Symbiotic variability in Vicia faba. 2. genetic variation in Vicia faba

Summary Eight varieties of Vicia faba were examined in association with a standard strain of Rhizobium leguminosarum and also with the application of mineral nitrogen. Large differences in dry matter yield, nitrogen percentage and total nitrogen uptake were apparent between varieties and between rhizobium and mineral nitrogen treatments; the interaction was also significant. Genotypic differences in both the effectiveness of the symbiosis with this strain of rhizobium and the efficiency of utilization of nitrogen were thus demonstrated. The implications of this variability on the assessment of varieties and the possiblity of exploiting it by plant breeding techniques are discussed.     

Symbiotic variability in Vicia faba. 3. Genetic effects of host plant,rhizobium strain and of host × strain interaction

Summary Six Vicia faba populations were grown in all possible combinations with six Rhizobium leguminosarum strains in order to estimate the relative importance of the three genetic components of symbiotic variability. Additive genetic effects of host genotype and rhizobium genotype accounted for only 8.9% and 11.8% of the total phenotypic variation. Non-additive variation attriabutable to specific host genotype × rhizobium genotype interactions was by far the largest component of variation and accounted for 73.8% of phenotypic differences. Therefore the greatest improvement in symbiotic nitrogen fixation is likely to arise from simultaneous selection of both symbionts.     

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.     

Screening of Portuguese cole landraces (Brassica oleracea L.) with Leptosphaeria maculans and Xanthomonas campestris pv. campestris

Summary Fifty-six landraces of Portuguese coles (Brassica oleracea l.), representing fourteen morphotypes, were screened for resistance to Leptosphaeria maculans at the cotyledon stage. Three isolates of the pathogen belonging to three pathogenicity groups were used in the screening. The mean disease interaction phenotype of the B. oleracea landraces with the L. maculans isolates confirmed their differences in pathogenicity. When compared to pathogenicity grouping using a B. napus differential set, a different classification was observed on B. oleracea for the three L. maculans isolates, suggesting possible differences in the genetic interaction between the two plant species and the pathogen. Most of the Portuguese cole landraces were found to be susceptible to all three isolates of L. maculans, although a few have shown intermediate interaction phenotype. Some accessions, on the other hand, elicit a differential reaction with the three isolates tested, and can be further selected to be used in host differential sets or in breeding for resistance to black leg.Fifty-four landraces of Portuguese coles, representing eleven morphotypes, were also screened in the field for resistance to Xanthomonas campestris pv. campestris. A good level of resistance to the bacterium was found in several of the morphotypes.Accessions of the Penca type, particularly, were identified as highly resistant. Landraces belonging to the same Portuguese cole group presented a similar range of mean interaction phenotype with the bacterium. Some Portuguese landraces of B. oleracea are potential sources for use in breeding programs for black rot resistance.     

Stable resistance in barley to Pyrenophora teres f. teres isolates from the Nordic-Baltic region after increase on standard host genotypes

Results from tests of a mixture of Finnish net blotch, Pyrenophora teres Drechs. f. teres Smedeg., isolates on a differential series of barley seedlings, comprising 17genotypes, indicated that patterns of infection response (IR)and percentage leaf area damaged (PLAD) were unaffected by differences in seedling size. Variation of the concentration of inoculum between 1,250 conidia ml^-1 and 20,000 conidia ml^-1 produced similar patterns of IR and PLAD on the differential series. IR and PLAD scored on the second seedling leaf differentiated resistance to P. teres f. teresamong the genotypes better than on the first seedling leaf. Ina second experiment, 120 single-spore P. teres f. teres isolates from Finland, Sweden, Norway, Latvia, Estonia and Ireland were used in tests conducted in the greenhouse to differentiate them in terms of virulence reaction on seedlings of six differential barley genotypes. Each isolate was tested directly following isolation from the leaf material and after having passaged each through barley cvs. Arve or Pohto, to produce 360 isolates in total. Virulence of the isolates differed significantly on the members of the differential series, but differences associated with country of origin and passaging, and interactions, were small. It is concluded that little variation between virulence of P. teres f. teres isolates is evident over a large geographic area, incorporating Nordic and Baltic countries, and Ireland. Barley genotype response to P. teres f. teres appeared to be of more significance than relative virulence of the pathogen isolates. This could simplify breeding barley for improved resistance to this phytopathogen. This revised version was published online in August 2006 with corrections to the Cover Date.     

