Genetic analyses of resistance of the wheat differential cultivars Carstens V and Spaldings Prolific to two races of Puccinia striiformis

Genetic analysis of resistance of wheat seedlings to two races of Puccinia striiformis was conducted on F₁, F₂ and F₃ generations from crosses Carstens V (CV) × Lee, Spaldings Prolific (SPA) × Lee and CV × SPA. F₂ generations from crosses of CV and SPA with Strubes Dickkopf (SD) were also studied. The plants were classified into six resistance classes and analysed by factorial correspondence analysis and nonhierarchical classification. The two P. striiformis isolates tested were a French isolate of race 43E138 and a Lebanese isolate of race 2E16, selected for the differences in their virulence spectra for the common differential cultivars Strubes Dickkopf and Nord Desprez. Resistance of CV and SPA was recessive and dominant to races 43E138 and 2E16, respectively. CV possessed three or four resistance genes, one of them being expressed with both races. Two genes of CV had a cumulative effect for resistance to 43E138 and two or three gave dominant resistance to 2E16. SPA had three resistance genes, all of which gave resistance to 2E16 and two of which also gave resistance to 43E138. SPA had one gene in common with CV for resistance to both races. Furthermore, the gene for resistance to race 2E16 in CV and SPA was allelic with a gene in SD, and was probably Yr25 .     

Genetics of resistance to cotton leaf curl disease in Gossypium hirsutum

Twenty-two cotton varieties were screened for resistance to cotton leaf curl disease (CLCuD), a disease of viral origin, using three procedures: field evaluation, whitefly transmission assay and graft inoculation. Viral infection of cotton varieties was determined by visual symptom assessment as well as dot-blot and multiplex PCR diagnostic techniques. Crosses were made between the most susceptible variety (S-12) and highly resistant varieties (CP-15/2, LRA-5166 and CIM-443). All F₁ plants of these crosses were resistant, showing dominant expression of the resistance as well as the absence of extrachromosomal inheritance. The F₂ plants of the crosses CP-15/2 × S12, LRA-5166 × S-12 and CIM-443 × S12 exhibited a ratio of 13 resistant (symptomless) to three susceptible (with symptoms). Screening of the F₂ generation for virus infection by multiplex PCR further subdivided the resistant class into those exhibiting a high level of resistance (HR; PCR-negative) and those exhibiting resistance (R; symptomless, yet showing virus replication by PCR analysis). Hence, the final ratio was 3:10:3 (HR:resistant:susceptible). The F₃ progeny of susceptible F₂ plants segregated for resistance, indicating the probable presence of a suppressor gene ( S ). These findings are consistent with three genes being involved in G. hirsutum resistance to CLCuD, two for resistance ( R _1CLCuDhir and R _2CLCuDhir ) and a suppressor of resistance ( S _CLCuDhir ).     

Complementation between genes for resistance to race 1 of Fusarium oxyspomm f.sp. ciceri in chickpea

The reactions of parents and F₁ and F₂ generations of crosses of chickpea cultivars K-850 with C-104 and JG-62 and F₃ progenies of K-850 × C-104 to race 1 of Fusarium oxysporum f.sp. ciceri were studied. The results indicate that K-850 carries a recessive allele for resistance at a locus different from and independent of that carried by C-104 and recessive alleles at both loci together confer complete resistance. The possible contribution of this recessive gene to late wilting in K-850 is discussed. These observations have important implications in breeding for resistance to wilt in chickpea.     

Specific resistance to soybean stem canker conferred by the Rdm4 locus

The Rdm4 gene from soybean cv. Hutcheson has been extensively used to incorporate resistance to soybean stem canker (SSC), caused by Diaporthe phaseolorum var . meridionalis (Dpm), into soybean commercial cultivars. The objective of this work was to characterize the inheritance of the Rdm4 locus in different populations derived from the cross: J77-339 ( rdm / rdm , susceptible) × Hutcheson ( Rdm4 / Rdm4 , resistant) in independent interactions with two local isolates of Dpm. Four F₂ populations were obtained and two were advanced to the F₃ generation as separate F_2:3 families to perform progeny tests. Each population was inoculated with the CE109 and/or CE112 isolates of Dpm. Within each plant–pathogen interaction, the resistance gene segregated as completely dominant. However, cross resistance, or opposite disease reactions, to CE109 and CE112 isolates of Dpm were observed in four F_2:3 families, indicating an intergenic recombination event between two nonallelic genes interacting specifically with each isolate of Dpm. The distance between them, estimated as the recombination fraction, was 29%, suggesting that both genes were not tightly linked, but close enough to segregate together in most crosses. Results indicated the existence of a genomic region in cv. Hutcheson composed of race-specific resistance loci with at least two Rdm genes: the previously recognized Rdm4 and a novel gene, tentatively named Rdm5 , conferring specific resistance to Dpm isolates CE109 and CE112.     

