Resistance of nineteen cultivars of subterranean clover to four races of Phytophthora clandestina

Summary The resistance of 19 cultivars of subterranean clover was screened against 4 races of P. clandestina by mycelial inoculation of roots of 10-day-old seedlings growing in water agar and by growing seedlings in pasteurised potting mix containing infested vermiculite in controlled conditions. The cultivars showed differential resistance (vertical resistance) to races of the pathogen and can be divided into 4 resistance groups. Cultivars Clare, Esperance, Green Range, Junee, Mount Barker, Rosedale, Woogenellup and Yarloop were susceptible to all races. Cultivars Bacchus Marsh, Denmark, Enfield, Gosse, Goulburn, Karridale, Larisa, Leura and Trikkala were susceptible to races 1 and 3, but resistant to races 0 and 2. Cultivar Meteora was susceptible to races 2 and 3, but resistant to races 0 and 1. Cultivar Seaton Park (LF) was resistant to all races. Cultivars also varied in their race-non-specific (horizontal) resistance: cultivars that were susceptible to particular races usually varied in their degree of susceptibility to those races. In particular, Junee was more resistant to all four races than the other cultivars within its group. Similarly, cultivars Gosse, Larisa, Denmark and Trikkala were more resistant to races 1 and 3 than the other cultivars in their group. Races of the pathogen varied in their aggressiveness as well as in their virulence, as shown by the variation in aggressiveness of different isolates of race 0.     

Crosses between red clover (Trifolium pratense L.) Varieties to find a suitable combination for reciprocal recurrent selection

Summary As an introduction to a program of reciprocal recurrent selection in red clover crosses of varieties were tested. Five varieties were crossed with Leisi and five with Groninger red clover. Flowering showed that the crosses were successful. Varieties were tested under different conditions. A significant positive difference in dry matter yield between the cross and the better parent of the pair occurred in one of the ten crosses namely Red clover CB × Groninger red clover and under particular conditions. Only a poor comparison was possible in this case because Groninger red clover was attacked by eelworms.It was concluded that none of the tried crosses warranted a program of reciprocal recurrent selection.The data of flowering of the crosses with Leisi showed that in most cases there is intermediate inheritance of the number of flowering heads. In the cross Lembke × Leisi rich flowering was dominant to less rich flowering.     

The expression of resistance of subterranean clover cultivars to races of Phytophthora clandestina

Infection and sporulation of races of Phytophthora clandestina in susceptible and resistant cultivars of subterranean clover and the effect of the races on growth of the cultivars in pots were studied using a total of five races of the pathogen and eight host cultivars. The pathogen caused severe disease where races and cultivars were compatible, although a significant reduction in root and shoot dry weight occurred only on Woogenellup inoculated with race 2. There was little increase in disease severity after 1 wk from the time of inoculation. Sporangia and oospores were formed at high frequency in tap and lateral roots in all compatible combinations, except in Woogenellup in which sporangia were produced inconsistently. The pathogen caused negligible disease and produced very few sporangia in incompatible combinations. Plant age had a strong effect on disease development. Disease severity caused by virulent races on cultivars Gosse, Trikkala, Meteora and Woogenellup was less on older than on younger plants. A further experiment using aggressive isolates of races 0, 1, 2, 3 and 4 revealed that cultivar Gosse showed no reduction in root and shoot weight, despite it being susceptible to races 1, 3 and 4. This provided evidence of the existence of race-non-specific or horizontal resistance to P. clandestina in cultivar Gosse. This revised version was published online in July 2006 with corrections to the Cover Date.     

