Partial resistance to anthracnose in Brazilian land races of dry beans shows race specificity

Summary Fifty-four land races of dry beans (Phaseolus vulgaris), indigenous to areas of Brazil where anthracnose (caused by Colletotrichum lindemuthianum) is a common problem, were evaluated in field nurseries for partial resistance to race Brazilian 1 (B1) of C. lindemuthianum using symptom severity classes (SSC) from 0 to 6. Plants were selected if symptoms were present and the SSC was less than the 95% confidence interval of the mean SSC of the susceptible cultivar Carioca. S₁ progeny from selected plants were evaluated in air-conditioned chambers for partial resistance to races B1, delta, and kappa of C. lindemuthianum. Of 246 S₁ families evaluated, 145 families were partially resistant to one or two of the races [symptoms present, but S₁ family mean significantly (p<0.05) less than the mean of Carioca] and susceptible to the third. Six families were partially resistant to all three races. The remaining families were either susceptible or segregated for reaction to race B1. Partial resistance to C. lindemuthianum showed race specificity in the air-conditioned chambers and field nurseries.     

Monogenic resistance in tomato to Fusarium oxysporum f. sp. lycopersici race 3

Summary Resistance to fusarium wilt, incited by Fusarium oxysporum (Schlecht.) f. sp. lycopersici (Sacc.) Snyder & Hansen race 3 in tomato (Lycopersicon esculentum Mill.) was discovered in LA 716, a L. pennellii accession. A resistant BC₁F₃ breeding line, E427, was developed from LA 716. E427 was crossed with the susceptible cv. Suncoast and F₁, BCP₁, BCP₂ (to Fla 7155, a susceptible parent) F₂, F₃, and BCP₂S₁ seeds were obtained. Segregation for resistance following root dip inoculation over three experiments indicated a single dominant gene controlled resistance. Five of the 12 BCP₁S₁'s segregated more susceptible plants, whereas one of the 12 segregated more resistant plants than expected (P<0.05). Three of 23 F₃ lines segregated more susceptible plants than expected while 1 of the 23 had more resistant plants than expected (P<0.05). Segregation in all other lines fit expected ratios. Five of the 23 F₃'s were homozygous resistant which was an acceptable fit to expectations (P=0.1–0.5). The gene symbol I ₃ is proposed for resistance to race 3 of the wilt pathogen. Deviations from expected ratios in data reported here and for other breeding lines indicate an effect of modifier genes and/or incomplete penetrance. Plant age at inoculation and seed dormancy did not affect results.Florida Agricultural Experiment Station Journal Series No. 8101.     

Rubus coreanus as donor of resistance to cane diseases and mildew in red raspberry breeding

Summary Strong resistance to the cane diseases Elsinoë veneta, Didymella applanata and Botrytis cinerea, and to Sphaerotheca macularis, occurred in F₁ and BC₁ derivatives of an accession of Rubus coreanus. Resistance to cane spot (E. veneta) was polygenic.In eight out of ten BC₁ progenies, average grades for cane spot infections were significantly higher in white-flowered (an ₁) than pink-flowered (An ₁) plants and in hairy-caned (H) than in glabrous (h) seedlings. It is postulated that in R. coreanus factors controlling resistance are linked with An ₁ and. probably, with h.Average grades for spur blight (D. applanata) were significantly higher in white-flowered plants in nine out of ten BC₁ progenies. Spininess (S) was associated with greater susceptibility in six out of eight BC₁ families, although this difference was not statistically significant.Plants with the phenotype hAn ₁ on average provided the best source of resistance to both cane spot and spur blight.     

Apium graveolens PI 181714 is a source of resistance to Fusarium oxysporum f. sp. apii race 4 in celery (A. graveolens var. dulce)

Celery has little genetic diversity and is highly susceptible to the new fungal pathogen Fusarium oxysporum f. sp. apii (Foa) race 4. After screening an Apium graveolens germplasm collection for resistance to Foa race 4, we crossed celery cv. 'Challenger', which is Foa race 2-resistant but Foa race 4-susceptible and A. graveolens PI 181714, which is Foa races 2- and 4-resistant but non-celery type. After selfing F₁s, we screened the F₁S₁ for race 4-resistance and celery-type and then selfed selected F₁S₁. Greenhouse and field trials indicate that three selected F₁S₂ families (76–8-4, 76–8-27 and 76–8-36) are suitable as germplasm for celery breeders for resistance to Foa race 4. A F1S3 76–8–36-124 is either fixed or nearly so for resistance to Foa races 4 and 2. Furthermore, quantitative PCR indicates that PI 181714 is resistant, rather than tolerant, to Foa races 4 and 2, and that this resistance has been introgressed into F1S3 76–8–36-124.     

