Stem Canker Resistance Research Articles

Genetic Dissection of Phomopsis Stem Canker Resistance in Cultivated Sunflower Using High Density SNP Linkage Map.

Phomopsis stem canker (PSC) caused by Diaporthe helianthi is increasingly becoming a global threat for sunflower production. In this study, the genetic basis of PSC resistance was investigated in a recombinant inbred line (RIL) population developed from a cross between HA 89 (susceptible) and HA-R3 (resistant). The RIL population was evaluated for PSC disease incidence (DI) in seven screening trials at multiple locations during 2016–2018. The distribution of PSC DI in the RIL population was continuous, confirming a polygenic inheritance of the trait. A moderately high broad-sense heritability (H2, 0.76) was estimated for the trait across environments. In the combined analysis, both the genotype and the genotype × environment interactions were highly significant. A linkage map spanning 1505.33 cM was constructed using genotyping-by-sequencing derived markers. Marker–trait association analysis identified a total of 15 quantitative trait loci (QTL) associated with PSC resistance on 11 sunflower chromosomes, each explaining between 5.24 and 17.39% of the phenotypic variation. PSC resistance QTL were detected in two genomic regions each on chromosomes 3, 5, 13, and 17, while one QTL each was detected in the remaining seven chromosomes. Tightly linked single nucleotide polymorphism (SNP) markers flanking the PSC resistance QTL will facilitate marker-assisted selection in PSC resistance sunflower breeding.

Association mapping of a locus that confers southern stem canker resistance in soybean and SNP marker development

BackgroundSouthern stem canker (SSC), caused by Diaporthe aspalathi (E. Jansen, Castl. & Crous), is an important soybean disease that has been responsible for severe losses in the past. The main strategy for controlling this fungus involves the introgression of resistance genes. Thus far, five main loci have been associated with resistance to SSC. However, there is a lack of information about useful allelic variation at these loci. In this work, a genome-wide association study (GWAS) was performed to identify allelic variation associated with resistance against Diaporthe aspalathi and to provide molecular markers that will be useful in breeding programs.ResultsWe characterized the response to SSC infection in a panel of 295 accessions from different regions of the world, including important Brazilian elite cultivars. Using a GBS approach, the panel was genotyped, and we identified marker loci associated with Diaporthe aspalathi resistance through GWAS. We identified 19 SNPs associated with southern stem canker resistance, all on chromosome 14. The peak SNP showed an extremely high degree of association (p-value = 6.35E-27) and explained a large amount of the observed phenotypic variance (R2 = 70%). This strongly suggests that a single major gene is responsible for resistance to D. aspalathi in most of the lines constituting this panel. In resequenced soybean materials, we identified other SNPs in the region identified through GWAS in the same LD block that clearly differentiate resistant and susceptible accessions. The peak SNP was selected and used to develop a cost-effective molecular marker assay, which was validated in a subset of the initial panel. In an accuracy test, this SNP assay demonstrated 98% selection efficiency.ConclusionsOur results suggest relevance of this locus to SSC resistance in soybean cultivars and accessions from different countries, and the SNP marker assay developed in this study can be directly applied in MAS studies in breeding programs to select materials that are resistant against this pathogen and support its introgression.

Genetic control, combining ability and heritability of resistance to stem canker in cacao (Theobroma cacao L.)

Stem canker caused by Phytophthora palmivora and Phytophthora megakarya is a serious disease of cacao (Theobroma cacao L.). Studies of the genetics of resistance to this disease is important to identify an effective procedure to use in breeding resistant varieties. The objective of this study was to determine mode of inheritance, combining ability and heritability of resistance to stem canker in cacao. A 6 × 6 full diallel mating design was used to generate 36 F1 hybrids which were evaluated for resistance to stem canker with three replications under artificial inoculation with 1 mm2 agar plug taken from actively growing colonies of 10-day old cultures of P. palmivora and P. megakarya. Twelve months later, the canker lesions on outer bark of the stem of the seedlings were measured and the canker lesions inside was also measured after scraping the bark. There were significant (p 50%) of GCA suggest that additive gene effects were more important than non-additive gene effects. The overall direction of results showed that using one of the following parents: P1 (Alpha B36), P4 (T17/524) and P5 (T65/238) as the female produced progenies with better resistance to stem canker disease than when they were used as the male parent. The majority reflected the resistance level of the female parent, suggesting that resistance to stem canker in cacao is influenced by maternal inheritance. Cacao genotypes Alpha B36, T17/524 and T65/238 showed significant negative GCA effects for resistance to stem canker suggesting that these genotypes will transmit the trait to their progenies and therefore recommended for cacao breeding. Narrow sense heritability for stem canker resistance was low indicating large component of dominance effects and influence of environment in inheritance of resistance to stem canker in cacao. However, the very high broad sense heritability (0.99) coupled with large variation within the cacao tree population indicate that progeny testing and within family selection would be appropriate in breeding against stem canker in cacao.

