Resistance to a single UK isolate ofColletotrichum acutatum in strawberry germplasm from Northern Europe

Summary Colletotrichum acutatum on strawberry was first introduced to the UK on runners of cultivar Brighton, which were imported from California in 1983. Recently further outbreaks have been reported following the importation of infected runner plants from mainland Europe. Phytosanitary precautions have so far been successful in controlling the disease but the current trend in the UK is for increasing late season production. This increases the likelihood of there being fruiting plants when the environmental conditions are favourable toC. acutatum. Fifteen cultivars and 19 HRI breeding lines were tested for susceptibility toC. acutatum using the most pathogenic isolate found on plants imported into the UK. It is likely this isolate was originally of Californian origin. Potted plants were spray inoculated and subsequently rated in the glasshouse using a disease severity rating (DSR) with the range from 0–4, where a rating of 4 indicated severe disease symptoms. In contrast to results from California and Italy standard deviations were variable and often large and consequently it was considered unsafe to classify the susceptibility of the genotypes on the basis of mean DSR alone. Classification was thus based on a combination of the mean value and the distribution of the scores but in some cases the results were inconclusive. It was possible to classify five lines as having a high level of resistance (Elvira, Honeoye, EM99, EM224, EM255); six lines had a useful level of resistance but did show symptoms on some plants (Cambridge Favourite, Gorella, Pandora, Pantagruella, EM17 and EM290); five lines were highly susceptible (Elsanta, Redgauntlet, Tamella, EM237 and EM319). The remaining 18 lines could not be classified with the same degree of certainty but 10 were probably resistant.     

Utilizing Arachis cardenasii as a source of Cercospora leafspot resistance for peanut improvement

Summary Advanced generation 40-chromosome hybrids between A. hypogaea (2n=4x=40) and a wild diploid species. A. cardenasii were evaluated for early leafspot, Cercospora arachidicola resistance and agronomic potential. The objective of this investigation was to determine if early leafspot resistance derived from a wild species could be incorporated into the A. hypogaea genome. Interspecific hybrid selections were made and then compared in the field and greenhouse to susceptible cultivars and to A. hypogaea lines which are reported to be resistant to early leafspot. Significantly higher levels of resistance were found in five hybrid selections than in cultivated lines based on numbers of lesions per leaf. In a greenhouse study, several hybrid selections also had greatly reduced sporulation from lesions as compared to A. hypogaea. Several mechanisms of resistance are believed to be present. Although hybrid selections had small seeds and low yields as compared to A. hypogaea, a new and valuable source of early leafspot resistance derived from the species A. cardenasii is present.Paper no. 8814 of the journal series of the North Carolina Agricultural Research Service, Raleigh, NC 27695. This research was partially funded by AID-Peanut CRSP grant DAN-4048-G-SS-2065-00.     

The effect of the ‘Golden Promise’ dwarfing gene on partial resistance to leaf rust and powdery mildew in spring barley

Summary F₂-plant progenies, derived from seven crosses between susceptible erectoides breeding lines and conventional, nutans breeding lines with partial resistance to leaf rust incited by Puccinia hordei Otth., derived from Vada x Cebada Capa, were examined in a field and a glasshouse experiment. In the field experiment, the nutans plant progenies generally were more resistant to leaf rust and powdery mildew than the erectoides progenies. In the glasshouse experiment, the components of partial resistance to leaf rust of six erectoides and six nutans progenies derived from one cross were studied. The nutans progenies generally had lower infection frequencies than erectoides progenies with a similar infection level in the field experiment. The variation for leaf rust and powdery mildew infection in the field was similar for the nutans and erectoides progenies. This suggests that erectoides lines with a fairly high level of partial resistance to leaf rust and powdery mildew can be selected from these populations.     

The inheritance of stem rust and leaf rust resistance in some Ethiopian wheat collections

Summary Common and durum wheat populations obtained from Sweden and originally collected in Ethiopia were screened for resistance to steum rust and leaf rust. Resistant selections of common wheat were crossed and backcrossed with either stem rust susceptible RL6071, or leaf rust susceptible Thatcher. Genetic studies, based largely on tests of backcross F₂ families, showed that four of the selections had in common a recessive gene SrA. Plants with this gene were resistant (1⁺ infection type) to all stem rust races tested. This gene was neither Sr26 nor Sr29. The resistance of other selections, based on tests with an array of rust isolates, was due to various combinations of Sr6, 8a, 9a, 9d, 9c, 11, 13, 30, and 36. One of the selections had linked genes, Lr19/Sr25. Another selection had a dominant gene for resistance (;1 infection type) to all the races of leaf rust. With the possible exception of this gene for leaf rust resistance and SrA, no obviously new resistance was found.     

