which varies with disease resistance levels. This is the first report of a molecular marker associated with resistance to Sclerotinia blight in peanut. The identification of this marker and development of a PCR-based screening method will prove to be extremely useful to peanut breeders in screening germplasm collections and segregating populations as well as in pyramiding S. minor resistance with other desirable traits into superior peanut lines.     

Possibilities of family selection in a cultivar of lucerne,Medicago sativa L., for resistance to the stem nematode,Ditylenchus dipsaci (Kühn) Filipjev

Summary Half-sib families representing the lucerne cultivar Resis were tested for resistance to a stem nematode population, recovered from lucerne. The percentage resistance of the HS-families varied from 13 to 88%. The mean resistance for the cultivar was approximately 50%. Half-sib family selection would be more efficient than recurrent mass selection. The observed data suggest the action of a few genes with relatively large effects.     

Nematode resistance in bananas: screening results on some wild and cultivated accessions of Musa spp.

Bananas cultivated for export all belong to Cavendish cultivars and are all recognized as very susceptible to nematodes, particularly to the burrowing nematode Radopholus similis and the lesion nematode Pratylenchus coffeae. Even if there have been many changes in the management of banana nematodes in large commercial banana plantations, chemical control still remains most often the last resort method to manage the nematodes, although the number of registered products is definitely declining. Therefore, nematode control though genetic improvement is gaining new interest worldwide. In this study, 55 banana accessions mostly diploids from the Musa acuminata genome group (AA) but including some triploid accessions (AAA), some diploids of the Musa balbisiana genome group (BB) and some interspecific hybrids (AAB, AB) were evaluated for resistance to four nematode species R. similis, P. coffeae, Meloidogyne incognita and M. arenaria. These experiments were conducted in a growth chamber under controlled conditions. All banana accessions were susceptible to nematode species, although many different levels of susceptibility were detected. This study confirmed the good resistance status to R. similis of some cultivars from the Pisang jari buaya and Pisang batuau subgroups and the partial resistance of 17 diploid accessions significantly different from the susceptible reference cv. Grande Naine. This study also showed that 12 diploid accessions exhibited a partial resistance to P. coffeae, including some usual or potential genitors belonging to the wild diploids subspecies burmannica (cvs. Long Tavoy 1 and 2) and burmannicoides (cv. Calcutta 4). No source of resistance to Meloidogyne spp. was found. These screening results, combining for the first time four nematode species, are discussed within the scope of banana breeding in order to produce parental diploid lines with single or combined nematode resistances and further develop triploids that can substitute existing susceptible commercial cultivars.     

Interfering with regular meiotic behaviour in Avena sativa as a method of incorporating the gene for mildew resistance from A. barbata

Summary The regular meiotic behaviour of the cultivated oat Avena sativa (2n=6x=42) is genetically controlled. The factors which control the diploid-like meiotic behaviour also restrict the amount of pairing that occurs between alien chromosomes and their homoeologues in A.sativa, and hence increases the difficulties of introducing desirable variation from wild species into the cultivated oat. A genotype of the diploid species A.longiglumis which interferes with the regular meiotic behaviour of A. sativa can be used to induce pairing between alien chromosomes and their corresponding chromosomes in A. sativa. Using this procedure the dominant gene conferring mildew resistance has been transferred from the tetraploid weed species A. barbata into the cultivated oat.     

The yield stability of CIMMYT'S maize germplasm

Summary Relationships between whole-plant and germination NaCl tolerance and two in vitro measures of NaCl tolerance were evaluated in lucerne (Medicago sativa L.). No significant correlations were observed between relative NaCl tolerance of non-regenerating callus cultures, herbage regrowth, seed or pollen germination, and pollen tube growth. Simulated selection using indirect measures of NaCl tolerance produced small expected gains in relative NaCl tolerance during herbage regrowth and seed germination. These gains were less than 30% of those expected with direct selection for regrowth or germination tolerance. Selection based on the apparent NaCl tolerance of callus tissue or male gamephytes would appear unproductive. Direct selection for NaCl tolerance both at germination and during mature plant regrowth would appear to be necessary to rapidly develop lucerne populations with field-relevant levels of tolerance to NaCl stress.Approved by the Arizona Agric. Expt. Stn. as Journal Series no. 5015.     