Selection for resistance to Plasmodiophora brassicae Wor. in oriental subspecies of Brassica rapa L.

Summary Selection for resistance to Plasmodiophora brassicae Wor. in oriental groups of Brassica rapa L.Two hundred and sixty-five cultivars of leafy, oriental bassicas were tested for resistance to 18 collections of Plasmodiophora brassicae, the causal agent of clubroot. The tests were conducted in the greenhouse at low and high level inoculum concentrations. Eleven cultivars of B. rapa pe-tsai, five cultivars of B. rapa pak-choy and three cultivars of B. rapa choy-sum consistently segregated for resistance at the lower concentration of inoculum (1000 spores/ml). All 265 cultivars were susceptible at the higher concentration (1 000 000 spores/ml). Three cultivars were used in pedigree and recurrent selection schemes for increased resistance. After three cycles of selfing resistant individuals, significantly more resistant S₃ lines were derived from each cultivar. Lines derived from two cultivars. Chinese White and PI 257236, continued to improve with each cycle of selection and demonstrated increased resistance to higher levels of inoculum (up to 1 000 000 spores/ml) New cultivars based on intercrosses of S₂ resistant individuals also had significantly better resistance than the original cultivar. After two cycles of selection in the third cultivar, PI 419007, resistance did not increase and its S₂ mass did not differ significantly from the original cultivar. Evidence that indicates resistance is pathotype-non-differential and offers an alternative to major gene, pathotype-differential types of resistance currently being introduced to the leafy oriental brassicas from other Brassica rapa groups.     

Inheritance of resistance to the chlorosis component of tan spot of wheat caused by Pyrenophora tritici-repentis, races 1 and 3

The fungus Pyrenophora tritici-repentis, causal agent of tan spot of wheat, produces two phenotypically distinct symptoms, tan necrosis and extensive chlorosis. The inheritance of resistance to chlorosis induced by P. tritici-repentis races 1 and 3 was studied in crosses between common wheat resistant genotypes Erik, Hadden, Red Chief, Glenlea, and 86ISMN 2137 and susceptible genotype 6B-365. Plants were inoculated under controlled environmental conditions at the two-leaf stage and disease rating was based on presence or absence of chlorosis. In all the resistant × susceptible crosses, F₁ plants were resistant and the segregation of the F₂ generation and F₃ families indicated that a single dominant gene controlled resistance. Lack of segregation in a partial diallel series of crosses among the resistant genotypes tested with race 3␣indicated that the resistant genotypes possessed␣the same resistance gene. This resistance gene was effective against chlorosis induced by P.␣tritici-repentis races 1 and 3.     

Transfer of resistance to race 2 of Plasmodiophora brassicae from Brassica napus to cabbage (B. oleracea ssp. capitata). III. First backcross and F2 progenies from interspecific hybrids between B. napus and B. oleracea ssp. capitata

Summary The first backcross and F₂ progenies from triploid F₁ and tetraploid F₁ hybrids between B. napus and 2x and 4x B. oleracea ssp. capitata (cabbage) were studied for their general morphology, resistance to race 2 of the clubroot pathogen, chromosome number and meiotic chromosome behavior. No linkage was apparent between resistance and the major morphological characters. Unreduced gametes played a large part in the successful formation of seed of the B₁ and F₂ progeny. B₁ plants with low chromosome numbers were selected for use in recurrent backcrosses. The potential use of anther culture to extract gametic progenies from resistant B₁ and F₂ plants with higher chromosome numbers was suggested. The presence of homoeologous pairing observed in all the plants is considered advantageous for selecting suitable progeny in later generations.     