Genetic analysis of partial resistance to coffee leaf rust ( Hemileia vastatrix Berk & Br.) introgressed into the cultivated Coffea arabica L. from the diploid C. canephora species

The objective of this study was to identify polymorphic molecular markers associated with partial resistance to coffee leaf rust, Hemileia vastarix. A segregating F ₂ population derived from a cross between the susceptible Coffea arabica cv. Caturra and a C. canephora-introgressed Arabica line exhibiting high partial resistance was analyzed. Rust resistance measured as rust incidence (RI) and defoliation (DEF) was evaluated in field conditions in three consecutive years (2003–2005). During the 2003 season, which was characterized by favorable conditions for a rust epidemic, the F ₂ plants exhibited different levels of resistance ranging from very susceptible (50.1% for DEF and 49.5% for RI) to highly partial resistance (9.1% for DEF and 3.7% for RI). Molecular analysis enabled identification of seven polymorphic markers (5 AFLP and 2 SSR) exhibiting significant association with partial resistance. Coexistence of resistance homozygous alleles (RR) at codominant SSR loci was correlated with high resistance. This study is the first attempt to develop PCR-based sequence specific markers linked to partial rust resistance in coffee.     

Partial resistance in lettuce to downy mildew (Bremia lactucae). 1. Search for partially resistant genotypes and the influence of certain plant characters and environments on the resistance level

Summary In three bicyclic inoculation experiments in the field 756 genotypes from the IVT Lactuca gene bank were screened for differences in partial resistance against Bremia lactucae. Significant differences between genotypes were found. No linkage was found between the presence or absence of R-genes and partial resistance nor was there a clear relationship between plant characters such as size, erectness and lettuce type and resistance level. Comparison of test results for genotypes which were tested twice and in different years and/or environments revealed a rather good relationship. So no clear genotype × environment interactions for resistance level were found. Results imply that the bicyclic inoculation experiments are useful to determine levels of partial resistance.     

Studies on field resistance of strawberry varieties to Phytophthora fragariae

Summary Strawberry varieties were grown in a field infected with several races of Phytophthora fragariae and scored for resistance to red core disease caused by this fungus. Field resistance of a variety assessed in this way was not related to its pattern of immunity to specific races of the fungus. Possible mechanisms of field resistance were investigated by examining primary root regeneration and the amount of red steles developed in plants grown in infected soil. The least red steles were found in the roots of 53Q13, a highly field resistant Fragaria virginiana derivative. In the fairly field resistant varieties Crusader, Talisman and Templar, resistance was associated with a high capacity for root regeneration. No explanation was found for the field resistance of Cambridge Favourite.The design of a successful screening technique needs to take account of top growth because no single plant characteristic was universally linked with field resistance.     

Identification of growth stage dependent expression of partial resistance of barley to powdery mildew

Summary Genotypic differences for growth stage dependent expression of partial resistance to barley powdery mildew have been identified on the basis of two components of resistance in the glasshouse and the pattern of epidemic development in the field.Differences for infection frequency and proportion of sporulating colonies were highly significant between the 16 genotypes investigated at four stages of plant development in the glasshouse. Both resistance components were significantly correlated mutually (r=0.73 to 0.86) and with the infection level on leaves developed at comparable growth stages in the field (r=0.52 to 0.73). The infection level of seedlings in the glasshouse was significantly correlated with the infection level of the leaves of the first until the third node in the field (r=0.70 to 0.73). Adult plant resistance was predominantly expressed at the uppermost leaf. A clear genotype × growth stage interaction was apparent for infection level; some genotypes showed partial resistance predominantly at the seedling stage and others predominantly at the adult plant stage.Abbreviations IF Infection Frequency, number of colonies per cm² leaf area, glasshouse experiment - IL Infection Level, number of colonies per cm² leaf area, field experiment - PSC Proportion Sporulating Colonies from total number of colonies, glasshouse experiment     