Transgressive segregation for resistance to Pyrenophora teres in barley

Populations of F₁, F₂, F₁× parent 1 (BC1), F₁× parent 2 (BC2), and F₃ resulting from two crosses between four susceptible varieties of barley used in national and international breeding programmes were tested at the seedling stage for their resistance to an isolate of Pyrenophora teres which is virulent to the parental varieties. Infection type, average lesion size, and number of lesions per unit leaf area were used to assess disease reaction. In the two crosses, F₂ and especially F₃ generations (produced by self pollination of selected resistant F₂ plants) showed more resistance expressed by infection type and average lesion size than either parent. The frequency distributions of F₂ and F₃ generations derived from these crosses were continuous and showed transgressive segregation for resistance. The results indicated that the four cultivars used in this study possessed additive genes for resistance to P. teres. Thus useful resistance could be obtained following crossing of susceptible cultivars and selection in succeeding generations.     

A new blast resistance gene identified in the Indian native rice cultivar Aus373 through allelism and linkage tests

Blast, caused by Pyricularia grisea , is a major constraint on rice production. To widen genetic diversity for disease resistance, the Indian native rice cultivar Aus373 was screened by F₂ segregation analyses to investigate the genetic basis of its high resistance. Aus373 was crossed with a series of Japanese differential cultivars (JDCs) and the Chinese susceptible cultivar Lijiangxintuanheigu (LTH). The resistance ratios of subsequent F₂ progenies were used to determine the number of blast-resistance loci present as well as allelic relationships with known loci. Resistance of Aus373 was governed by dominant alleles at two loci, one at the Pi-k locus and the second apparently at a new locus linked to an isozyme gene Amp-1 with a recombination fraction of 37.9 ± 3.0% on chromosome 2. This putative new locus and allele were designated Pi16 (t).     

Cultivar reactions and the genetic basis of resistance to alternaria stem canker (Alternaria alternata f.sp. lycopersici) in tomato

The reactions of several tomato cultivars and the genetic basis of resistance to an isolate of Alternaria alternata f. sp. lycopersici were examined. Among the 105 cultivars tested only two, Regal and Toper, were susceptible, while the remainder were highly resistant. The resistant cultivar ACE 55 VF (P_;) was crossed with the susceptible cultivar Earlypak No. 7 (P₁)- When F₁, F₂, F₃, BC₁, BC₁ selfed, F₂ X P₂ and BC₁P₂ x P₁ progenies were inoculated at the two-leaf stage with a suspension of spores of the pathogen, the ratios of resistant to susceptible plants indicated that resistance was conferred by a single dominant gene.     

Identification and mapping of a rice blast resistance gene Pi-g(t) in the cultivar Guangchangzhan

A single dominant blast resistance gene was identified in Chinese indica rice ( Oryza sativa ) cv. Guangchangzhan (GCZ), which shows complete resistance to Japanese isolate Ken53-33 of Magnaporthe grisea . Genetic analysis of the backcross (BC₁) and second-generation (F₂) populations from a cross between susceptible cv. Lijiangxintuanheigu (LTH) and GCZ indicated that the resistance was conferred by one dominant gene. This gene was mapped on the long arm of chromosome 2 and flanked by RM166 and RM208 at distances of 4·00 ± 4·90 and 6·30 ± 4·89 cM (centiMorgans), respectively. It was designated tentatively as Pi-g(t) .     