Inheritance of resistance in soybean PI 567516C to LY1 nematode population infecting cv. Hartwig

Worldwide, cyst nematode (SCN) Heterodera glycines is the most destructive pathogen on cultivated soybean (Glycine max (L.) Merr.). In the USA yield losses in 2001 were estimated to be nearly 60 million dollars. Crop losses are primarily reduced by the use of resistant cultivars. Nematode populations are variable and have adapted to reproduce on resistant cultivars overtime because resistance primarily traces to two soybean accessions. Recently cv. Hartwig was released which has comprehensive resistance to most SCN populations. A virulent nematode population LY1 was recently selected for its reproduction on Hartwig. LY1 population originated from a mass mating of Race 2 (HG Type 1.2.5-) females with Race 5 (HG Type 1.2-) males. LY1 nematode population infects currently known sources of resistance except PI 567516C. The female indices obtained on PI 567516C and Hartwig were 7% (resistant) and 155% (susceptible), respectively. However, the genetic basis of LY1 resistance in soybean PI 567516C is not known. Resistant PI line 567516C was crossed to susceptible cultivar Hartwig to generate 105 F_2:5 families. These families together with parents, seven indicator lines and a susceptible control cv. Lee-74 were evaluated for response to LY1 nematode population in the greenhouse. Chi-square analysis showed resistance in PI567516C to LY1 was conditioned by one dominant and two recessive genes (Rhg, rhg, rhg). Chi-square value was 0.15 and P = 0.70. This information will be useful to soybean researchers for developing resistant cultivars to nematode population that infects Hartwig.     

Resistance of wild emmer wheat to stem rust: Ecological,pathological and allozyme associations

Summary Seedling resistance to wheat stem rust was determined in populations of wild emmer wheat, Triticum dicoccoides, and characterized by means of ecological factors and allozyme genotypes. Reactions to wheat stem rust were studied in 102 single plant accessions of T. dicoccoides from ten populations by inoculation with Puccinia graminis tritici race 14, isolate GSR-739. Six populations displayed different degrees of response polymorphism with reactions ranging from high resistance to complete susceptibility, whereas four populations contained only susceptible plants. In some of the accession, unexplained intrasib variation in resistance and intraplant variation of infection-types were found. Resistance to stem rust was negatively correlated with two ecological factors, altitude and number of Sharav (hot-dry) days which are unfavorable to disease development. Variation in stem rust response was shown to exist in ecogeographic regions where climatic variables enhanced the development of the fungus, conceivably maintained by natural selection. Likewise, allozyme genotypes, single or in multiple loci combinations, appeared to be associated with resistance or susceptibility to rust. Such association need to be verified by genetic studies in order to become established as useful markers.     

The potential for breeding an improved lucerne-rhizobium symbiosis. 1. Assessment of genetic variation

Summary Six cultivars of lucerne (Medicago sativa L.) were grown in all possible combinations with eight strains of Rhizobium meliloti in order to assess genetic variation in symbiotic nitrogen fixation. Host genotype and Rhizobium genotype effects on nitrogen fixation were respectively 4.8% and 21.0% of total phenotypic variance. Genetic variation due to host cultivar × Rhizobium strain interactions accounted for a further 6.0% of phenotypic variance. The results indicated low heritability of general symbiotic effectiveness in the host with interaction effects being large enough to suggest that plant performance may be unpredictable with populations of R. meliloti in field soils. Joint regression analysis showed that about 50% of the interactive variation could be explained by generalized differences in the sensitivity of cultivars to alterations in the genotype of Rhizobium strains. One cultivar, Siriver, was relatively insentitive to changes in the Rhizobium genotype whilst still maintaining high average yield. The implications of the results for lucerne breeding are discussed.     

Resistance of lettuce (Lactuca) to the leaf aphid Nasonovia ribis nigri. 1. Transfer of resistance from L. virosa to L. sativa by interspecific crosses and selection of resistant breeding lines

Summary Interspecific crosses were carried out between the three Lactuca species L. virosa, L. serriola and L. sativa to transfer resistance to the leaf aphid Nasonovia ribis nigri from L. virosa to the cultivated lettuce. L. sativa, L. serriola was used as an intermediate parent between the other two species. Many irregularities were observed in the interspecific hybrids, ranging from premature dying of F₁ plants resulting from a kind of bastard necrosis till complete male and female sterility and deviating microsporogenesis. Using in vitro culture and after several backcrosses male and female fertile plants were obtained with a L. sativa habit and with resistance to the leaf aphid.     

