Inheritance of foliar resistance to Phytophthora palmivora (butler) Butler in cacao (theobroma cacao L.)

The genetic nature of resistance in leaves of cacao (Theobroma cacao L.) to Phytophthora palmivora (Butler) Butler was investigated in a 3 × 8 and a 4 × 4 mating design. Leaf resistance was assessed at the penetration and post-penetration stages of infection. Wide variability was observed among the 32 families examined for lesion frequency (penetration resistance) and lesion size / spread (post-penetration resistance). Segregation within families showed a continuous distribution for both forms of resistance, suggesting that resistance is polygenic. The poor association between resistance at the penetration stage and that at the post-penetration stage suggested that they were independently inherited and could be selected independently with the aim of accumulating genes for resistance. Genetic complementation between the two forms of resistance and transgressive segregation were evident within the families studied. The additive effects were predominant for lesion size, while both additive and non-additive effects were important in the inheritance of lesion frequency. The narrow sense heritabilities obtained from North Carolina Model II analysis were as high as 0.61 and 0.40 for lesion size and lesion frequency respectively. Possible breeding strategies based on these findings are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Evaluation and utilisation of cacao (Theobroma cacao L.) germplasm at the International Cocoa Genebank, Trinidad

The aim of this study was to identify promising genotypes in the International Cocoa Gene bank, Trinidad (ICG,T) for use in cacao breeding. Subsets of the germ plasm collection were evaluated for bean number, bean weight, pod index (581 genotypes) and resistance to Phytophthora pod rot(500 genotypes). Among three groups of cacao assessed (For astero, Refractario and Trinitario), Trinitario had the highest percentage of genotypes (36.9%) with large bean weight (> 1.2 g), while For astero possessed the highest proportion of genotypes (22.6%) with a large number of beans (> 45). The ICS population had the highest percentage (44.1%) of genotypes with heavy beans, and IMC the highest percentage of genotypes with large bean number (68.6%). A low, but significant negative correlation (r = –0.19, p≤ 0.001) between bean number and bean weight suggests that an increase in bean number may repress an increase in bean weight and vice versa. However, inten genotypes good values of bean weight and number were combined demonstrating that selection for both large bean number and bean weight is possible. The highest percentage (28.1%) of genotypes with low pod index (< 20.1) was observed in the Trinitario group (mainly ICS). The Forastero group had the highest percentage of Phytophthora resistant accessions(18.0%). The PA population had the highest proportion of resistant (24.0%) and moderately resistant (38.0%)genotypes. Sixty-five and 99 genotypes were categorised with large bean weight and high bean number, respectively, and 60 genotypes were found with a low pod index. Seventy-eight genotypes were identified with resistance to Phytophthora podrot. A significant relationship was observed between resistance to Phytophthora pod rot (measured as the frequency of localised lesions) and bean number (r = –0.45, p ≤ 0.001)showing that the two traits may complement each other. The combination of low to intermediate pod index with moderate to high resistance to Phytophthora podrot was found in 87 genotypes, 12 of which were also reported to have resistance to witches’ broom disease. These genotypes are of high potential value in cacao breeding and their inclusion in working and corecollections would be justified. This revised version was published online in July 2006 with corrections to the Cover Date.     

Quantitative trait loci analysis in Theobroma cacao using molecular markers. Inheritance of polygenic resistance to Phytophthora palmivora in two related cacao populations