Occurrence and inheritance of resistance to powdery mildew (Oidium lycopersici) in Lycopersicon species

Among 146 accessions of Lycopersicon pimpinellifolium , 132 of L. esculentum var. cerasiforme and 53 of L. peruvianum screened for resistance to powdery mildew, caused by Oidium lycopersici , a wide variation in reactions was found. Two plants of L. esculentum var. cerasiforme accession LA-1230 were resistant. One resistant symptomless plant of accession LA-1230, designated LC-95, produced homozygous resistant progenies. LC-95 was crossed with cv. Marmande (susceptible parent) and F₁, F₂ and backcrosses to the resistant and the susceptible parents were derived. These genotypes were grown in glasshouses at 23°C and 95–100% RH and inoculated with O. lycopersici . The F₁ plants were susceptible. F₂ and backcross segregations fitted the hypothesis of a single recessive gene which is here designated ol-2 .     

Evaluation of carrot resistance to alternaria leaf blight in controlled environments

The objective of this study was to find a technique for plant resistance screening to alternaria leaf blight (ALB), caused by the fungus Alternaria dauci , in controlled environments. Glasshouse and laboratory screening methods were compared using three cultivars and F₂ genotypes segregating for ALB resistance evaluated against self-pollinated F₃ field-grown plants. Plant disease was assessed through a disease index obtained from the size and number of symptoms on carrot leaves. The results indicated the value of glasshouse evaluation and the inadequacy of detached leaf and hypocotyl assays for carrot screening for ALB resistance. Spearman's rank correlation, applied to results obtained with both F₂ plants and their progeny, indicated that the optimal evaluation stage for ALB resistance in carrot is 20 days after inoculation. This test was powerful enough to be used as a prescreening test in breeding programmes.     

Genetics of avirulences in Erysiphe graminis f.sp. hordei

The genetics of avirulences towards barley mildew resistances were analysed in crosses of the Ervsiphe graminis f.sp. hordei isolate DH14 with CC107 and with CC138. Nine avirulences, Av r_a9, Avr _a10, Avr _a11, Avr _a12, Avr _Ab, Avr _CP, Avr _h, Avr _k and Avr _La, segregated as single genes in one or other cross. However. F₁ segregation data were consistent with avirulence matching the Mla7 resistance gene being controlled by two genes, designated Avr _a7 1 and Avr _a7 2. Infection types of avirulent isolates differed on varieties in which Mla7 had been derived from each of the four known sources of that resistance. Linkage was detected between Avr _a7 1 and Avr _h in the cross CC107 × DH14, and between Avr _a10 and Avr _k, Avr _a11 and Avr _La, and Avr _h and the triadimenol response gene Tdl2 in CC138 × DH14.     

Monogenic resistance to a new fungicide, JS399-19, in Gibberella zeae

The fungicide JS399-19 is a novel cyanoacrylate fungicide active against Gibberella zeae , and has been marketed in China for control of fusarium head blight (FHB) on wheat. Forty-three isolates sensitive to fungicide JS399-19 were collected from three commercial wheat fields in China. Forty-five isolates resistant to JS399-19, obtained from five sensitive isolates by selection for resistance to JS399-19, were selected. Three sensitivity levels were identified: sensitive (S), moderately resistant (MR) and highly resistant (HR) to JS399-19, based on a previous study. Eight isolates representing the three sensitivity-level phenotypes were randomly selected for a study on the inheritance of JS399-19 resistance by analysing the sensitivity of hybrid F₁ progeny. A nitrate-non-utilizing mutant ( nit ) was used as a genetic marker to confirm that individual perithecia were the result of outcrossing. Five crosses were assessed: S × S, S × HR, MR × HR, HR × HR and MR × S. In crosses between parents with different sensitivity levels, such as S × HR, MR × HR and MR × S, the progeny fitted a 1:1 segregation ratio of the two parental phenotypes. No segregation was observed in the crosses S × S and HR × HR. It was concluded that the MR and HR phenotypes in G. zeae were conferred by different allelic mutations within the same locus. In these isolates, resistance to JS399-19 was not affected by modifying genes or cytoplasmic components.     

Genetic variation in Danish populations of Erysiphe graminis f.sp. hordei: estimation of gene diversity and effective population size using RFLP data

Genetic variation of the barley powdery mildew fungus ( Erysiphe graminis f.sp. hordei ) was estimated in three Danish local populations. Genetic variation was estimated from the variation amongst clones of Egh , and was therefore an estimate of the maximum genetic variation in the local populations. The average gene diversity, Ĥ , was estimated as 0.84. The effective population size was estimated as: log₁₀ ( N^ _e ) = 0.64 − log₁₀(μ), or 4.4 × 10⁹, assuming a nucleotide mutation rate ( μ) of 10⁻⁹ per base per generation. There was no significant genetic differentiation between locations.     