Pedigree selection for Gibberella ear rot resistance in maize

The pedigree method is often used for developing inbred lines in maize (Zea mays L.). This study was conducted to assess the effectiveness of pedigree selection for improving resistance to Gibberella ear rot in four maize populations. Selection was based on the severity of ear rot symptoms after inoculation with macroconidial suspensions of Fusarium graminearum (Schwabe) into the silk channel (for two populations) and into the developing kernels (for two other populations). Samples of the selfed families (S₁ to S₅), recovered from remnant seed from the selection programs, were evaluated for disease resistance during three years, using inoculation and evaluation protocols similar to those used during selection. Among-family selection was effective in both of the populations selected after silk inoculation and in one of the populations selected after kernel inoculation. Responses to selection were more evident in later than in earlier generations for both types of inoculation. Changes in the estimated genetic gain over generations were consistent with changes in the variances among families, which tended to increase in early generations and to decrease in later generations. Selection after kernel inoculation seemed to have been more effective than selection after silk inoculation in developing families with more stable resistance. Based on the results obtained here, it seems that responses to family selection could be accelerated without increasing operational costs by increasing selection intensity in later generations and inoculating fewer plants per family.     

Inheritance of resistance to wildfire and angular leaf spot derived from Nicotiana rustica var. Brasilea

Summary The introgression of wildfire (races 0 and 1) and angular leaf spot (ALS) resistance from N. rustica var. Brasilea into N. tabacum has proved economically useful in Zimbabwe although the mode of inheritance of, and genetic relationships between the resistance are unknown. This study was undertaken to (1) examine the mode of inheritance of the resistance to races 0 and 1 of wildfire, and ALS, (2) determine the genetic relationship between the resistances and (3) establish whether the N. rustica-derived wildfire race 0 resistance is allelic to that obtained from N. longiflora. Inheritance was examined under greenhouse and field conditions by studying disease reactions in the parental, F1, F2 and backcross generations derived from crosses of three susceptible lines to a resistant line Nr-7. Three-point backcrosses to the susceptible parent were examined for linkage and segregating generations from a cross of Nr-7 to Burley 21 which carries the N. longiflora race 0 resistance were used to test for allelism. In general, we observed that all resistances are determined by a single dominant gene although some incosistent ratios were obtained likely due to misclassification of disease reactions and erratic transmission. All resistances showed linkage although pleiotropism cannot be ruled out. Allelism tests demonstrated that the N. rustica race 0 resistance is not allelic to that obtained from N. longiflora. Our findings are examined in relation to the efficacy of indirect selection for resistance.     

Transfer of resistance to race 2 of Plasmodiophora Brassicae from Brassica napus to cabbage (B. oleracea var. Capitata). I. Interspecific hybridization between B. napus and B. oleracea var. Capitata

Summary Interspecific hybridization between Brassica napus L. (2n=38, a₁a₁c₁c₁) and B. oleracea var. capitata L. (2x- and 4x-cabbage; 2n=2x=18, cc and 2n=4x=36, cccc) was carried out for the purpose of transferring clubroot disease resistance from the amphidiploid species to cabbage. Nineteen hybrids with three different chromosome levels (2n=28, a₁c₁c; 2n=37, a₁c₁cc and 2n=55, a₁c₁cccc) were obtained. The F₁ plants were mostly intermediate between the two parents but as the number of c genomes in the hybrids increased, the more closely the hybrids resembled the cabbage parent. All F₁ hybrids were resistant when tested against race 2 of Plasmodiophora brassicae wor. The complete dominance of resistance over susceptibility suggested that the gene(s) controlling resistance to this particular race of the clubroot pathogen is probably located on a chromosome of the a genome in Brassica.Contribution No. J654.     

Quantitative resistance to Botrytis cinerea from Solanum neorickii

Tomato (Solanum lycopersicum) is susceptible to gray mold (Botrytis cinerea). Quantitative resistance to B. cinerea was previously identified in a wild relative, S. neorickii G1.1601. The 122 F₃ families derived from a cross between the susceptible S. lycopersicum cv. Moneymaker and the partially resistant S. neorickii G1.1601 were tested for susceptibility to B. cinerea using a stem bioassay. Three putative quantitative trait loci (pQTL) were detected: pQTL3 and pQTL9 reducing lesion growth (LG) and pQTL4 reducing disease incidence (DI). For each pQTL, a putative homologous locus was identified recently in another wild tomato relative, S. habrochaites LYC4. pQTL3 was confirmed by assessing disease resistance in BC₃S₁ and BC₃S₂ progenies of S. neorickii G1.1601. pQTL4 was not statistically confirmed but the presence of the S. neorickii resistance allele reduced DI in all three tested populations. The reduction in LG of pQTL9 was not confirmed but rather, this locus conferred a reduced DI, similar to observations in the QTL study using S. habrochaites. The results are discussed in relation to other disease resistance loci identified in studies with other wild tomato relatives.     

