Genetic analysis of resistance to tomato late blight in Solanum pimpinellifolium accession PI 163245

Late blight (LB), caused by Phytophthora infestans, is one of the most devastating diseases of tomato (Solanum lycopersicum) worldwide. Due to the emergence of new and aggressive P. infestans isolates, identifying new genetic resistance to LB is a priority in tomato breeding. Recently, we reported the identification of several Solanum pimpinellifolium accessions with strong LB resistance. In this study, we investigated the utility of resistant accession PI 163245 for tomato breeding by examining heritability (h²) of resistance and the response to selection for resistance. Estimates of h² based on F₂ : F₃ and F₃ : F₄ parent : offspring correlation analyses averaged 0.79 and 0.94, respectively, suggesting the heritable nature of LB resistance in PI 163245. Analysis of response to selection for resistance from F₂ to F₄ generations indicated a realized h² of 0.63, confirming the utility of this resistance in tomato breeding. Two methods of estimating the minimum number of loci involved indicated the presence of one major resistance locus. Currently, genetic mapping and breeding efforts are underway to further confirm the viability of this accession for improving tomato LB resistance.     

Genetic control of resistance to Meloidogyne incognita race 1 in the Brazilian common bean (Phaseolus vulgaris L.) cv. Aporé

The use of resistant cultivars is one of the best methods for nematode control and reduction of economic losses caused by these pathogens. Studies of inheritance of nematode resistance in common bean (Phaseolus vulgaris L.) are nonetheless scarce. The present paper reports on the estimation of genetic parameters associated with resistance to the root nematode Meloidogyne incognita race 1 in common beans. Two contrasting bean lines, ‘Aporé’ (P₁ = nematode resistant) e ‘Macarr?o Rasteiro Conquista’ (P₂ = susceptible), and the generations F₁ (P₁ × P₂), F₂ (P₁ × P₂), BC₁(P₁) = (F₁ × P₁) and BC₁(P₂) = (F₁ × P₂), were assessed 45 days after nematode inoculation, through a scale related to the number of eggs per gram of root tissue. Dominant genetic effects were inferior in magnitude to additive effects, indicating incomplete dominance of nematode resistance. Dominance was in the direction of increased nematode resistance (i.e., lower number of eggs per g root). Resistance to Meloidogyne incognita race 1 in common bean is under control of a single gene locus, with incomplete dominance of the resistance allele present in ‘Aporé’, but modifier genes affecting its expression appear to be present in the susceptible parent ‘Macarr?o Rasteiro Conquista’.     

Allelic relationships at the pvr1 locus in Capsicum annuum

Diseases caused by members of the Potyviridae family currently threaten pepper crops (Capsicum annuum L.) worldwide. A series of monogenic recessive resistance genes that control potyvirus resistance at the pvr1 locus in Capsicum species are widely known and used in pepper breeding programs, each allele with a differential resistance spectrum affecting a distinct range of viral strains across three viruses, Tobacco etch virus (TEV), Pepper mottle virus and Potato virus Y. In this study, we systematically analyzed the resistant spectra, and the level of the resistance each allele confers using a set of pepper genotypes homozygous or heterozygous for the following alleles; Pvr1 ⁺, pvr1, pvr1 ¹ and pvr1 ². The resistance alleles at the pvr1 locus show recessive inheritance when combined with a susceptible allele in F1 progenies. However, our results show that resistance in this system is, in fact, not always fully recessive and establish a hierarchy of allelic interactions We identified the resistance phenotype in F₁ progenies generated by combining a complete resistant allele with an incomplete resistance allele against TEV strains implying that the resistance alleles at the pvr1 shows dominant inheritance when combined with an incomplete resistance allele. Resistance alleles against TEV-HAT, pvr1 and pvr1 ², show dominance inheritance when each is combined with pvr1 ¹. The resistance allele, pvr1, shows dominance inheritance when combined with pvr1 ² against TEV-N. These results clarify the allelic relationship between resistance alleles at the pvr1 locus displaying different resistance spectra, and will assist breeders to select the preferred combinations of the resistance alleles to obtain durable resistance against multiple potyviral strains.     