Two related segregating populations of Theobroma cacao L. were analysed for their resistance to Phytophthora palmivora. The first F₁ population was obtained by crossing two susceptible cacao clones of Catongo (a highly homozygous genotype) and Pound 12(a highly heterozygous genotype) and the second population was obtained by backcrossing a single F₁ tree with Catongo. The genetic maps obtained for each population were compared. The F₁ map includes 162 loci and the backcross has 140 loci. The two maps, F₁ and BC₁, exhibit high co-linear loci organization covering respectively, 772 and 944 cM.Phytophthora resistance was assessed by measuring the size increase of a lesion at five (DL5)and ten days (DL10) after pod inoculation. Six different QTL were detected in the F₁ and BC₁ populations. One QTL was found in both populations, and appeared to be a major component of disease resistance, and explaining nearly 48% of the phenotypic variance in the F₁ population. The absence of some yield QTL detection in the BC₁ in comparison with the F₁ population is due to the lack of transmission of the favouring alleles for these QTL from the single F₁ tree used for the backcross. The phenotypic variance explained by the action of the quantitative trait alleles indicated that genetic factors of both major and minor effects were involved in the control of the character studied. QTL conferring increased resistance to Phytophthorawere identified in both susceptible parents, suggesting the presence of transgressive traits and the possibility of selection in cacao. Pleiotropic and epistatic effects for the QTL were also detected. Finally, the use of marker assisted selection (MAS) in cacao breeding programs is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Expected selection efficiency for resistance to cacao pod rot (Phytophthora palmivora) comparing leaf disc inoculations with field observations

Summary A genetic analysis of resistance of cacao to Phytophthora palmivora was carried out in a 5 × 5 diallel and in a 4 × 2 NC II factorial design, involving cross-progenies and parental clones planted in the nursery and field, respectively. Resistance was scored in the laboratory by inoculation of leaf discs with P. palmivora spores with four replicates and, for the factorial design, results were compared with the percentage of rotten pods in the field assessed over a seven-year period. Significant differences between genotypes were observed for both laboratory and field evaluation methods. For the laboratory test, no reciprocal effect was observed and plant effects within seedling progenies were more significant than plant effects within clones. General combining ability was the main source of variation but specific combining ability was also significant for the leaf disc test applied to the diallel. Heritabilities increased with the number of replicates, reaching values of 0.34 and 0.67 for narrow sense heritability, and 0.60 and 0.67 for broad sense heritability, for the diallel and the factorial design, respectively. These values were higher than observed for the percentage of rotten pods in the field in the factorial design (0.42 and 0.47, respectively). For the leaf test, the expected genetic gains were around 30% with a selection intensity of 5%. The existence of significant genetic (0.71) and phenotypic (0.39) coefficients of correlation between resistance on leaves and percentage of rotten pods in the field in the factorial design confirms the feasibility of using the leaf disc test for early selection of resistance to Phytophthora pod rot of cacao.     

Genetic differentiation among Ecuadorian Theobroma cacao L. accessions using DNA and morphological analyses

The quality of Ecuadorian cacao is presently threatened by the introduction of hybrid material. An estimation of genetic diversity in Ecuador is required in order to avoid the loss of fine flavored cocoa. Genetic variability amongst 60 Ecuadorian genotypes of Theobroma cacao has been evaluated using molecular and phenotypic markers. The two distance matrices derived from the molecular and phenotypic data were found to be correlated (R² = 0.5). Dynamic clustering analyses classified the genotypes in two or three groups depending on the markers used. The genotypes coming from Sebasti{àn Arteaga (SA) and Balao Chico (BCH) plantations appeared related to each other suggesting a common genetic origin. They also may be considered as a distinct group with high RFLP homozygosity. The EETP (Estací}on Experimental Tropical Pichilingue of Ecuador) collection was comprised of more variable genotypes possessing variable heterozygosity levels. The low heterozygous genotypes may be genetically related to SA and BCH trees, whereas the higher heterozygous genotypes may have resulted from hybridizations between original Nacional material of Ecuador and genotypes imported from Trinidad at the beginning of the century. Thus genetic introgression may have occurred giving rise to a range of variation between Nacional and hybrid forms. This revised version was published online in July 2006 with corrections to the Cover Date.     

Inheritance of foliar resistance to <Emphasis Type="Italic">Phytophthora</Emphasis> species in cacao (<Emphasis Type="Italic">Theobroma cacao</Emphasis> L.): can it be influenced by CSSV infection?