Resistance of lettuce (Lactuca) to the leaf aphid Nasonovia ribis nigri. 2. Inheritance of the resistance

Summary The inheritance of resistance to Nasonovia ribis nigri in L. sativa was investigated. Parents and F₁ and F₂ populations from crosses between the susceptible cultivar Ravel and two resistant breeding lines were tested. In both breeding lines one dominant gene appeared responsible for resistance.     

Resistance to Rhopalosiphum padi (Homoptera: Aphididae) in Hordeum vulgare subsp. spontaneum and in hybrids with H. vulgare subsp. vulgare

Summary Accessions of Hordeum vulgare subsp. spontaneum, the progenitor of cultivated barley, were screened in field and glasshouse trials for resistance to the aphid Rhopalosiphum padi. A few selected lines were furthermore hybridized with a modern barley variety and the resulting populations evaluated. High levels of resistance were found among some of the spontaneum accessions resulting in lower aphid growth rates (maximum reduction 57%). Segregation patterns among siblings in F2 populations were continuous, indicating the presence of several genes with possibly additive effects. The usefulness of H. vulgare subsp. spontaneum for breeding aphid resistant barley is discussed.     

The introgression of characters of the wild oat Avena magna (2n=4x=28) into the cultivated oat A. sativa (2n=6x=42)

Summary A study of derivatives from the hybrid Avena sativa × A. magna backcrossed to A. sativa clearly indicate that it is possible to introduce characters of the wild oat into the cultivated oat. Progeny with improved grain characters and crude protein content of the groat have been isolated and their use as breeding material is discussed.     

The importance of wild potato species resistant to the potato cyst nematode,Globodera pallida,pathotypes P4A and P5A,in potato breeding. I. Resistance studies

Summary Seven wild diploid potato species, Series Tuberosa, representing 1023 clones were screened for resistance to the potato cyst nematode, Globodera pallida. Over 25% of the clones were resistant to pathotype P₄A and almost 30% were resistant to pathotype P₅A. The resistance in hybrid progenies of these and other resistant species with cultivated potatoes was evaluated, and over 2200 seedlings were screened. High frequencies of resistance (>50%) to P₄A were found in progenies with Solanum leptophyes, S. vernei, S. gourlayi and S. capsicibaccatum, whereas resistance to P₅A was found in these species as well as S. sparsipilum. The importance of nematode resistant wild species for potato breeding is discussed.     

Genotypical differences in nitrate content inLactuca sativa L. and related species and correlation with dry matter content

Summary Nitrate content was measured in 135 genotypes of cultivated lettuce (Lactuca sativa L.) and 21 genotypes of wildLactuca spp. grown in two experiments. In Experiment 1 plants were grown on nutrient film. In Experiment 2 plants were grown in large pots of potting soil. Within cultivated lettuce five plant types were distinguished and within each of them genotypes were found with low nitrate content. The coefficient of variation in the experiment with plants growing in large pots was not larger than in the experiment with plants growing on nutrient film. In butterhead genotypes nitrate content was negatively correlated with dry matter content and positively with plant fresh weight.     

Stability of maize resistance to the ear rots caused by Fusarium graminearum and F. verticillioides in Argentinian and Canadian environments

Sources of resistance to Fusarium spp. are needed to develop maize hybrids resistant to the accumulation of fungal mycotoxins in the grain. In a search for resistant germplasm in 1999 and 2000, a set of Argentinian maize populations was evaluated in Ottawa, Canada, for resistance to ear rots after inoculation with local isolates of Fusarium verticillioides and F. graminearum. Sixteen of these populations, varying in observed resistance levels, were re-evaluated in 2003 and 2004 in Pergamino, Argentina, using local isolates of the same fungi. Conidial suspensions of each fungal species were inoculated into the silk channel of primary ears. Disease severity was assessed after physiological maturity using a scale based on the percentage of visibly infected kernels. Genotype effect was more important than genotype-by-fungal species or genotype-by-fungal species-by-environment interaction effects. In addition, disease severity levels associated with each fungal species were positively correlated (P < 0.05) (r = 0.90, r = 0.81, r = 0.87 and r = 0.53, in Ottawa 1999 and 2000, and Pergamino 2003 and 2004, respectively). Populations ARZM 01107, ARZM 07138, ARZM 10041, ARZM 13031, ARZM 16002 and Pora INTA exhibited the highest and most stable resistance to both species. Considering that disease resistance exhibited low specificity to the environment and to the fungal species in evaluations conducted in a wide range of environments and with fungal isolates collected from different hemispheres, the most resistant populations are potential sources of genes for stable resistance to these Fusarium spp.