Heritability of late blight resistance in tomato conferred by Solanum pimpinellifolium accession PI 224710

Late blight (LB), caused by the oomycete Phytophthora infestans, is one of the most destructive diseases of tomato and potato worldwide. Identifying and characterizing new sources of resistance is essential given the emergence of new aggressive and fungicide‐resistant P. infestans isolates. Recently, we reported identification of several new sources of LB resistance within the tomato wild species, Solanum pimpinellifolium. In this study, we examined heritability (h²) of LB resistance conferred by the S. pimpinellifolium accession PI 224710 using a parent–offspring regression (correlation) analysis. F₂ and F₃ progeny populations, derived from crosses between PI 224710 and a LB‐susceptible tomato breeding line, were evaluated for response to LB infection. To obtain a better estimate of h², the F₃ progeny were evaluated for LB resistance in two separate replicated experiments. The h² estimates were similar in the two experiments and averaged ~0.87, suggesting that this resistance was highly heritable. Two different methods estimated involvement of one resistance locus. Breeding and mapping efforts are underway to further assess the viability of this newly reported LB resistance.     

Inheritance of resistance to the peach root‐knot nematode (Meloidogyne floridensis) in interspecific crosses between peach (Prunus persica) and its wild relative (Prunus kansuensis)

The peach root‐knot nematode, Meloidogyne floridensis (MF), infects majority of available nematode‐resistant peach rootstocks which are mostly derived from peach (Prunus persica) and Chinese wild peach (P. davidiana). Interspecific hybridization of peach with its wild relative, Kansu peach (P. kansuensis), offers potential for broadening the resistance spectrum in standard peach rootstocks. We investigated the inheritance of resistance to MF in segregating populations of peach (‘Okinawa’ or ‘Flordaguard’) × P. kansuensis. A total of 379 individuals from 13 F₂ and BC₁F₁ families were challenged with a pathogenic MF isolate “MFGnv14” and were classified as resistant (R) or susceptible (S) based on root galling intensity. Segregation analyses in F₂ progeny revealed the involvement of a major locus with a dominant or recessive allele determining resistance in progeny segregating 3R:1S and 1R:3S, respectively. Testcrosses with a homozygous‐susceptible peach genotype (‘Flordaguard’ or ‘UFSharp’) confirmed P. kansuensis as a source of new resistance and the heterozygous allelic status of P. kansuensis at the locus conferring resistance to MF. We propose a single‐locus dominant/recessive model for the inheritance of resistance.     

Inheritance of resistance to the bipartite Tomato chlorotic mottle begomovirus derived from Lycopersicon esculentum cv. ‘Tyking’

Severe outbreaks of bipartite begomoviruses (family Geminiviridae) have been observed on tomatoes after the introduction of the whitefly Bemisia tabaci (biotype B) in Brazil. The Lycopersicon esculentum line ‘TX 468-RG’ was identified as one of the best sources of broad-spectrum resistance to species comprising the tomato-infecting Begomovirus complex in Brazil. The genetic basis of resistance to one Begomovirus isolate was investigated using populations from the cross between ‘TX 468-RG’ (P₁) and the susceptible line ‘Ohio 8245’ (P₂). Parental lines, F₁, backcross (BC) to P₁ and BC to P₂ and F₂ generations were inoculated at the two true-leaf stage using 20 viruliferous whiteflies per plant. Assessment was done two weeks after inoculation based upon visual analysis of symptom expression. The ratio of resistant to susceptible plants closely fit to a single recessive gene (locus) model. The sequence analysis indicated that the Begomovirus isolate used in this assay was closely related to the bipartite Tomato chlorotic mottle virus. Therefore, this gene/locus, was tentatively named tcm-1 (tomato chlorotic mottle virus resistance-1). This locus has been transferred to distinct tomato cultivars and levels of resistance similar to that of ‘TX 468-RG’ were observed in advanced (F₈ and F₉) generations. In addition, breeding lines carrying the tcm-1 locus were also resistant to other Brazilian bipartite tomato-infecting Begomovirus species.     

Identification and genetics of resistance to cercospora leaf spot (<Emphasis Type="Italic">Cercospora zonata</Emphasis>) in faba bean (<Emphasis Type="Italic">Vicia faba</Emphasis>)