Interspecific transfer of Brassica juncea-type high blackleg resistance to Brassica napus

Summary Complete resistance to Leptosphaeria maculans, the cause of blackleg of oilseed rape (Brassica napus), was transferred from B. juncea to B. napus through an interspecific cross. B. juncea-type complete resistance (JR) was recognized first in one F₃ progeny (Onap^JR) by the absence of leaf-lesions on seedlings and canker-free adult plants. The commercially important characters of B. napus were retained in advanced lines of Onap^JR, which combined JR with low erucic acid levels (<0.5%), high seed yield and variable maturity dates.JR appeared to be inherited as a major gene or genes. Segregation for resistance and susceptibility contintied to occur during later generations of selection of Onap^JR. JR was readily transferred from Onap^JR to other suitable B. napus cultivars or lines with partial resistance to blackleg and resulted in highly vigorous carly generation selections adapted to cold, wet situations along with complete resistance to blackleg.     

The assessment of variation in two populations of Hordeum bulbosum L. for improving success rates in a doubled haploid barley programme

Summary Improvements in the success of doubled haploid production have been achieved partly by screening new accessions of Hordeum bulbosum. In order to assess levels of variation within stocks of this species, rates of embryo differentiation from the cross between H. vulgare × H. bulbosum were recorded for selections derived from two different stocks of H. bulbosum. There was little difference within stocks for this character despite variation in the morphology and banding patterns of two enzyme systems of one of the stocks. It is proposed that to obtain further increases in success rates a few selections from many accessions of H. bulbosum should be screened rather than many selections from a few stocks.     

Genetic analysis of resistance to Karnal bunt (Tilletia indica,Mitra) in bread wheat

Summary Karnal bunt caused by Tilletia indica in wheat seriously affects the quality of the grains. It is important to generate information on the genetics of resistance to this pathogen so as to aid resistance breeding. For this purpose, four Karnal bunt-resistant lines from China, Brazil and CIMMYT (International Maize and Wheat Improvement Center) and a susceptible Indian cultivar, WL711, were used. The parents, F1 and F3 progenies of five parental diallel crosses revealed that independently segregating loci with three partial dominant resistance alleles were involved in the resistance of Karnal bunt. Lines RC7201/2^*BR2 and Roek//Maya/NAC carried one locus for resistance while Shanghai#7 and Aldan/IAS58 have two and three loci, respectively. One common locus was present in all four resistant parents, which imparted a high level of resistance.     

The transfer of characters from Brassica campestris L. to Brassica napus L.: Production of clubroot-resistant oil-seed rape (B. napus ssp oleifera)

Summary Four oil-seed rape lines were crossed with a clubroot resistant Brassica campestris line from the European Clubroot Differential sct. The allotriploid hybrids were backcrossed to the rape lines to introgress clubroot resistance into oil-seed rape. Using a combination of screening for disease resistance and chromosome number, a high proportion of 38-chromosome, clubroot resistant selections were obtained.     

The oat line Pc54 as a source of resistance to crown rust,stem rust and powdery mildew in Europe

Summary The oat line Pc54 was found to be resistant to powdery mildew under both field and glasshouse conditions. The ratio of resistant to susceptible F₂ and F₂ progeny of a cross between a selection from the Pc54 line (Cc7422) and a susceptible cultivar (Selma) showed that, in addition to carrying the crown rust resistance gene Pc54 and the pg15 gene for stem rust resistance, the mildew resistance of the Pc54 line was conditioned by a single incompletely dominant gene along with additional factors which modified the expression of resistance. Previous results, that there was no linkage between genes Pc54 and Pg15, were confirmed. In addition, there was no evidence of linkage between the mildew resistance gene and gene Pc54. Evaluation of selections from within the Pc54 line showed that the expression of both stem rust and mildew resistance was modified by, or linked to, plant height. The effectiveness of genes Pc54 and Pg15, as measured by virulence frequencies, in central and eastern Europe is described.     