Pathogenic variation among isolates of Rhynchosporium secalis from barley grass growing in South Eastern Australia

Summary The pathogenicity of 182 single spore isolates of Rhynchosporium secalis from Hordeum leporinum and 94 isolates from H. vulgare collected from throughout southeastern Australia was tested on 15 barley varieties, each having different combinations of resistance genes. Forty five percent of the barley grass isolates were pathogenic on 5 or more varieties but only 6% of the cultivated barley isolates attacked this range of varieties. On the basis of reaction type 20 different pathogenicity groups were recognised, with barley grass isolates being classified into 19 and the cultivated barley isolates into 5, four of which were the same as the barley grass isolates. Numerical analysis of data on leaf area damage inferred 33 groups, 24 of which were unique to barley grass isolates, two to cultivated barley isolates and 9 common to both groups. There was as much variation in pathogenicity among single spore isolates from the same lesion as between isolates from different lesions collected from the same or different locations.     

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.     

Resistance in Lactuca spp. to Microdochium panattoniana (lettuce anthracnose)

Summary Four hundred and forty-nine lines of Lactuca spp. were screened for resistance to an isolate of Microdochium panattoniana. Eighty-seven lines were resistant. Forty-four lines were then screened for resistance to four isolates; twelve groups of lines were identified on the basis of their pattern of resistance. A differential set of lines was used to analyse nine further isolates; this revealed five races of M. panattoniana. Salad Bowl was the only cultivar found to be resistant to three races. A line of L. saligna, UC83US1, was the only line found to be resistant to all the isolates tested.     

Genes for resistance in barley to Finnish isolates of Rhynchosporium secalis

Summary Twenty Finnish isolates of Rhynchosporium secalis (Oud.) J.J. Davis, the causal agent of scald, were taken from infected barley (Hordeum vulgare L.) plants and inoculated on to seedlings of a differential series of barley containing a range of major genes for resistance to the fungus, as well as on to six Nordic 6-row spring barleys and three winter ryes (Secale cereale L.). These fungal isolates derived from four sites and three host varieties. Disease development was monitored on two leaves of seedlings in the greenhouse employing a standard scale, and on adult plants in the field by assessing the diseased area on the three uppermost leaves. A comparison was also made between the pathogenicity and virulence of ten Finnish and ten Canadian R. secalis isolates. The Finnish isolates varied in virulence, but with the exception of Algerian (CI 1179) seedlings and adult La Mesita (CI 7565) all seedlings and adult plants of the entire differential series were resistant to all isolates. Canadian isolates were, on average, less virulent than Finnish isolates. All the Nordic checks were susceptible to all Finnish and seven Canadian isolates, but differences in the degree of susceptibility were evident. Isolates of R. secalis from barley were non-pathogenic on rye, isolates from Elymus repens L. were non-pathogenic on barley and rye, and isolates from rye were only pathogenic on rye. Finnish R. secalis isolates contain no redundant pathogenic diversity. The differential series represents a useful, but as yet untapped, source of resistance to R. secalis for Finnish barley breeders.     

Resistance of Coffea arabica cv. Ruiru 11 tested with different isolates of Colletotrichum kahawae, the causal agent of coffee berry disease

Seven single conidia isolates of Colletotrichum kahawae varying in pathogenicity were used to inoculate hybrid progenies from 66 crosses ofCoffea arabica cv. Ruiru 11. The objective of this study was to investigate the effect of pathogen variation on resistance of the Ruiru 11 cultivar. The main effects of crosses and isolates were significant (p ≤0.05) while their interaction effects were non-significant. Partitioning variance components indicated that the proportion of phenotypic variance for resistance that is due to genetic effects was low. It was concluded that variation for resistance among hybrid progenies of the Ruiru 11 cultivar was probably due to differences in aggressiveness of the pathogen as reflected by the significant main effects of crosses and isolates in combination with other environmental factors which influence disease epidemics. The coffee berry disease pathogen is unlikely to have adapted to the cultivar because of the non-significant crosses × isolates interaction effects. This revised version was published online in August 2006 with corrections to the Cover Date.     