Studies on quantitative genetics of foliar resistance to black pod disease in cacao could inadvertently use cocoa swollen shoot virus (CSSV) infected leaves which could bias the results especially in West Africa where the virus is prevalent. However, effects of CSSV on inheritance and heritability of foliar resistance to Phytophthora species is not known. Choice of an efficient breeding method requires an accurate estimation of genetic effects in selection schemes for foliar resistance to Phytophthora species in cacao. The objective of this study was to investigate the effect of CSSV infection on quantitative genetic parameters of foliar resistance to cocoa black pod disease in a population of 36 F1 hybrids developed by mating six cacao genotypes using a diallel method. The generated F1s and their parents were evaluated for foliar resistance to P. palmivora and P. megakarya using a randomized complete block design (RCBD) with three replications. 1A CSSV and Nsaba CSSV strains were used to infect the cacao genotypes using the patch graft method. The parents chosen showed significant variations for scores of leaf discs after inoculation with P. palmivora and P. megakarya. The leaf disc scores of CSSV infected crosses were lower than leaf disc scores of CSSV-free crosses. Genetic component analysis showed that the effects of GCA and SCA was significant for both CSSV-free and CSSV-infected crosses in resistance to P. palmivora and P. megakarya. The significant GCA and SCA for both CSSV-free and CSSV-infected crosses strongly suggest that both additive and non-additive genetic effects play an important role in the determination of inheritance of foliar resistance to Phytophthora species in cacao. There was significant variability in mean squares of GCA and SCA of CSSV-free and CSSV-infected crosses indicating that CSSV infection modifies GCA and SCA of affected plants. Narrow sense heritability was relatively low (0.31) for foliar resistance to P. palmivora and P. megakarya under CSSV-free and 1A CSSV strain infected conditions. However, heritability for foliar resistance to P. palmivora (0.43) and P. megakarya (0.36) was significantly higher under Nsaba CSSV infected condition. The modifications of mean squares of GCA and SCA and narrow sense heritability due to CSSV infection could mislead in choice of breeding methods indicating that attention must be paid to the infection status of plants when conducting quantitative genetics studies using diseased and healthy plants. CSSV status of leaf samples should be known before using them in leaf disc test. Genotypes Pa7/808 and Pound 7 had high negative GCA effects and are promising parents for enhancement of resistance to black pod disease in cacao.     

Residual effects of the Xa-4 resistance gene in three rice cultivars when exposed to a virulent isolate of Xanthomonas campestris pv. oryzae

Summary F₂ plants of five, and F₃ plants of three, crosses between genotypes carrying the race-specific resistance gene Xa-4 and genotypes not carrying this gene were inoculated with two isolates of Xanthomonas campestris pv. oryzae. Half the tillers of each plant received isolate PX061, avirulent on the Xa-4 gene, the other half of the tillers received isolate PX099, virulent for the Xa-4 gene. The F₂ and F₃ populations segregated for a single dominant resistance gene, Xa-4.The parental, F₂ and F₃ genotypes not carrying Xa-4 had mean lesion lengths between 28 and 29 cm for both isolates. The Xa-4 carrying parents showed a mean lesion length of 2.7 cm with the avirulent isolate and of 12.4 cm with the virulent isolate. The Xa-4 carrying F₂ and F₃ genotypes had mean lesion lengths of 5.2 and 20.1 cm for the two isolates, respectively. These observations strongly indicate that the Xa-4 gene, carried by the rice genotypes studied (IR28, Cisadane and BR51-282-8), had a considerable residual effect when exposed to virulent isolate PXO99.     