Regenerative trait and cold hardiness in highly productive cultivars of alfalfa and red clover

Summary In earlier work on improvement of persistance in forage legumes, we selected genotypes from highly productive cultivars of alfalfa, Algonquin and Apica (Euphytica 45: 105–112, 1990) and cv. Florex red clover (Plant Cell Reports 8: 395–398, 1989) capable of in vitro regeneration from callus and cell culture. The alfalfa germplasm and its F₁ progeny as well as an F₂ red clover population were tested for cold stress tolerance. Plantlets were hardened in culture tubes at 2 or 5°C, 8h photoperiod, for at least four weeks and then subjected to freezing temperatures, –16 or –10°C for alfalfa and red clover, respectively. Survival of regenerative genotypes was significantly higher than of the non-regenerative ones in both species. A strong oositive correlation (r=0.78) between the regenerative trait and plant survival was found in alfalfa. The experiments indicate that in vitro selection for regenerative trait may improve cold stress tolerance of alfalfa and red clover.     

The effect of induced autotetraploidy on resistance to clover rot (Sclerotinia trifoliorum erikss.) in red clover

In infection experiments with clover rot (Sclerotinia trifoliorum Erikss.) on 9 pairs of diploid and corresponding autotetraploid families arisen from chimaera plants the tetraploid families proved to be more resistant than their corresponding diploid families. Significant interaction between family pairs and ploidy level was demonstrated and this is an indication that there may be differences between different genotypes with respect to the effect of autotetraploidy on resistance to clover rot in red clover.

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Samenvatting Bij kunstmatige infectie met klaverkanker van 9 paren (uit chimaere planten van rode klaver afkomstige) diploide en daarmede corresponderende autotetraploide families bleken de tetraploide families grotere resistentie te vertonen dan de daarmede corresponderende diploide families.Uit een vergelijking van de families bleek dat er verschillen in invloed van de autotetraploidie op de resistentie tegen klaverkanker zijn.

     

Influence of recurrent selection for flowering on flowering and yields in kura clover

Summary These investigations were conducted to determine the effectiveness of first- and second-season selection programs for increasing forage and seed yield in kura clover (Trifolium ambiguum Bieb.).Rhizo kura clover, the base population, was grown as spaced plants in two programs for six generations of selection for flowering in the seedling and second seasons. Remnant seed for each generation of selection was used to establish spaced plant and broadcast evaluation trials.The first-season selection program was effective in increasing flowering not only in the first season, but for two subsequent seasons. The second-season selection for flowering had little or no effect. However, vigor, forage and seed yields were not increased by either program, and the first-season program progressively reduced vigor and forage yields as generations of selection increased. The cause of this is unknown but may be associated with inbreeding depression or detrimental physiological effects of first-season flowering. Reselection to eliminate possible inbreeding effects is being conducted to further examine the possibility of increasing seedling and aftermath vigor in kura clover.     

Influence of sample size on precision of heritability and expected selection response in red clover

Limited attention has been given to the effect of sample size on the precision of heritability estimates and expected selection responses. The objectives of this study were to evaluate the effect of sample size on the point estimate, its sampling error and interval width. Monte Carlo simulation and non-parametric bootstrap were used to estimate parameters in a red clover (Trifolium pratense L.) data set. We conducted separate analyses for two and four replications, and for combinations of six, 12, 24, 42 and 66 families, each having three, six, nine, 12 and 18 plants. Results showed that the effect of increasing number of families and replications on reduction of sampling error and interval width was greater than that of increasing plants per family. The sampling error and interval width of parameters decreased at a decreasing rate as sample size increased. To reduce the chances of having negative estimates of family variance components and to achieve a narrow confidence interval, an experiment of 42 families, each having six seedlings in four replications would be required.     

A simple method for the evaluation of red clover (Trifolium pratense L.) resistance to Sclerotinia trifoliorum

Sclerotinia crown and stem rot is the most destructive disease on red clover in areas with cool seasons including North America and Europe. An artificial inoculation test in controlled conditions has been developed in order to evaluate resistance to this fungus in individual plants. Measured drops of ascospore suspension were applied to detached clover leaflets and disease severity scored on a visual scale. Clovers from 16 cultivars and 12 ecotypes were tested for resistance to Sclerotinia trifoliorum. The results obtained for cultivar resistance are in agreement with those of other tests described in the literature. Moreover, repeated individual evaluations on detached leaflets from the same plant was successful in detecting genotypic variation within clover populations.     