The fungal disease cercospora leaf spot CLS (Cercospora zonata) has affected major faba bean (Vicia faba) production regions in southern Australian in the last several years. This study offers the first report of sources of resistance to CLS in faba bean and describes techniques to evaluate resistance to C. zonata in faba bean genotypes within a controlled environment. The method was rapid (43 days), repeatable (R ² > 0.74) and demonstrated positive correlations (R ² > 0.45–0.80) to data collected from field disease nurseries under naturally established CLS epiphytotics. All faba bean cultivars currently adopted by the Australian industry were found to be susceptible to CLS and defoliation was found to be an important component of disease expression. Genetic analysis of segregation patterns in F ₂ derived F ₃ families of 1322/2*Farah (resistant*susceptible) showed the mode of inheritance of resistance to C. zonata was monogenic dominant. F ₃ families were shown to segregate in the ratio of 1:2:1 for homozygous resistant: heterozygous: homozygous susceptible (χ₂² = 2.78; P > 0.05) and individual plants within heterozygous F ₃ families segregated in the ratio of 3:1 for resistant: susceptible responses (χ₁² = 2.93; P > 0.05). Monogenic dominant inheritance also explained the change in frequency of resistant and susceptible plants within a population of cv. Cairo following one generation of self-pollination (χ² = 0.88, 0.3 < P < 0.5). The sources of resistance identified in this study are being used to transfer CLS resistance to adapted faba bean genotypes for future cultivar releases to the southern Australian industry.     

Development of molecular markers linked to the <Emphasis Type="Italic">Fom-1</Emphasis> locus for resistance to Fusarium race 2 in melon

Fusarium wilt, caused by Fusarium oxysporum f. sp. melonis (F.o.m), is a worldwide soil-borne disease of melon (Cucumis melo L.). The most effective control measure available is the use of resistant varieties. Resistance to races 0 and 2 of this fungal pathogen is conditioned by the dominant gene Fom-1. An F₂ population derived from the ‘Charentais-Fom1’ × ‘TRG-1551’ cross was used in combination with bulked segregant analysis utilizing the random amplified polymorphic DNA (RAPD) markers, in order to develop molecular markers linked to the locus Fom-1. Four hundred decamer primers were screened to identify three RAPD markers (B17₆₄₉, V01₅₇₈, and V06₁₀₉₂) linked to Fom-1 locus. Fragments amplified by primers B17₆₄₉ and V01₅₇₈ were linked in coupling phase to Fom1, at 3.5 and 4 cM respectively, whereas V06₁₀₉₂ marker was linked in repulsion to the same dominant resistant allele at 15.1 cM from the Fom-1 locus. These RAPDs were cloned and sequenced in order to design primers that would amplify only the target fragment. The derived sequence characterized amplified region (SCAR) markers SB17₆₄₅ and SV01₅₇₄ (645 and 574 bp, respectively) were present only in the resistant parent. The SV06₁₀₉₂ marker amplified a band of 1092 bp only in the susceptible parent. These markers are more universal than the CAPS markers developed by Brotman et al. (Theor Appl Genet 10:337–345, 2005). The analysis of 24 melon accessions, representing several melon types, with these markers revealed that different melon types behaved differently with the developed markers supporting the theory of multiple, independent origins of resistance to races 0 and 2 of F.o.m.     

Inheritance and allelic relationship among gene(s) for blast resistance in pearl millet [Pennisetum glaucum (L.) R. Br.]

Six blast‐resistant pearl millet genotypes, ICMB 93333, ICMB 97222, ICMR 06444, ICMR 06222, ICMR 11003 and IP 21187‐P1, were crossed with two susceptible genotypes, ICMB 95444 and ICMB 89111 to generate F₁s, F₂s and backcrosses, BC₁P₁ (susceptible parent × F₁) and BC₁P₂ (resistant parent × F₁) for inheritance study. The resistant genotypes were crossed among themselves in half diallel to generate F₁s and F₂s for test of allelism. The F₁, F₂ and backcross generations, and their parents were screened in a glasshouse against Magnaporthe grisea isolates Pg 45 and Pg 53. The reaction of the F₁s, segregation pattern of F₂s and BC₁P₁ derived from crosses involving two susceptible parents and six resistant parents revealed the presence of single dominant gene governing resistance in the resistant genotypes. No segregation for blast reaction was observed in the F₂s derived from the crosses of resistant × resistant parents. The resistance reaction of these F₂s indicated that single dominant gene conferring resistance in the six genotypes is allelic, that is same gene imparts blast resistance in these genotypes to M. grisea isolates.     

Identification and mapping of a novel Turnip mosaic virus resistance gene TuRBCS01 in Chinese cabbage (Brassica rapa L.)