Identification of <Emphasis Type="Italic">Ascochyta rabiei</Emphasis> disease resistance in chickpea genotypes

Ascochyta blight (AB) disease, caused by the fungus Ascochyta rabiei, is a major yield limiting factor of chickpea in Australia and around the world. The aggressiveness of six A. rabiei isolates was identified using 3 chickpea varieties (Jimbour, Flipper and Yorker). These AB isolates were isolated from chickpea fields in northern NSW, one of the major chickpea production regions in Australia. Each of the six isolates produced a different aggressiveness pattern and isolate 4859 was found to be the most aggressive. The AB resistance in 16 international and Australian chickpea genotypes was then investigated by inoculating the plants with the most aggressive isolate and a mixture of the other 5 isolates. Resistance to both the most aggressive isolate and the mixed isolates has been identified in 5 genotypes (ICCV 98813, Flipper, ICCV 05111, ICCV 98801, Jimbour #1) while 10 entries (Howzat, ICCV 06108 and ICCV 98818, Jimbour, ICCV 96852, ICCV 06107, ICCV 98816, Yorker, FLIP97-114C, ICCV 96853) were moderately resistant. Only one genotype (Bumper) appeared to be susceptible to both inoculums. There was large variation observed in the pathogenicity of the isolates suggesting that the six AB isolates represent several different pathogen strains. Significant differences in leaf infection rate, plant infection rate, plant death rate and disease development were identified among the chickpea genotypes tested. These findings suggest that these chickpea genotypes carry different resistant genes, which can be exploited in breeding programmes to develop high levels of disease resistance.     

A differential series of pepper (<Emphasis Type="Italic">Capsicum annuum</Emphasis>) lines delineates fourteen physiological races of <Emphasis Type="Italic">Phytophthora capsici</Emphasis>

The study of the genetics of resistance in pepper to the oomycete pathogen Phytophthora capsici has been complicated due to a lack of use of a common set of pathogen isolates and host genotypes. We have developed a differential series for this system using eleven pepper genotypes and thirty-four isolates of the pathogen from California, New Mexico, North Carolina, and Turkey. Through differential patterns of virulence of the isolates on the hosts, we identified fourteen different physiological races of P. capsici. There appears to be no restriction of races to particular geographical locations. Isolate mating types were also determined, and both mating types were found in one field in California. The significance of the characterization of physiological races and existence of both mating types in the field to pepper growers and breeders is discussed.     

QTL mapping and identification of corresponding genomic regions for black pod disease resistance to three <Emphasis Type="Italic">Phytophthora</Emphasis> species in <Emphasis Type="Italic">Theobroma cacao</Emphasis> L.

The cacao tree (Theobroma cacao L.) is a species of great importance because cacao beans are the raw material used in the production of chocolate. However, the economic success of cacao is largely limited by important diseases such as black pod, which is responsible for losses of up to 30–40% of the global cacao harvest. The discovery of resistance genes could extensively reduce these losses. Therefore, the aims of this study were to construct an integrated multipoint genetic map, align polymorphisms against the available cacao genome, and identify quantitative trait loci (QTLs) associated with resistance to black pod disease in cacao. The genetic map had a total length of 956.41 cM and included 186 simple sequence repeat (SSR) markers distributed among 10 linkage groups. The physical “in silico” map covered more than 200 Mb of the cacao genome. Based on the mixed model predicted means of Phytophthora evaluation, a total of 6 QTLs were detected for Phytophthora palmivora (1 QTL), Phytophthora citrophthora (1 QTL), and Phytophthora capsici (4 QTLs). Approximately 1.77–3.29% of the phenotypic variation could be explained by the mapped QTLs. Several SSR marker-flanking regions containing mapped QTLs were located in proximity to disease regions. The greatest number of resistance genes was detected in linkage group 6, which provides strong evidence for a QTL. This joint analysis involving multipoint and mixed-model approaches may provide a potentially promising technique for detecting genes resistant to black pod and could be very useful for future studies in cacao breeding.     