Screening for resistance to clover rot (Sclerotinia spp.) among a diverse collection of red clover populations (Trifolium pratense L.)

Red clover (Trifolium pratense) is an important perennial forage crop that is widely cultivated in Europe. Clover rot remains a major disease in red clover, but resistance breeding is hampered by the lack of available sources of resistance. Moreover, little is known about the factors that influence clover rot resistance. In this paper we evaluated the variation in clover rot susceptibility among a diverse collection of 113 red clover accessions, with the aim of identifying more resistant accessions. Clover rot susceptibility was assessed with a high throughput bio-test on young plants. We found significant variation in clover rot susceptibility, within and among accessions. ‘Tedi’, ‘Maro’ and ‘No 292’ were the most resistant accessions. Fifteen diploid accessions were more susceptible than the average accession with the cultivar ‘Nemaro’ being the most susceptible. Clover rot susceptibility was not correlated with isoflavone levels from Mullaney et al. (Agronomy abstract. ASA, Madison, p 195, 2000). Cultivars were more resistant than landraces and wild accessions and tetraploid cultivars were more resistant than diploid cultivars. Besides the in-depth analysis for clover rot susceptibility, possible correlations with plant architecture and other diseases were investigated. Growth habit, branching, plant yield, flowering date and susceptibility to mildew, virus and rust diseases were investigated in a 3-year field trial. Unlike previously suggested, clover rot susceptibility was not correlated with branching or with plant yield over three years. On the other hand, late flowering accessions and accessions with erect growth habit were less susceptible to clover rot. Clover rot susceptibility was not correlated with susceptibility to rust disease (Uromyces trifolii) or viral diseases, but negatively with susceptibility to mildew (Erysiphe polygoni). Because no completely resistant accessions were found, the best way to improve clover rot resistance would be to select recurrently for resistant genotypes among diverse cultivars and landraces with lower susceptibility. Tetraploidisation of diploid populations with a higher resistance level can provide an additional level of protection.     

Number of involved genes and heritability of clover rot (Sclerotinia trifoliorum) resistance in red clover (Trifolium pratense)

European red clover (Trifolium pratense) crops are challenged by clover rot, a devastating disease caused by Sclerotinia trifoliorum or, in some cases by S. sclerotiorum. No completely resistant cultivars are available and resistance breeding is hampered by the lack of knowledge on the number of involved resistance genes and the heritability of clover rot resistance. In this study, we estimated the number of major genes contributing to clover rot resistance by analysing 15 F₁ progeny populations from pair crosses between ramets of resistant and susceptible genotypes. Parent plants were chosen from diverse, diploid populations, including wild material, landraces and cultivars. Young progeny plants were inoculated with ascospores, evaluated phenotypically and the segregation of disease scores was studied. Our results indicated that clover rot resistance may be conferred by three major effect genes, although segregation patterns suggested that there may be numerous minor effect genes involved as well. No proof was found for a maternal inheritance of clover rot resistance. To get insight in the heritability of clover rot resistance, we applied divergent selection by our high-throughput bio-test on an experimental diploid population: the original population (70.5 %), the first generation after selection for susceptibility (79.2 %) and the first generation after selection for resistance (62.3 %) differed significantly in susceptibility (p < 0.001). The second generation after selection for resistance (60.0 %) was not more resistant than the first generation after selection for resistance. In the first generation of selection the heritability (h²) was on average 0.34. In the second generation of selection h² was 0.07. These findings have important implications for resistance breeding.     

Preliminary evaluation of turfgrass (perennial ryegrass and red fescue) cultivars for resistance to red thread disease using artificial infection

Selection of main cool‐season turfgrasses (perennial ryegrass and red fescue) for resistance to red thread disease caused by Laetisaria fuciformis has not progressed during the last 30 years because of low reproducibility of experiments carried out under natural infection. In this study, evaluation of a set of the turfgrass cultivars, including non‐host fodder grasses as control, was performed in a climatic chamber, a cold house and in the field using three types of artificial inoculum, and correlations between trials were analysed. In general, cultivars of Festuca rubra were more susceptible to the disease but differentiated better than cultivars of Lolium perenne. Significant interactions between cultivars of L. perenne and F. rubra and isolates of L. fuciformis were found. The fungus was also able to colonize leaves of sow thistle and wheat seedlings. In field conditions and in the glass cold‐house Dactylis glomerata was found to be relatively resistant to red thread disease. A possible selection programme is proposed.     