In vitro techniques for selecting wheat (Triticum aestivum L.) for Fusarium-resistance. II. Culture filtrate technique and inheritance of Fusarium-resistance in the somaclones

Summary Calli of resistant, intermediary and susceptible wheat (Triticum aestivum L.) varieties were selected using culture filtrates of Fusarium graminearum and F. culmorum and the regenerants were evaluated for resistance up to R₃. Czapek-Dox broth medium was inoculated with mycelia of Fusarium isolates and incubated for 2–6 weeks. Filtrates were added to MS callus growing medium, then 5 weeks-old calli were transferred onto this medium (MST) for 4–5 weeks. MST containing 30% filtrate was found to be suitable for selection. Resistant calli were transferred again to fresh MST for further two selection cycles. The surviving calli produced less fertile regenerated lines (R₀) than the non-selected ones. Among 18 R₁ lines tested for Fusarium-resistance in the seedling stage by artificial inoculation in the greenhouse, two (11.1%) were significantly more resistant, one (5.6%) was more susceptible than the original cultivar and the rest (83.3%) behaved similarly to the donor plants. Among unselected R₃ lines of three varieties, practically the same number of resistant plants were found as among the related selected ones. When the R₃ selfed generations obtained through double-layer and culture filtrate selection techniques were tested for Fusarium-resistance, 35.7% of the lines were found to be more resistant than the original cultivars, none was more susceptible and 64.3% had a reaction similar to that of the source materials. Thus, inheritance of the disease reaction was not stable in all cases. Success of in vitro selection for Fusarium-resistance depended also on the genotype, and toxin analysis showed that although being effective, the selective media contained deoxynivalenol only exceptionally. In selecting wheat for Fusarium-resistance in vitro, the culture filtrate technique proved better than the double-layer procedure.     

Pathogenic variation among isolates of Rhynchosporium secalis from cultivated barley growing in Victoria,Australia

Summary Three hundred and nineteen Rhynchosporium secalis isolates from cultivated barley were divided into five groups on the basis of their virulence on 15 differential barley varieties. Pathogenic variation was also demonstrated for isolates from different scald lesions within the same crop and amongst different spores from the same lesions.     

A set of differential Glycine hosts for the identification of races of Phakopsora pachyrhizi Syd.

Summary Variation in the responses of a wide range of accessions of four native Australian species of Glycine (viz. G. canescens, G. clandestina, G. tabacina and G. tomentella) to infection by eight Australian isolates of Phakopsora pachyrhizi was analyzed.Differences in the infection type responses of the various wild Glycine species were sufficient to recognize six different virulence combinations amongst the eight pathogen isolates.A set of differential hosts useful in the identification of different races of the pathogen is presented to facilitate further examination of the virulence structure of the pathogen population.     

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.     

Variations in pathogenicity patterns ofMycosphaerella graminicola withinTriticum spp. in Israel

Summary Pathogenicity patterns of 42Mycosphaerella graminicola (Septoria tritici) isolates secured from the major wheat growing regions in Israel, were assessed on seedlings of 16 bread and durum wheats. The spring bread wheat cultivar Titan (CI12615), the winter bread wheats Bezostaya 1, Kavkaz, NE7060 and the durum wheats Nursit 163 and Zenati Bouteille, all exhibited a high level of resistance. Significant cultivar × isolate interactions were recorded for isolates from the different regions in relation to the response of the 16 cultivars organized in 6 reponse classes to the test isolate ISR8036. Number of genes for resistance in the 16 cultivars was estimated, based on the assumption of a gene-for-gene relationship. Twelve complementary genes were hypothesized in the 42M. graminicola isolates × 16 wheat cultivars matrix. There was considerable variation in virulence frequencies between regions and between locations within the same region. The overall frequency of Bezostaya-Kavkaz virulence was low throughout the country (5–7%). The frequency of designated hypothesized virulence genes VST OLAF, VST COLOTANA, VST IAS 20 in Israel, was high in most locations. The reported analytical approach enables the identification of virulence hot-spots and assists in designing breeding for resistance strategies.     