Effects of glandular trichomes on the development of Phytophthora infestans infection in potato (S. tuberosum)

S. berthaulltii Hawkes, a wild potato species, possesses two types of glandular trichomes on its leaf surface: types A and B wich confer resistance to insects. Preliminary results indicated that the presence of glandular trichomes also confers resistance to Phytophthora infestans, the causal agent of late blight. In this paper we report the evaluation of trichome characteristics (density and exudate activities) performed on 8 lines of potato, including the control varieties Desirée and Tropicana, 5 hybrid lines derived from intercrosses and backcrosses between S. tuberosum and S. berthaultii and one accession of S. berthaultii. Whole plants of the 8 genotypes were inoculated with a spore suspension of Phytophthora infestans in order to evaluate their resistance to late blight. The resultant disease incidence correlated negatively with type A trichome density and with polyphenol-oxidase (PPO) activity of type A trichome glands. The removal of type B trichomes exudate by leaf dipping in ethanol did not influence the disease development after artificial inoculation of detached leaves. Type B trichome exudate was extracted and the fraction corresponding to Fatty Acids Sucrose Esters (FASE) was recovered after Thin Layer Chromatography and tested on the fungal mycelium in vitro. The FASE exhibited a slight inhibitory effect on mycelium growth. The results of the experiments are discussed, and several hypotheses regarding the possible role of types A and B trichomes are formulated. This revised version was published online in July 2006 with corrections to the Cover Date.     

Evaluation of French Brassica oleracea landraces for resistance to Plasmodiophora brassicae

A total of 240 kale, 38 cabbage and 126 winter cauliflower French landraces from the B. oleracea genepool of INRA were assessed for resistance to clubroot caused by Plasmodiophora brassicaeWoron. Two French isolates of the pathogen (K and SJ) were used in the experiments under controlled conditions. The reaction of the 126 cauliflower accessions to naturally occurring clubroot was also evaluated in field trials. Kales exhibited considerable variation for expression of disease resistance and high levels of resistance were found in several accessions. In this group, single resistant plants were observed in most of the morphological types and from quite different geographical origins. Cabbage accessions were moderately to highly susceptible to both isolates. All cauliflower populations proved to be highly susceptible to K isolate and moderately susceptible to SJ isolate. In field trials, cauliflowers were also severely infected. Two lines selected from a resistant kale population were highly resistant against a large range of pathotypes of the pathogen. These lines presented a sufficient level of resistance to be directly useful in the breeding program in order to develop cauliflower and broccoli hybrids resistant to clubroot. This revised version was published online in July 2006 with corrections to the Cover Date.     

Molecular mapping of a new gene for resistance to rice blast (Pyricularia grisea Sacc.)

A single dominant blast resistance gene conferring resistance to a Korean rice blast isolate was identified in rice variety `Suweon 365'. We report the chromosomal localization and molecular mapping of this blast resistance gene designated as Pi-18, which confers resistance to Korean isolate `KI-313' of the blast pathogen. To know whether there is a relationship among genes conditioning resistance to location-specific isolates of the blast pathogen and thereby to identify linked markers to resistance gene for isolate KI-313 collected in Korea, RFLP markers previously reported to be linked to major blast resistance genes in different rice germplasm and other markers mapped to nearby regions were surveyed for polymorphism between a resistant (`Suweon 365') and a susceptible (`Chucheongbyeo') parent. Linkage associations of the RFLP markers with the resistance gene were verified using an F2 and F3 segregating population of known blast reaction. RFLP analysis showed that Pi-18 was located near the end of chromosome 11, linked to a single copy clone RZ536 at a distance of 5.4 centiMorgans (cM) and that this gene was different from Pi-1(t). An allelism test revealed that this gene was also different from Pi-k. Currently, a combination of RAPD and microsatellite primers is being employed to find additional markers in this region. Tightly linked DNA markers will facilitate selection for resistant genotypes in breeding programs and provide the basis for map based cloning of this new blast resistance gene. This revised version was published online in July 2006 with corrections to the Cover Date.     

Rust infections of some Allium species: An assessment of germplasm for utilizable rust resistance

Summary Isolates of three species of rust (Puccinia allii, P. mixta and Uromyces ambiguus) were each inoculated onto a range of potential host plants, selected to include the major European crop Allium species together with the commoner wild species in the section Allium. Measurements of incubation period, lesion type, latent period and pustule quantity were made in a single environment. Each isolate was able to sporulate on a range of Allium species. There was evidence of specialisation to particular sections of the genus; all the leek isolates performed well on the majority of hosts in section Allium, whilst the chive isolate grew poorly on most members of that section. Exceptions to this are exemplified by A. fistulosum being attacked by all three rust species and A. sativum being attacked by both leek and chive isolates. A continuum of reaction types was found which, for the leek isolates, ranged from the low levels of quantitative resistance in leek cultivars, through higher quantitative levels within the A. ampeloprasum complex, to qualitative resistance in several interactions in non-host sections of Allium, where the pathogen was able to infect and form colonies of some size without sporulating.     