We aimed to identify Turnip mosaic virus (TuMV) resistance genes in Chinese cabbage by analysing the TuMV resistance of 43 P₁ (resistant), 88 P₂ (susceptible), 26 F₁, 104 B₁ (F₁ × P₁), 108 B₂ (F₁ × P₂) and 509 F₂ individuals. All parents and progeny populations were mechanically inoculated with TuMV‐C4. Both F₁ and B₁ populations showed TuMV resistance. Resistant: susceptible ratios in the B₂ and F₂ populations were 1 : 1 and 3 : 1, respectively. TuMV resistance in P₁ was controlled by a dominant gene, TuRBCS01. Bulked segregation analysis was performed to identify simple sequence repeat or insertion or deletion markers linked to TuRBCS01. Data from 108 B₂ individuals with resistant or susceptible phenotypes were analysed using mapmake r/exp 3.0. Polymorphic marker sequences were blast searched on http://brassicadb.org/brad/ . TuRBCS01 was found to be linked to eight markers: SAAS_mDN192117a_159 (3.3 cM), SAAS_mDN192117b_196 (4.0 cM), SAAS_mDN192403_148 (13.0 cM), SAAS_mGT084561_233 (6.8 cM), BrID10723 (3.3 cM), mBr4041 (3.3 cM), SAAS_mBr4055_194 (2.6 cM) and mBr4068 (4.0 cM). Further, TuRBCS01 was mapped to a 1.98‐Mb region on chromosome A04 between markers BrID10723 and SAAS_mBr4055_194.     

Application of marker‐assisted backcross to introgress Bph3, Bph14 and Bph15 into an elite indica rice variety for improving its resistance to brown planthopper

To improve brown planthopper (Nilaparvata lugens Stål; BPH) resistance of an elite indica cultivar of South China, Hemeizhan (HMZ), we applied marker‐assisted backcross (MABC) to incorporate three BPH‐resistance genes (Bph3, Bph14 and Bph15) into the genetic background of HMZ. In the third backcross (BC₃) generation, we obtained near‐isogenic lines (Bph3‐NIL, Bph14‐NIL, Bph15‐NIL and Bph14 + Bph15‐NIL) with more than 96% recovery of recurrent parent genome, and pyramided lines (Bph3 + Bph14‐PYL, Bph3 + Bph15‐PYL and Bph3 + Bph14 + Bph15‐PYL) with more than 89% recovery of recurrent parent genome. These lines showed stronger resistance against BPH than HMZ at seedling and booting stages. The rank of resistance gene effect was Bph3 + Bph14 + Bph15 ≥ Bph3 + Bph15 ≥ Bph3 +Bph14 ≥ Bph14 + Bph15 ≥ Bph3 ≥ Bph15 ≥ Bph14 > none. Compared with HMZ, only Bph3 + Bph14 + Bph15‐PYL had a significant difference in yield per plant, and the lines carrying Bph3 had higher amylose contents, indicating that Bph3 was tightly linked to Wx^a allele. These improved lines are good intermediate sources of broad‐spectrum and durable BPH resistance to improve other indica cultivars. Our results demonstrate that MABC is a very efficient approach to improve BPH resistance of elite rice cultivar.     

Genetics of Resistance in Vicia sativa L. to Orobanche crenata Forsk.

The genetics of resistance of common vetch (Vicia sativa L.) to broomrape (Orobanche crenata Forsk.) was studied for two years by using the P₁, P₂, F₁, BC¹, BC₂, F₂ F₃, and F₄ generations obtained from crosses between resistant and susceptible lines. Resistant lines were selected by screening a world collection m a naturally infested plot. Resistance was tested both under field and greenhouse conditions. The best index to measure resistance was the number of emerged broomrapes per host plant. The results fit the additive-dominance model. The main component of the variation was additivity; dominance and interaction effects seemed to depend on the environment. When dominance is expressed, a low number is dominant over a high number of broomrapes per host plant.     

Development of SCAR markers linked to the Ns locus in potato

A random amplified polymorphic DNA marker OPG17₄₅₀ linked to the Ns gene that confers resistance of potato to potato virus S (PVS), was used to develop sequence‐characterized amplified region (SCAR) markers. After cloning and sequencing of OPG17₄₅₀ new polymerase chain reaction (PCR) primers were designed to generate dominant (SCG17₃₂₁) and codominant (SCG17₄₄₈) markers. For SCG17₄₄₈, polymorphism between susceptible and resistant genotypes was recovered after digestion of the marker with the restriction enzyme Muni. In addition to the band corresponding to ‘susceptible’ allele that does not contain the Muni cleavage site, two bands of approximately 251 bp and 197 bp were observed in the resistant genotypes. The usefulness of these SCAR markers was verified in diploid potatoes possessing the Ns locus from clone G‐LKS 678147/60, and in tetraploid potatoes derived from G‐LKS 678147/60 and from clone MPI 65118/3.     