Inheritance of resistance to angular leaf spot in Nicotiana tabacum L. cultivars burley 21 and kentucky 14

Summary The genetics of resistance to angular leaf spot caused by Pseudomonas syringae pv. tabaci in Nicotiana tabacum cultivars Burley 21 and Kentucky 14 was investigated by studying disease reactions to three isolates of parental, F₁, F₂ and backcross generations derived from crosses between the resistant cultivars and the susceptible cultivar Judy's Pride. Studies were conducted in the greenhouse and in field plant beds. Chi-square values were computed to determine whether the observed ratios for disease reactions deviated from expected Mendelian ratios for a single, dominant gene controlling resistance to angular leaf spot in tobacco. Based on the resistance of the F₁ and the backcross generation to the resistant parent (BC-R), a 3 resistant: 1 susceptible segregation ratio in the F₂, and a 1 resistant: 1 susceptible segregation ratio in the backcross generation to the susceptible parent (BC-S), it was concluded that resistance to three isolates of Pseudomonas syringae pv. tabaci is governed by a single, dominant gene.     

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.     

Evaluation of the virulence of races 1, 2 and 4 of Fusarium oxysporum f.sp. dianthi in carnation

Summary A simultaneous analysis of the virulence of races 1, 2 and 4 of Fusarium oxysporum f.sp. dianthi to a series of nine carnation cultivars revealed the presence of different interactions between races and cultivars, as well as differences in pathogenesis between race 1 on the one hand and race 2 and 4 on the other.The most common race 2 induced typical symptoms of Fusarium wilt in all susceptible cultivars. The cultivars showed considerable variation in resistance to race 2. Only Novada remained free of external symptoms throughout the experiment. In diseased plants of all cultivars studied, infected vascular tissue was white with dark brown margins, and heavy degradation of the cell walls and xylem parenchyma cells had occurred. All Dutch isolates corresponded with race 2.Race 4 induced wilt symptoms similar to those induced by race 2, and there was a similar variation in resistance to race 2 and 4 in the cultivars. On average, the race 4 isolates were less aggressive than those of race 2. Compared with race 2, there was evidence of some genotype × race interactions: Pallas proved to be considerably more susceptible, and Lena more resistant to race 4 than to race 2. The isolates of race 4 induced a nistopathology similar to that induced by race 2, but with less vascular browning.Race 1 induced atypical but severe wilt symptoms and unusual vascular discoloration in Elsy, Niky and Sam's Pride only. The vascular tissue in these cultivars turned pale brown; in spite of heavy colonization of these tissues virtually no degradation of cell walls was observed. All other cultivars tested proved virtually resistant to race 1, providing further evidence for genotype × race interactions.Within races, limited but statistically significant genotype × isolate interactions were found as well, in particular within race 4. These are tentatively attributed to independent variation of two (or more) resistance components.     

Assessment of quantitative resistance of rice cultivars to Xanthomonas campestris pv. oryzae: A comparison of assessment methods

Summary Lesion length, leaf length, and leaf width were measured on infected leaves two weeks after clip inoculation of 64 rice cultivars with two virulent isolates of Xanthomonas campestris pv. oryzae (X.c. pv. oryzae). No significant correlation was found between the lesion length and the leaf dimensions, indicating that physical leaf size does not affect the spread of the bacteria once these have entered the leaf. Lesion length is therefore an acceptable parameter for assessing resistance to (X.c. pv. oryzae), and is to be preferred above the parameter percentage diseased leaf area (% DLA), especially when small differences between genotypes are to be assessed. The confounding influence of differences in leaf length can cause large changes in the ranking order of cultivars when assessed by the % DLA. For this reason lesion length is a better assessor of the value of a quantitative resistance for breeding and research purposes than % DLA.     

Variation in the expression of specificity in two maize diseases

Summary Ten inbred lines from the open-pollinated maize variety Jarvis were selected from 51 randomly collected lines to represent a wide range of susceptibility to one isolate each of Bipolaris maydis or Colletotrichum graminicola. Ten isolates of each pathogen were selected for a range of virulence on a maize line with average resistance. Resistance and virulence ratings were based on lengths of lesions that developed on leaves of greenhouse-grown seedlings inoculated with 5 l droplets of suspensions of known spore concentrations. For each disease the ten maize lines were inoculated in all possible combinations with the ten pathogen isolates. The experiment was run six times with each pathogen. Analysis of variance for individual trials indicated a significant interaction between maize lines and B. maydis isolates in all six trials and between maize lines and C. graminicola isolates in four of six trials. For both diseases. the combined analysis over all six trials revealed no significant interaction. Apparently the expression of specificity in these host-pathogen interactions is variable.