Diversity among Finnish net blotch isolates and resistance in barley

Summary Seedlings of a differential barley (Hordeum vulgare L.) series (21 genotypes) and of six check genotypes were used in the greenhouse to assess variation in virulence among 20 single-spore isolates of the net blotch pathogen. Pyrenophora teres Drechs. f.teres Smedeg., collected from various sites in Finland. The experiment was run twice and symptom expression was recorded on the first three leaves. Analysis of second leaf symptom scores from Run 1 indicated differences in virulence between isolates, all of which were pathogenic, and differential resistance among the barleys. The virulence of P. teres isolates appeared to be conditioned by the host barley from which the isolate derived; the average virulence of isolates collected from a susceptible host was greater than that of isolates collected from a less susceptible host. Results from Run 2 were similar regarding resistance within the barleys, but variation in virulence among the P. teres isolates was not consistent with that from Run 1. CI 9819 caries duplicate genes for resistance to P. teres (Rpt1b and Rpt1c), and CI 7548 possesses Rpt3d. Both genotypes were highly resistant to all isolates. The Rpt1a gene of Tifang (CI4407) conferred resistance to all isolates in Run 2, but only to about half of the isolates in Run 1. The checks, including two of the symptomatically most resistant Nordic barley genotypes, were universally susceptible during these stringent tests. No selective pressure has been placed on Finnish isolates of P. teres through previous deployment of major resistance genes, and it is speculated that any variation in virulence among isolates is likely to be due to a combination of evolutionary forces including, natural selection, random genetic drift and gene flow.     

Isozyme variation in a number of populations of Theobroma cacao L. obtained through various sampling regimes

Summary Isozyme polymorphism in four enzyme systems was used to estimate the genetic diversity in nine populations of Theobroma cacao L. The distribution of allelic variation observed in this study was inconsistent with the currently held view that the Upper Amazon region of Peru is the centre of cacao genetic diversity. All the populations analysed from this area of Peru were genetically similar, and contained a low level of isozyme polymorphism. It is argued that this is a result of genuine low diversity in the region rather than a sampling artifact. If a centre of diversity of wild cacao truly exists, this work indicates that it lies further north in Ecuador and Colombia.Although they are morphologically highly variable, Trinitario cacao populations derived from cultivation are currently accorded low priority for conservation. This phenotypic variation was observed to be reflected in high levels of isozyme polymorphism. It is suggested that the present strategy for the conservation of cacao germplasm, which accords priority to the Upper Amazon region of Peru while neglecting the Carribbean should be reconsidered.     

Repeatability and minimum harvest period of cacao (Theobroma cacao L.) in Southern Bahia

Classically, selection for superior genotypes in cacao has been based on the successive harvest records across a number of years. Little information on the minimum duration of these harvest periods is available in the literature. The repeatability coefficient (ρ) was used to estimate this period. Twenty five cacao genotypes were assayed in a randomized block design with four replications and 16-plant plots. The following yield components were studied: number of healthy fruits per plant, number of collected fruits per plant, weight of humid seeds per plant and per fruit, and percentage of diseased fruits per plant, over 5 years (1986–90). Repeatability estimates were higher than 0.84 for all components, except percentage of diseased fruits per plant (^ρ - 0.41). With such estimates, it is possible to select genotypes on the basis of only two years of successive harvests, with a determination coefficient of 90%. The advantages of applying the repeatability coefficient to the cacao breeding program are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Variation in virulence and resistance in the bean-bean rust pathosystem in Ecuador