Evaluation of seedling and adult plant resistance to <Emphasis Type="Italic">Puccinia hordei</Emphasis> in barley

The responses of 92 barley genotypes to selected Puccinia hordei pathotypes were assessed in greenhouse tests at seedling growth stages and in the field at adult plant growth stages to identify known and/or unknown resistances. On the basis of multipathotype tests, 35 genotypes were postulated to carry Rph1, Rph2, Rph4, Rph5, Rph12, RphCantala, alone or in combination (Rph2 + Rph4 and Rph1 + Rph2), whereas 52 genotypes lacked seedling resistance to P. hordei to the pathotypes used. Five genotypes carried seedling resistance that was effective to all pathotypes, of which four were believed to carry uncharacterised resistance based on pedigree information. Field tests at adult plant growth stages indicated that while 28 genotypes were susceptible, 57 carried uncharacterised APR to P. hordei. Pedigree analysis indicated that APR in the test genotypes could have been derived from three different sources. The resistant responses of seven cultivars at adult plant growth stages were believed to be due to the presence of seedling resistance effective against the field pathotypes.     

Genes for scald resistance from wild barley (Hordeum vulgare ssp spontaneum) and their linkage to isozyme markers

Summary Accessions of Hordeum vulgare ssp. spontaneum, the wild progenitor of barley, collected in Israel (70), Iran (15) and Turkey (6) were screened for seedling response to four isolates of Rhynchosporium secalis, the pathogen causing leaf scald in barley. Resistance was very common in the collection (77%) particularly among accessions from the more mesic sites (90%). The genetics of this resistance were investigated in fifteen backcross (BC₃) lines that contained an isozyme variant from H.v. ssp. spontaneum in a H.v. ssp. vulgare (cv. Clipper) background and were resistant to scald. Segregation in the BC₃F₂ families conformed with a single dominant resistance gene in 9 of the 15 lines. Scald resistance and the isozyme marker were closely linked in three of the BC₃-lines, loosely linked in four and unlinked in the remaining eight. Scald resistance genes were identified on barley chromosomes 1, 3, 4 and 6. Crosses between several of the scald resistant BC-lines together with the linkage data indicated that at least five genetically independent resistances are available for combining together for deployment in barley. The linkage of scald resistance in several BC₃-lines to the isozyme locus Acp2 is of special interest as this locus is highly polymorphic in wild barley.     

Genetic analysis of resistance to <Emphasis Type="Italic">Fusarium</Emphasis> root rot in common bean

Fusarium root rot (FRR) is a major disease of common bean worldwide. Knowledge of the inheritance of resistance to FRR would be important in devising strategies to breed resistant varieties. Therefore, a 12 × 12 full diallel mating scheme with reciprocal crosses was performed to generate 132 F₁ progenies, which were then advanced to the F₃. The progenies were evaluated for resistance to FRR under green house conditions in Uganda. General combining ability (GCA) effects were highly significant (P ≤ 0.01) for disease scores. Specific combining ability effects were not significant (P > 0.05) in the F₁, but were highly significant (P < 0.01) in the F₃ generation. These results indicate that resistance to FRR was governed by genes with additive effects in combination with genes with non-additive effects. Reciprocal differences were also significant (P = 0.01) at F₁ and F₃, primarily reflecting a large influence of maternal effects in both these generations. In fact, susceptible parents did not differ significantly (P > 0.05) for disease scores when used as paternal parents in the F₃, but differed strongly as maternal parents (P = 0.0002). Generally, the progenies were distinctly more resistant when the resistant parent was used as the female in crosses, especially as observed in the F₃. The maternal effects were strong in the F₃ generation, suggesting a complex form of cytoplasmic–genetic interaction. The non-maternal reciprocal effects in the F₃ were significant (P < 0.05) in both the resistant × resistant diallel, and in the resistant × susceptible crosses. Mid-parent heterosis (MPH) occurred in most crosses, with average heterosis approximately equal in each of the three generations, indicating that epistasis was probably more influential than dominance of individual genes. Gene-number formulas indicated that several genes were involved in resistant × susceptible crosses. Among resistant × resistant crosses, many produced continuous distributions of F₁ progeny scores, suggesting polygenic inheritance, while bi-modal distributions were characteristic of the F₃ distributions, and fit expected ratios for two or three loci segregating in each cross. Dominant forms of epistasis favoring resistance were strongly indicated. Parent–offspring heritability estimates were moderate. Overall, the results indicate that resistant parents contain a number of different resistance genes that can be combined with the expectation of producing strong and durable resistance. The lines MLB-49-89A, MLB-48-89, RWR719 and Vuninkingi, with large and negative GCA effects, contributed high levels of resistance in crosses and would be recommended for use in breeding programs.     