A qualitative quick-test for detection of herbicide resistance to tribenuron-methyl in Papaver rhoeas

A qualitative seed-based method useful for the detection of resistance to the herbicide tribenuron-methyl in Papaver rhoeas L. is described. Seeds were germinated on 35 mL of a 1.3% agar medium containing 2 g KNO₃ L^–1 in 8.5 cm Petri dishes in a growth chamber under 20 μmol s^–1 m^–2 of fluorescent light. When 0.24 μM tribenuron-methyl or more was added, growth in susceptible plants stopped after the cotyledon stage and they turned chlorotic. The resistant plants continued developing new leaves. The same effect was achieved when 0.2 g gibberellin (GA₃) L^–1 and 7.68 μM tribenuron-methyl or 0.5 g GA₃ L^–1 and 61.44 μM tribenuron-methyl were added. Germination percentage rose with gibberellin in the presence or absence of the herbicide. Plants developed rapidly, with only about 14 d needed to finish the test but sometimes root growth was reduced because of the addition of gibberellin. In the absence of gibberellin but in the presence of the herbicide, plants grew more slowly and developed smaller leaves with a 17-d evaluation period requirement. The test was validated with pot experiments in a greenhouse and also with field trials. The best combination was found to be 0.2 g GA₃ L^–1 and 7.68 μM tribenuron-methyl, assuring homogenous germination and testing of dormant seeds but avoiding root inhibition associated with too much gibberellin.     

The genetic analysis of resistance to fusarium crown and root rot of tomato

The tomato line IRB-301-31, resistant to fusarium crown and root rot ( Fusarium oxysporum f.sp. radicis-lycopersici ) was crossed with two susceptible cultivars, Motelle and Earlypak No. 7. When F₁, F₂ and backcross progenies were inoculated at the one-leaf stage with a suspension of spores of the pathogen, all could be classified as either resistant (healthy) or susceptible (dead). The ratios of resistant to susceptible plants indicated that resistance was conferred by a single dominant gene, designated Fr1.     

Evaluation of variability in Striga aspera, Striga hermonthica and their hybrids using morphological characters and random amplified polymorphic DNA markers

Striga aspera and Striga hermonthica are recognized as separate species, but their close morphological similarity causes difficulty in distinguishing between them in areas where they coexist in Africa. In this study, crosses between the species were made using randomly selected morphologically typical parental plants collected from different locations in Nigeria. Genetic analysis of both species and their reciprocal F₁ hybrids were determined using cluster analysis of DNA profiles derived from genetic polymorphism (RAPD)-polymerase chain reaction (PCR) markers. Principal component and hierarchical cluster analyses were used to separate parental and hybrid populations based on 13 morphological characteristics. Morphological data from wild samples of both species were compared with the hand-pollinated parental, F₁ and F₂ hybrids, and back-crosses. Results showed that S. aspera and S. hermonthica were genetically and morphologically distinct. Morphological and genetic analyses revealed two major clusters: a S. aspera cluster and a S. hermonthica cluster. Genetically, the F₁ hybrids showed closer affinity to their maternal parents, while morphologically, the F₁ hybrids formed distinct clusters intermediate to the parents. Most F₂ plants and back-crosses were morphologically similar to S. hermonthica . Comparative morphological analysis of wild and hand-pollinated populations showed some samples from the wild clustered with the hybrids, suggesting that hybrids may exist in nature.     