Heritability estimates of spot blotch resistance and its association with other traits in spring wheat crosses

Summary Spot blotch caused by Cochliobolus sativusis considered a major disease problem of wheat(Triticum aestivumL.) in the warm areas of South Asia. This study estimated heritability (h ²) of resistance to spot blotch and its correlation with days to heading DH) and maturity (DM), one-hundred-kernel weight (HKW), and plant height (PHT) in 14 crosses involving four resistant (‘Attila’, ‘Chirya 7’, ‘G 162’, and ‘SW89.5422’) and two susceptible (‘Sonalika’ and ‘HD2329’) wheat genotypes. Data were recorded on F₅and F₆lines in fields under natural epidemics of spot blotch in 2003 and 2004, respectively. Heritability was estimated for area under disease progress curve (AUDPC), AUDPC/day, and the highest disease score (HDS) using offspring-parent regression (h _op ²) and realized heritability (h ² _R) procedures. Heritability estimates were low to high in terms of AUDPC (0.21 < h _op ²< 0.64; 0.32 < h _R ²< 0.70), AUDPC/day (0.40 < h _op ²< 0.96; 0.42 < h _R ²< 0.99), and HDS (0.29 < h _op ²< 0.92; 0.32 < h _R ²< 0.95). The h ²estimates for AUDPC/day were higher than for AUDPC and HDS. Estimates of h _R ²were by and large higher than h _op ²in the same cross. A weak negative or nonsignificant correlation of spot blotch score with HKW, DH, DM, and PHT indicated that independent selection for resistance and these agronomic traits is possible.     

Genetics of Cyst Nematode Resistance in Soybean PIs 467312 and 507354

Soybean Cyst nematode (SCN) Heterodera glycines Ichinohe is the most serious pest of soybean [Glycine max (L.) Merr.] in the world and genetic resistance in soybean cultivars have been the most effective means of control. Nematode populations, however, are variable and have adapted to reproduce on resistant cultivars over time due mainly to the narrow genetic base of SCN resistance in G. max. The majority of the resistant cultivars trace to two soybean accessions. It is hoped that new sources of resistance might provide durable resistance. Soybean plant introductions PI 467312 and PI 507354, are unique because they provide resistance to several nematode populations, i.e. SCN HG types 0, 2.7, and 1.3.6.7 (corresponding to races 3, 5, and 14) and HG types 2.5.7, 0, and 2.7 (corresponding to races 1, 3, and 5), respectively. The genetic basis of SCN resistance in these PIs is not yet known. We have investigated the inheritance of resistance to SCN HG types 0, 2.7, and 1.3.6.7 (races 3, 5, and14) in PI467312 and the SCN resistance to SCN HG types 2.5.7 and 2.7 (races 1 and 5) in PI 507354. PI 467312 was crossed to ‘Marcus’, a susceptible cultivar to generate F₁ hybrids, 196 random F₂ individuals, and 196 F_2:3 families (designated as Pop 467). PI 507354 and the cultivar Hutcheson, susceptible to all known SCN races, were crossed to generate F₁ hybrids, 225 random F₂ individuals and 225 F_2:3 families (designated as Pop 507). The F_2:3 families from each cross were evaluated for responses to the specific SCN HG types in the greenhouse. Chi-square (χ²) analyses showed resistance from PI 467312 to HG types 2.7, and 1.3.6.7 (races 5 and 14) in Pop 467 were conditioned by one dominant and two recessive genes (Rhg rhg rhg) and resistance to HG type 0 (race 3) was controlled by three recessive genes (rhg rhg rhg). The 225 F_2:3 progenies in Pop 507 showed a segregation of 2:223 (R:S) for response to both HG types 2.5.7 and 2.7 (corresponding to races 1 and 5). The Chi-square analysis showed SCN resistance from PI 507354 fit a one dominant and 3 recessive gene model (Rhg rhg rhg rhg). This information will be useful to soybean breeders who use these sources to develop SCN resistant cultivars. The complex inheritance patterns determined for the two PIs are similar to the three and four gene models for other SCN resistance sources known to date.     

Parent-offspring correlation estimate of heritability for early blight resistance in tomato, Lycopersicon esculentum Mill.