Genetic analysis of late blight resistance in a new RIL population of tomato derived from LB-resistant Solanum pimpinellifolium accession PI 270443

Late blight (LB), caused by Phytophthora infestans, is one of the most devastating diseases of tomato (Solanum lycopersicum) worldwide. Aggressive pathogen isolates resistant to fungicides have driven research in favour of finding new sources of host resistance for tomato breeding. Recently, we reported S. pimpinellifolium accession PI 270443 exhibiting LB resistance stronger than all commercial LB-resistant tomato cultivars. The purpose of this study was to examine the inheritance of LB resistance conferred by this accession. An interspecific cross was made between PI 270443 and a LB-susceptible tomato breeding line and advanced to F₁₀ generation. A total of 166 F₉ and corresponding F₁₀ recombinant inbred lines (RILs) were evaluated for response to LB in four replicated greenhouse experiments. Estimates of heritability (h²) of LB resistance, determined by parent–offspring (F₉:F₁₀) correlation analysis, ranged from 0.66 to 0.81, with an average of 0.76. The moderately high h² of LB resistance in PI 270443 suggests the utility of this accession for tomato breeding. Molecular mapping and RNA-sequencing efforts are underway to identify genes underlying LB resistance in PI 270443.     

Mapping of a novel,major late blight resistance locus in the diploid (1EBN) Mexican Solanum pinnatisectum Dunal on chromosome VII

Late blight caused by the oomycete Phytophthora infestans (Mont.) de Bary (Pi) is the most important foliar disease of potato worldwide. An intraspecific hybrid between individuals of a resistant and a susceptible S. pinnatisectum accession was backcrossed to the susceptible parent to generate a segregating population for late blight resistance consisting of 84 plants. In detached‐leaflet assays, reaction to late blight segregated in a 1r:1s manner in BC₁ progeny indicating the presence of a single dominant resistance gene. A genetic map was constructed based on 1,583 DArT/SSR markers which were allocated to 12 linkage groups, covering 1,793.5 cM with an average marker distance of 1.1 cM. The late blight resistance locus derived from S. pinnatisectum was mapped on chromosome VII. In comparison with the previously reported resistance genes Rpi1 and Rpi2, the new target resistance locus most likely is located on the opposite arm of chromosome VII. Results of this study will serve as a basis for future fine mapping of the late blight resistance locus and the development of locus‐specific markers for marker‐assisted selection.     

The importance of wild potato species resistant to the potato cyst nematode,Globodera pallida,pathotypes P4A and P5A,in potato breeding. I. Resistance studies

Summary Seven wild diploid potato species, Series Tuberosa, representing 1023 clones were screened for resistance to the potato cyst nematode, Globodera pallida. Over 25% of the clones were resistant to pathotype P₄A and almost 30% were resistant to pathotype P₅A. The resistance in hybrid progenies of these and other resistant species with cultivated potatoes was evaluated, and over 2200 seedlings were screened. High frequencies of resistance (>50%) to P₄A were found in progenies with Solanum leptophyes, S. vernei, S. gourlayi and S. capsicibaccatum, whereas resistance to P₅A was found in these species as well as S. sparsipilum. The importance of nematode resistant wild species for potato breeding is discussed.     

Analysis of mode of inheritance of Fusarium wilt resistance in castor (Ricinus communis L.)

Castor (Ricinus communis L.) is an important industrial oilseed crop grown worldwide. Wilt caused by Fusarium oxysporum f.sp. ricini is a devastating disease in castor. The inheritance mode of wilt resistance was investigated. The F₁, F₂ and backcross generations of four crosses involving four resistant and three susceptible parents were developed. The role of digenic (R¹ and R²) epistatic interactions on wilt resistance was confirmed. The 15 : 1, 9 : 7 and 13 : 3 ratios indicated duplicate dominant, duplicate recessive and dominant and recessive epistatic interactions, respectively. Castor parents used in the crosses exhibited varied inheritance modes. All generations of a cross exhibited similar inheritance mode when parents were comparable. However, generations varied in inheritance mode when parents were not comparable in inheritance mode. These results would have practical interest when decisions are required regarding the choice of parents and methodology in resistance and hybrid breeding. The results also provided a basis for investigating molecular genetics of wilt resistance mechanisms.