Host–parasite relationships of Meloidogyne incognita on spinach

Host–parasite relationships in root-knot disease of spinach caused by Meloidogyne incognita race 1 were studied under glasshouse conditions. Nematode-induced mature galls were large and usually contained one or more females and egg masses with eggs. Feeding sites were characterized by the development of giant cells containing granular cytoplasm and many hypertrophied nuclei. The cytoplasm in these giant cells was aggregated alongside the thickened cell walls. Stelar tissues within galls appeared disorganized. The relationship between initial nematode population density ( P _i) in a series from 0–128 eggs and second-stage juveniles per cm³ soil and growth of spinach cv. Symphony F₁ seedlings was tested under glasshouse conditions. A Seinhorst model [ y = m  + (1 −  m ) z ^P–T ] was fitted to fresh top- and total plant-weight data for inoculated and control plants. Tolerance limits ( T ) of spinach cv. Symphony F₁ to M. incognita race 1 for fresh top and total plant weights were 0·25 and 0·5 eggs and second-stage juveniles per cm³ soil, respectively. The minimum relative values for fresh top and total plant weights were zero in both cases at P _i ≥ 32 eggs and second-stage juveniles per cm³ soil. Root galling was least at low initial population densities and greatest at 16 eggs and second-stage juveniles per cm³ soil. Maximum nematode reproduction rate was 33·1-fold at the lowest P _i.     

Inheritance of resistance to carboxylic acid amide (CAA) fungicides in Plasmopara viticola

Mandipropamid is a new mandelic acid amide fungicide expressing high activity against foliar infecting oomycetes, including the grapevine downy mildew, Plasmopara viticola . Because cross-resistance with the valinamide fungicides iprovalicarb and benthiavalicarb and the cinnamic acid amide fungicides dimethomorph and flumorph was postulated, all five compounds are classified as carboxylic acid amide (CAA) fungicides. To support this classification, cross-resistance among these compounds with field isolates and the segregation of resistance in F₁ and F₂ progeny of P. viticola were evaluated. A bimodal distribution of sensitivity in field isolates and cross-resistance among all CAAs for the vast majority of isolates were detected. Crosses between sensitive (s) and CAA-resistant (r) isolates of opposite mating types, P1 and P2, yielded abundant oospores. All F₁-progeny isolates were sensitive to CAAs (s:r segregation 1:0), whereas in F₂ progeny segregation of about 9:1 (s:r) was observed suggesting that resistance to CAA fungicides is controlled by two recessive nuclear genes. Mating type segregated in a ratio P1:P2 of c . 2:1 in F₁ and 1:1 in F₂ progeny. In the same crosses, resistance to the phenylamide fungicide mefenoxam segregated in a ratio of c . 1:3:2 (sensitive:intermediate:resistant), reflecting the monogenic, semidominant nature of resistance. The risk of resistance in P. viticola was classified as high for phenylamide and moderate for CAA fungicides. This is the first report on the inheritance of phenotypic traits in P. viticola .     

Response of interspecific Brassica juncea/Brassica rapa hybrids and their advanced progenies to Albugo candida (white blister)

Transfer of factors for resistance to white blister disease caused by Albugo candida between Brassica species involving two genotypes each of B. juncea and B. rapa was studied in hybrids. More hybrids were obtained by in vivo than in vitro techniques, although an in vitro phase was a prerequisite for the establishment of in vivo hybrids. Hybrids were identified by PCR-based inter-simple sequence repeat (ISSR) markers with both male and female species-specific bands being identified. There was a positive correlation between disease severity and number of days after sowing ( r  > 0·93), the highest being towards pod formation and plant maturity at 110 days after sowing. The plants from F₂ and BC₁ progeny showed higher resistance to A. candida than either of the parents. Plants of B. juncea and B. rapa with high field resistance (disease index < 1·0) were selected from BC₂ and F₂BC₁ generations. The frequency of plants classified as resistant in BC₂ progeny ranged from 4·5 to 39·0% in cross-combinations involving B. juncea genotypes as female parent, compared with 100% in the reciprocal cross involving B. rapa as female parent.     

The identification of a gene for race-specific resistance to Peronospora parasitica (downy mildew) in Brassica napus var. oleifera (oilseed rape)

The oilseed rape cultivar Cresor was resistant to 14 isolates of Peronospora parasitica derived from crops of Brassica napus in the UK. Segregation for resistance to one isolate among F₂ plants and F₃ progeny of crosses between Cresor and the susceptible cultivars Victor and Jet Neuf indicated that resistance was controlled by a single gene. There was evidence that genetic background and environment could influence the phenotypic expression of this resistance. Two sexual progeny isolates derived from a homothallic isolate of P. parasitica avirulent on Cresor were completely virulent on this cultivar. This suggested that the parental isolate was heterozygous at a matching locus or loci for avirulence and demonstrated the race-specific nature of the resistance.