This study estimated the heritability (h ²) of early blight (EB) resistance in filial progeny of a cross between a susceptible (`NC84173';mid-season maturity) and a resistant (`NC39E'; late-season maturity)tomato breeding lines. It addition, it examined the potential of identifying progeny with mid-season maturity and EB resistance. A total of 162F₂ plants were grown under field conditions in 1998 and evaluated for disease symptoms three times during the season, and the area under the disease progress curve (AUDPC) and final percent defoliation (disease severity) were determined. The F₂ plants were self-pollinated and F₃ seeds produced. The 162 F₃ progeny families, consisting of 20 plants per family, were grown in a replicated field trial in 1999 and evaluated for EB resistance (final percent defoliation) and plant maturity(days to 50% ripe fruit). The distributions of the final percent defoliation values in the F₂ and F₃ generations indicated that resistance from `NC39E' was quantitative in nature. Estimates of h <sup>2</sup> for EB resistance, computed as the correlation coefficients between F<sub>3</sub>progeny family means and F<sub>2</sub> individual plant values, ranged from0.65 to 0.71, indicating that EB resistance of `NC39E' was heritable. Across F₃ families, a negative correlation (r = –0.46, p< 0.01) was observed between disease severity and earliness in maturity, indicating that plant maturity affected disease severity. However, several F₃ families were identified with considerable EB resistance and mid-season maturity, indicating that resistance from `NC39E' might be useful for the development of commercially acceptable EB resistant tomato cultivars. This revised version was published online in August 2006 with corrections to the Cover Date.     

Inheritance of resistance to two races of sunflower downy mildew (Plasmopara halstedii) in two Helianthus annuus L. lines

Sunflower downy mildew caused by Plasmopara halstedii is an important disease of sunflower capable of causing losses of more than 80% of production. Races 100, 300, 310, 330, 710, 703, 730 and770 of the fungus have been identified in Spain. Race 703, of high virulence, has been identified frequently in the northeast, while race 310 seems to occur over the south, the main sunflower growing region of the country. Oil sunflower lines RHA-274 and DM4 were studied for their resistance to races 310(RHA-274 and DM4) and 703 (DM4). In each cross, only one plant of the resistant parent was crossed to the inbred susceptible line HA-89 (or cmsHA-89).Plants from F₂ and backcross(BC₁F₁ to susceptible parent)generations were evaluated for fungal sporulation on true leaves and/or cotyledons. The resistant-to-susceptible ratios obtained in the F₂ and BC₁F₁ progenies from the crosses cmsHA-89 × RHA-274 and HA-89 × DM4suggested that one major gene in each line is responsible for resistance to race 703.The segregations of the progenies of the cross HA-89 × DM4 inoculated with race 703also fitted the ratios 1:1 and 3:1 (for BC₁F₁ and F₂, respectively)corresponding to control of resistance by a single dominant gene. In RHA-274, the gene for resistance to race 310 was designated Pl ₉, whereas Pl _v is tentatively proposed to designate the gene in DM4 responsible for resistance to races310 and 703. This revised version was published online in August 2006 with corrections to the Cover Date.     

Chromosomal location of fertility restoring genes for ‘wild abortive’ cytoplasmic male sterility using primary trisomics in rice

Summary Identification and location of fertility restoring genes facilitates their deployment in a hybrid breeding program involving cytoplasmic male sterility (CMS) system. The study aimed to locate fertility restorer genes of CMSWA system on specific chromosomes of rice using primary trisomics of IR36 (restorer), CMS (IR58025A) and maintainer (IR58025B) lines. Primary trisomic series (Triplo 1 to 12) was crossed as maternal parent with the maintainer line IR58025B. The selected trisomic and disomic F₁ plants were testcrossed as male parents with the CMS line IR58025A. Plants in testcross families derived from disomic F₁ plants (Group I crosses) were all diploid; however, in the testcross families derived from trisomic F₁ plants (Group II crosses), some trisomic plants were observed. Diploid plants in all testcross families were analyzed for pollen fertility using 1% IKI stain. All testeross families from Group I crosses segregated in the ratio of 2 fertile: 1 partially fertile+partially sterile: 1 sterile plants indicating that fertility restoration was controlled by two independent dominant genes: one of the genes was stronger than the other. Testcross families from Group II crosses segregated in 2 fertile: 1 partially fertile+ partially sterile: 1 sterile plants in crosses involving Triplo 1, 4, 5, 6, 8, 9, 11 and 12, but families involving triplo 7 and triplo 10 showed significantly higher X² values, indicating that the two fertility restorer genes were located on chromosome 7 and 10. Stronger restorer gene (Rf-WA-1) was located on chromosome 7 and weaker restorer gene (Rf-WA-2) was located on chromosome 10. These findings should facilitate tagging of these genes with molecular markers with the ultimate aim to practice marker-aided selection for fertility restoration ability.     