Experiences with the breeding of red clover resistant to the stem eelworm

Summary The methods used in collecting stem eelworms from diseased plants, in raising and inoculating plants, in assessing resistance and in crossing selected plants in cages using bumble-bees for pollination are described in detail.Research showed that in swollen seedlings the eelworms had multiplied (susceptible plants) whereas this is not the case in non-swollen plants with a stunted growth and brown stripes (resistant plants).The increase of eelworms in greatly swollen plants of Roosendaal red clover took place at a greater speed than in slightly swollen plants. In healthy-looking plants mostly no or few eelworms had invaded. It seems that these plants have escaped a degree of infection necessary to judge their resistance.It appeared that varieties of red clover which are susceptible or resistant to stem eelworms of a given origin from the Netherlands were susceptible or resistant in about the same degree to eelworms derived from a quite different part of this country. A good conformity between laboratory and field resistance was found.If in spring bumble-bee boxes are hung in trees according to the method described, in about 10 to 20% of the boxes a good bumble-bee nest is found in the beginning of July. These populations are suitable to use for the crossing of clover plants in cages. Inside the cages seed production per plant was generally much lower than outside.By intercrossing resistant plants of Dutch local varieties and selection in the progenies, fairly resistant families were obtained within some years.     

Components of resistance to Liriomyza trifolii in Chrysanthemum morifolium and Chrysanthemum pacificum

Summary In three experiments, Chrysanthemum morifolium Ramat. cultivars and C. pacificum Nakai, were exposed for four days to adult flies of Liriomyza trifolii (Burgess). The flies fed on all plants and laid eggs which were allowed to developed in a leafminer free greenhouse. Feeding level and oviposition varied between genotypes. The largest differences between entries were visible in the percentage of larvae that reached the pupal stage. In the most susceptible C. morifolium cultivar 97% of the larvae pupated against 1% in the most resistant cultivar Penny Lane. C. pacificum Nakai was also resistant with larval survival rates of 12%, 2% and 0% respectively in three consecutive experiments. Breeding prospects and areas for further study are discussed.     

Effects of recurrent selection for resistance to scab (Gibberella zeae) in wheat

Summary Scab caused by Gibberella zeae Petch., in common wheat, is one of the most severe diseases in China. A source population C0, bred for scab resistance, was developed through three cycles of multiple-parent crossing and intercrossing by means of the dominant male-sterile gene Ta1 (Ms2), according to Wu's scheme. Phenotypic recurrent selection methods for increasing the resistance to scab-infection of spikelets and seeds with the male-sterile plants were carried out simultaneously in Nanjing and Shanghai and at Jianyang, Fujian Province, for three cycles. The generations from C0 to C3 and two check cultivars were evaluated, using a randomized block design, under conditions of an artificially induced epidemic of scab during 1988–1990. The results indicate that there were significant differences in the resistance to scab between these generations. On average, the percentages of diseased spikelets and seeds of the male-fertile plants were reduced by 9% and 10%, respectively. The frequency of resistant plants was distinctly enhanced by recurrent selection. Analysis of variance showed that no significant differences existed between cycles of recurrent selection in agronomic characters such as plant height, spikes per plant, spike length, numbers of spikelets and seeds per spike, weight of seeds per spike and 100-kernel weight, days to heading and to maturity. Except for plant height, most of these traits tended to be slightly improved with improvement of resistance in the gene pool. The variance for resistance in the generations was decreased under selection. Recurrent selection for scab resistance using the dominant male-sterile gene Ta1 (Ms2) was both an effective and feasible breeding method for producing this character in wheat.     