Durable resistance to rice blast disease-environmental influences

Summary Blast is one of the most serious diseases of rice worldwide. The pathogen,Pyricularia grisea, can infect nearly all parts of the shoot and is commonly found on the leaf blade and the panicle neck node. Host resistance is the most desirable means of managing blast, especially in developing countries. Rice cultivars with durable blast resistance have been recognized in several production systems. The durable resistance of these cultivars is associated with polygenic partial resistance that shows no evidence of race specificity. This partial resistance is expressed as fewer and smaller lesions on the leaf blade but latent period does not appear to be an important component. Partial resistance to leaf blast is positively correlated with partial resistance to panicle blast, although some cultivars have been found showing leaf-blast susceptibility and panicle-blast resistance. A diverse set of environmental factors can influence the expression of partial resistance, including temperature, duration of leaf-wetness, nitrogen fertilization, soil type, and water deficit. Because of the great diversity of rice-growing environments, resistance that proves durable in one system may or may not prove useful in another. In highly blast-conducive environments, other means of disease management must be applied to assist host-plant resistance.     

Screening techniques and sources of resistance against parasitic weeds in grain legumes

Summary A number of parasitic plants have become weeds, posing severe constraints to major crops including grain legumes. Breeding for resistance is acknowledged as the major component of an integrated control strategy. However, resistance against most parasitic weeds is difficult to access, scarce, of complex nature and of low heritability, making breeding for resistance a difficult task. As an exception, resistance against Striga gesnerioides based on a single gene has been identified in cowpea and widely exploited in breeding. In other crops, only moderate to low levels of incomplete resistance of complex inheritance against Orobanche species has been identified. This has made selection more difficult and has slowed down the breeding process, but the quantitative resistance resulting from tedious selection procedures has resulted in the release of cultivars with useful levels of incomplete resistance. Resistance is a multicomponent event, being the result of a battery of escape factors or resistance mechanisms acting at different levels of the infection process. Understanding these will help to detect existing genetic diversity for mechanisms that hamper infection. The combination of different resistance mechanisms into a single cultivar will provide durable resistance in the field. This can be facilitated by the use of in vitro screening methods that allow highly heritable resistance components to be identified, together with adoption of marker-assisted selection techniques.     

Discovery,characterization and exploitation of Mlo powdery mildew resistance in barley

Summary Mlo resistance to barley powdery mildew is a relatively new kind of resistance. It was originally described in a powdery mildew resistant barley mutant in 1942 and has been mutagen-induced repeatedly since then. About 1970 it was also recognized in barley landraces collected in Ethiopia in the 1930s. It is unique in that 1) Mlo resistance does not conform to the gene-for-gene system; 2)mlo genes originating from different mutational events map as non-complementing recessive alleles in one locus; 3) all alleles confer the same phenotype, though with small quantitative differences; 4) it is effective against all isolates of the pathogen; and 5) the resistance is caused by rapid formation of large cell wall appositions at the encounter sites preventing penetration by the fungus. Powdery mildew isolates with elevated Mlo aggressiveness have been produced on barley in the laboratory, but have not been found in nature. Mlo resistance is considered very durable. The exploitation of Mlo resistance has been hampered by pleiotropic effects of themlo genes, vix. necrotic leaf spotting and reduced grain yield, but they have been overcome by recent breeding work. During the 1980s Mlo-resistant spring barley varieties have become cultivated extensively in several European countries, in 1990 on about 700,000 ha.     

Comparison of sources of resistance to leaf aphids in lettuce (Lactuca sativa L.)

Summary Three lines of lettuce with resistance to Nasonovia ribisnigri, based on the dominant Nr-gene, and four lines selected for partial resistance to Myzus persicae were tested against three species of leaf aphid: N. ribisnigri, M. persicae and Macrosiphum euphorbiae. The effect of the Nr-gene was also studied in a segregating F2 population.In the material tested, resistance to N. ribisnigri was exclusively based on the Nr-gene, lines selected only for resistance to M. persicae showed no resistance to N. ribisnigri. The Nr-gene also induces partial resistance to M. persicae, but the level of this resistance is influenced by other genes, because the lines with Nr-gene differed significantly from each other for reproduction of M. persicae. The Nr-gene had no effect on the resistance of lettuce to M. euphorbiae.In lines with the Nr-gene, levels of resistance to M. persicae and to M. euphorbiae were correlated, suggesting that the resistance may be determined by the same genes. The Nr-line with highest resistance to M. persicae was comparable for this characteristic to the lines selected for resistance to M. persicae.The cultivars Taiwan and Ravel possess a resistance factor to M. euphorbiae that has no effect on M. persicae or N. ribisnigri. Lines selected for resistance to M. persicae also showed partial resistance to M. euphorbiae. Based on the present results no conclusions can be drawn whether this resistance is based on the same genes that provide resistance to M. persicae, or on a resistance factor comparable to that found in Taiwan and Ravel.     