A quantitative trait locus on chromosome 5B controls resistance of <Emphasis Type="Italic">Triticum turgidum</Emphasis> (L.) var. <Emphasis Type="Italic">diccocoides</Emphasis> to Stagonospora nodorum blotch

Stagonospora nodorum blotch (SNB) is an important foliar disease of durum wheat (Triticum turgidum var. durum) worldwide. The combined effects of SNB and tan spot, considered as components of the leaf spotting disease complex, result in significant damage to wheat production in the northern Great Plains of North America. The main objective of this study was the genetic analysis of resistance to SNB caused by Phaeosphaeria nodorum in tetraploid wheat, and its association with tan spot caused by Pyrenophora tritici-repentis race 2. The 133 recombinant inbred chromosome lines (RICL) developed from the cross LDN/LDN(Dic-5B) were evaluated for SNB reaction at the seedling stage under greenhouse conditions. Molecular markers were used to map a quantitative trait locus (QTL) on chromosome 5B, explaining 37.6% of the phenotypic variation in SNB reaction. The location of the QTL was 8.8 cM distal to the tsn1 locus coding for resistance to P. tritici-repentis race 2. The presence of genes for resistance to both SNB and tan spot in close proximity in tetraploid wheat and the identification of molecular markers linked to these genes or QTLs will be useful for incorporating resistance to these diseases in wheat breeding programs.     

An allelic series at the Co-1 locus conditioning resistance to anthracnose in common bean of Andean origin

In this study, we characterized the genetic resistance of the Andean bean cultivars Kaboon and Perry Marrow and their relation to other sources of anthracnose resistance in common bean. Based on the segregation ratio (3R:1S) observed in two F₂ populations we demonstrated that Kaboon carries one major dominant gene conferring resistance to races 7 and 73 of Colletotrichum lindemuthianum. This gene in Kaboon is independent from the Co-2 gene and is an allele of the Co-1 gene present in Michigan Dark Red Kidney (MDRK) cultivar. Therefore, we propose the symbol CO-1 ² for the major dominant gene in Kaboon. The Co-1 is the only gene of Andean origin among the Co anthracnose resistance genes characterized in common bean. When inoculated with the less virulent Andean race 5, the segregation ratio in the F₂ progeny of Cardinal and Kaboon was 57R:7S (p = 0.38). These data indicate that Kaboon must possess other weaker dominant resistance genes with a complementary mode of action, since Cardinal is not known to possess genes for anthracnose resistance. Perry Marrow, a second Andean cultivar with resistance to a different group of races, was shown to possess another resistant allele at the Co-1 locus and the gene symbol Co-1 ³ was assigned. In R × R crosses between Perry Marrow and MDRK or Kaboon, no susceptible F₂ plants were found when inoculated with race 73. These findings support the presence of a multiple allelic series at the Andean Co-1 locus, and have major implications in breeding for durable anthracnose resistance in common bean. This revised version was published online in July 2006 with corrections to the Cover Date.     

The influence of disease complexes involving Leptosphaeria (Septoria) nodorum on detection of resistance to three leaf spot diseases in wheat

Summary In controlled inoculation studies with Septoria nodorum and Pyrenophora tritici-repentis, estimates of the relative proportion of each pathogen demonstrated differences in the responses of cultivars to pathogen mixtures that were not apparent from measurements of diseased leaf areas. Under field conditions estimates of the relative proportion of S. nodorum, P. tritici-repentis and S. tritici varied between field screening locations in Western Australian but also between lines within locations. Lines with known resistance to P. tritici-repentis and S. tritici, but susceptible to S. nodorum, could not be distinguished from susceptible lines on the basis of leaf area diseased or grain weight depression when S. nodorum was present in the disease complex. Such conditions, while suitable for the selection of combined resistance to these pathogens, were unsuitable for identifying resistance to individual pathogens. As symptoms were similar, the proportion of diseased leaf area sporulating with each pathogen provided a means of measuring the variation in disease development induced on lines varying in resistance. Knowledge of the components of disease and their relative importance were essential in understanding varietal response information under mixed infections of these leaf spot pathogens.     

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.