Twenty two Uromyces appendiculatus isolates were tested on 20 differential and 25 Ecuadorian Phaseolus vulgaris cultivars in the seedling stage. Based on the infection types 20 races could be discerned. The Ecuadorian cultivars differed greatly in their reaction to the isolates, from resistant to only one isolate (`Red Small Garden') to resistant to all isolates (’G2333‘). The isolates showed a wide range of virulence to the Ecuadorian cultivars, from virulent to only two cultivars (isolate 13) to virulent to 21 cultivars (isolates 5 and 23). Seven cultivars with a basically susceptible infection type appeared to differ greatly in quantitative resistance when tested in three consecutive crop cycles. The disease severities in percentage leaf area affected averaged over the three cycles ranged between 83.9% for ‘Red Small Garden’ and 13.1% for ‘INIAP-414’. Race-specific resistance does not seem an advisable breeding strategy, but the quantitative resistance offers a good alternative.     

Increasing cocoa butter yield through genetic improvement of Theobroma cacao L.: Seed fat content variability, inheritance, and association with seed yield

Cocoa butter is the major commercial product from seeds of Theobroma cacao (cacao). However, most cacao breeding programs have only considered as selection criteria total seed yield per plant and disease resistance. Progress in cocoa butter yield might be possible if variability for fat content exists and its associations with the other production components are established. The average fat content in dry unfermented seeds for 490 accessions of the Centro de Pesquisa do Cacau germplasm collection in Ilhéus, Bahia, Brazil was 53.2%, ranging from 45.4% in CC 57 to 60.3% in NA 312. Fat content averaged over series indicated that genotypes originally collected in the Upper Amazonian region had higher fat levels, while the Trinitario-Criollo and Bahian genotypes tended to have lower fat content. When considering total fat yield plant^-1 an opposite trend arose with the series with low fat content having the highest total fat yield. The correlation between fat content and fat yield plant^-1 was not significant, while there was a significant negative correlation between dry seed yield plant^-1 and fat content. Despite the significant negative association between fat content and dry seed yield plant^-1, the correlation coefficient was rather low, and genotypes could be identified with both high fat content and dry seed yield. A significant pollen effect on fat content was observed based on a complete diallel crossing scheme between three genotypes with high and three with low levels. The average fat content values in progenies were similar to the average of the parents, and the segregation range transgressed the distribution of the parents. This revised version was published online in August 2006 with corrections to the Cover Date.     

Expression of resistance to downy mildew at cotyledon and adult plant stages in Brassica oleracea L.

This research compared the expression of resistance to downy mildew at cotyledon and adult plant stages in seven Brassica oleracea genotypes against two P. parasitica isolates of different virulence. Seven day old seedlings were dual inoculated under controlled environment by depositing two 10 μl droplets of a spore suspension of a different isolate on each cotyledon and the interaction phenotype (IP) evaluated 7 days later using a 0–5 scale of increasing susceptibility. The seedlings were transplanted to 16 cm pots and grown in the greenhouse for 110 days (15 to 28 leaves). Adult plants were tested using a single leaf inoculation method that allowed the same plant to be simultaneously inoculated with the two P. parasitica isolates. Leaves were scored 10 days after inoculation using a 0–5 scale of increasing susceptibility. The inoculation of the same plant with the two isolates produced different combinations of cotyledon (CT) and adult-plant (AP) interaction-phenotype according to the genotype. CrGC 3.1 was susceptible at CT and AP stages, ‘Algarvia’ resistant at CT and AP stages, and broccoli ‘A’ susceptible at CT and resistant at AP stage against the two isolates. ‘Murciana’ and broccoli ‘B’ were differential at CT and AP stages. Savoy and Shetland cabbages were differential at CT stage and resistant to both isolates as adult plants. Cotyledon resistance could not be used to predict adult-plant resistance since the two types of resistance were very poorly correlated. Resistance can be race specific either at cotyledon or at adult-plant stage. This revised version was published online in July 2006 with corrections to the Cover Date.