浙江地区柑桔炭疽病对甲基硫菌灵的抗性水平及其分子机制

了解浙江地区柑橘炭疽病菌株对甲基硫菌灵的抗药性频率及其抗性分子机制。采用最低抑制浓度(MIC)法测定甲基硫菌灵对62株炭疽病菌株的敏感性,并利用PCR扩增的方法扩增12株代表性菌株的β-微管蛋白基因。结果表明：分离获得的菌株中,对甲基硫菌灵高抗菌株56株(90.32%),敏感菌株6株(9.68%)。通过对12株代表性的炭疽病菌株部分β-微管蛋白基因序列的分析。结果显示：甲基硫菌灵的抗性菌株发生单一的核苷酸突变(A→C),导致抗性菌株(HR)核苷酸所编码的198位氨基酸由敏感菌株的谷氨酸(GAG)突变为丙氨酸(GCG)。柑橘炭疽病菌对甲基硫菌灵的抗药性与β-微管蛋白198位氨基酸突变有关。     

Characterizing the molecular and morphophysiological diversity of Italian red clover

Red clover (Trifolium pratense L.) is the third major forage species in Europe, but there is limited information on the biodiversity and the genetic structure of landraces and natural populations which evolved in this region. The objective of this study was producing such information for Italian germplasm on the ground of molecular and morphophysiological diversity. The study included 16 Italian natural populations from a wide range of environments, four landraces representing the four traditional commercial ecotypes, and two varieties. Eight morphophysiological traits were assessed in a replicated trial under field conditions, whereas two AFLPs primer combinations with 140 polymorphic markers were recorded on a random sample of 13 genotypes per population. Ordination and classification results based on morphophysiological data clearly kept track of the type of germplasm (i.e. landrace or natural population) and its geographic origin, unlike results based on molecular markers. Euclidean distances among populations based on morphophysiological traits were not correlated with Nei’s genetic distances based on molecular markers according to Mantel’s test. Geographical distances among landrace or natural population material was correlated with distances based on morphophysiological traits but not with those based on molecular markers. The average within-population variation estimated via molecular markers was about 2.6-fold higher than that among populations, preventing an acceptable discrimination among most populations. On average, natural populations tended to have within-population variation similar to varieties and somewhat lower than landraces. Our results have implications for collection, conservation, exploitation and registration in a sui generis system of red clover genetic resources.     

Resistance of tomato cultivars and progenies to fruit hollowness

Summary In field experiments with fall and winter tomatoes, the incidence of fruit hollowness was found to range from 0 to more than 80%, depending both on the cultivar and the environment. The most resistant cultivars were Bonner Beste and Potentate; the most susceptible were Exhibiton, Moneymaker, E.S. 1, Berco, Cromco and Ailsa Craig. The rate of hollowness of F₁ and F₂ generations of crosses between resistant and susceptible parents suggests that resistance behaves as a partially dominant character governed by a small number of genes. Thus, commercial F₁ hybrids resistant to the disorder are easy to produce. The heritability of the resistance character was found to be rather high.Contribution from The Volcani Institute of Agricultural Research, Bet Dagan, Israel. 1969 Series, No. 1558-E.     

Breeding transgressive lines of pepper for resistance to Phytophthora capsici in a recurrent selection system

Summary In order to increase the resistance level of pepper (Capsicum annuum) to Phytophthora capsici the main sources of genetic resistance were intercrossed. The parents included 7 varieties from different origins of partial resistance to P. capsici and to other soilborne pathogens. Two cycles of recurrent selection were performed by 7 plant breeders who screened the plants at two developmental stages with various isolates and at two temperatures. Analysis of data revealed a strong influence of the testing conditions on the evaluation of the plants. The level of resistance of the progenies further depended on the pathogen isolate used to screen the plants. Improvement of the whole population was more significant during the first cycle and lines with enhanced level of resistance as compared to the parents i.e. transgressive lines were fixed from the two cycles. These lines were resistant to much higher inoculum concentrations than the original parents and the expression of resistance was stable at high temperature (32°C). These new sources of polygenic resistance were included into a new breeding program to improve the agronomic characters.