Screening techniques and sources of resistance to parasitic angiosperms

Parasitic angiosperms cause great losses in many important crops under different climatic conditions and soil types. The most widespread and important parasitic angiosperms belong to the genera Orobanche, Striga, and Cuscuta. The most important economical hosts belong to the Poaceae, Asteraceae, Solanaceae, Cucurbitaceae, and Fabaceae. Although some resistant cultivars have been identified in several crops, great gaps exist in our knowledge of the parasites and the genetic basis of the resistance, as well as the availability of in vitro screening techniques. Screening techniques are based on reactions of the host root or foliage. In vitro or greenhouse screening methods based on the reaction of root and/or foliar tissues are usually superior to field screenings and can be used with many species. To utilize them in plant breeding, it is necessary to demonstrate a strong correlation between in vitro and field data. The correlation should be calculated for every environment in which selection is practiced. Using biochemical analysis as a screening technique has had limited success. The reason seems to be the complex host-parasite interactions which lead to germination, rhizotropism, infection, and growth of the parasite. Germination results from chemicals produced by the host. Resistance is only available in a small group of crops. Resistance has been found in cultivated, primitive and wild forms, depending on the specific host-parasite system. An additional problem is the existence of pathotypes in the parasites. Inheritance of host resistance is usually polygenic and its transfer is slow and tedious. Molecular techniques have yet to be used to locate resistance to parasitic angiosperms. While intensifying the search for genes that control resistance to specific parasitic angiosperms, the best strategy to screen for resistance is to improve the already existing in vitro or greenhouse screening techniques.     

Transmission of Verticillium wilt resistance to tetraploid potato via unilateral sexual polyploidization

Summary Verticillium wilt is a serious disease of potato and is caused by the soil-borne fungi Verticillium dahliae and V. albo-atrum. No major cultivar is resistant to this disease. Two diploid interspecific potato clones, C287 and C545, were previously identified with consistently high levels of Verticillium wilt resistance and are thought to have the same genotype for the loci that confer resistance to V. dahliae stem colonization. The purpose of this study was to determine whether resistance to V. dahliae stem colonization could be transferred to the tetraploid level in potato via unilateral sexual polyploidization (USP). Progenies in eight families obtained by crossing C287 and C545 to two tetraploid breeding clones, S438 and S440, and the cultivar ‘Atlantic’ were planted in a V. dahliae-infested field and a field without a significant V. dahliae infestation. Resistance was evaluated relative to C545 and C287. There were differences among families for stem colonization and yield. No differences between the mean stem colonization of C545 and C287 progeny were detected. Family differences due to the tetraploid parents indicate that they contributed resistance to the progeny. Stem colonization data from this experiment were consistent with the proposed complementary two-gene model for Verticillium wilt resistance in the diploid parents. Unilateral sexual polyploidization is an effective method for transferring V. dahliae stem colonization resistance to the tetraploid level.     

Temperature-sensitive genes for resistance to Puccinia striiformis in Triticum dicoccoides

Summary Twenty-six selections of wild emmer collected at different sites in Israel and three entries obtained from Turkey were studied for the possible presence of temperature-sensitive genes controlling reaction to Puccinia striiformis. In tests carried out in Israel with a local P. striiformis isolate at two temperature regimes, 16 selections showed a change in infection rating toward resistance at the higher temperature regime. In comparable tests performed in the United States with a Montana P. striiformis isolate of different virulence pattern-including 14 of the same selection—ten entries displayed a temperature-sensitive reaction. Although slightly different temperature regimes were used in Israel and the United States, the results obtained were in general agreement. The shift toward resistance observed in the seedling tests at the higher temperature-profile was also evident in the field in the mature plant stage with increasing spring temperatures.