Development of southern stem canker disease on soybean seedlings in the greenhouse using a modified toothpick inoculation assay

Southern soybean stem canker, caused by Diaporthe aspalathi, has caused major soybean losses for growers in the Southeast U.S. The most effective disease management tool for growers is the use of stem canker resistant soybean varieties. A fast, reliable greenhouse assay for stem canker would facilitate identification of resistance in soybean germplasm. An existing toothpick assay was modified to include culturing D. aspalathi on oxgall agar on toothpicks autoclaved in oxgall liquid medium. Three week-old seedlings were inoculated with toothpicks inserted in the stem between the cotyledon and the first trifoliate leaf. Inoculation sites were sealed with petroleum jelly, and seedlings were incubated in humidity chambers for 72 h. Stem canker disease was highly consistent on susceptible lines four weeks post-inoculation and was not observed on soybean lines with known Rdm genes. High levels of disease (≥98.3%) were observed with cultivars Braxton, Davis, and Centennial previously reported to have resistance in field studies. Isolates of D. aspalathi were observed to differ in virulence. This modified greenhouse assay will assist in the efforts for breeding stem canker resistance and better understanding the differences in disease phenotypes for some cultivars.

The Correlation Between Septoria Leaf Spot and Stem Canker Resistance in Hybrid Poplar.

Sphaerulina musiva is an important fungal pathogen that causes a leaf spot and stem canker disease of hybrid poplar. Stem cankers are widely regarded as the greatest threat to hybrid poplar plantations because of their ability to cause tree mortality; thus, the efforts of breeding programs have been focused on stem canker resistance. To explore the relationship between resistance to leaf spot and stem canker in Populus nigra × P. deltoides hybrids, two experiments were conducted. Initially, comparisons among leaves of different ages indicated that younger leaves were more susceptible to leaf spot infection than older leaves. Correlations between leaf spot severity and stem canker severity for both individual leaves and all leaves averaged together indicated that, in 10 of 11 comparisons, there were no significant correlations. The lack of correlation suggests that deploying genotypes resistant to stem canker may not affect the pathogen population causing leaf spot disease. To our knowledge, this is the first study specifically designed to test the correlation between stem canker resistance and leaf spot resistance by inoculating whole trees with a spore suspension in a controlled environment.

Homoeologous duplicated regions are involved in quantitative resistance of Brassica napus to stem canker.

BackgroundSeveral major crop species are current or ancient polyploids. To better describe the genetic factors controlling traits of agronomic interest (QTL), it is necessary to understand the structural and functional organisation of these QTL regions in relation to genome duplication. We investigated quantitative resistance to the fungal disease stem canker in Brassica napus, a highly duplicated amphidiploid species, to assess the proportion of resistance QTL located at duplicated positions.ResultsGenome-wide association analysis on a panel of 116 oilseed rape varieties genotyped with 3228 SNP indicated that 321 markers, corresponding to 64 genomic regions, are associated with resistance to stem canker. These genomic regions are relatively equally distributed on the A (53%) and C (47%) genomes of B. napus. Overall, 44% of these regions (28/64) are duplicated homoeologous regions. They are located in duplications of six (E, J, R, T, U and W) of the 24 ancestral blocks that constitute the B. napus genome. Overall, these six ancestral blocks have 34 duplicated copies in the B.napus genome. Almost all of the duplicated copies (82% of the 34 regions) harboured resistance associated markers for stem canker resistance, which suggests structural and functional conservation of genetic factors involved in this trait in B. napus.ConclusionsOur study provides information on the involvement of duplicated loci in the control of stem canker resistance in B. napus. Further investigation of the similarity/divergence in sequence and gene content of these duplicated regions will provide insight into the conservation and allelic diversity of the underlying genes.Electronic supplementary materialThe online version of this article (doi:10.1186/1471-2164-15-498) contains supplementary material, which is available to authorized users.

Identification and mapping of a novel blackleg resistance locus LepR4 in the progenies from Brassica napus × B. rapa subsp. sylvestris

Blackleg, caused by Leptosphaeria maculans, is one of the most economically important diseases of Brassica napus worldwide. Two blackleg-resistant lines, 16S and 61446, were developed through interspecific hybridization between B. napus and B. rapa subsp. sylvestris and backcrossing to B. napus. Classical genetic analysis demonstrated that a single recessive gene in both lines conferred resistance to L. maculans and that the resistance alleles were allelic. Using BC(1) progeny derived from each resistant plant, this locus was mapped to B. napus linkage group N6 and was flanked by microsatellite markers sN2189b and sORH72a in an interval of about 10 cM, in a region equivalent to about 6 Mb of B. rapa DNA sequence. This new resistance gene locus was designated as LepR4. The two lines were evaluated for resistance to a wide range of L. maculans isolates using cotyledon inoculation tests under controlled environment conditions, and for stem canker resistance in blackleg field nurseries. Results indicated that line 16S, carrying LepR4a, was highly resistant to all isolates tested on cotyledons and had a high level of stem canker resistance under field conditions. Line 61446, carrying LepR4b, was only resistant to some of the isolates tested on cotyledons and was weakly resistant to stem canker under field conditions.

Defoliation of Brassica napus increases severity of blackleg caused by Leptosphaeria maculans: implications for dual-purpose cropping

Canola (Brassica napus) crops for grazing and grain (dual-purpose) production provide an economic break-crop alternative for dual-purpose cereals in Australian mixed farming systems. Infection by Leptosphaeria maculans is the most prevalent disease in Australian canola crops with airborne inoculum released throughout the autumn and winter when crops are grazed. Glasshouse and field experiments were conducted to investigate the effect of mechanical defoliation (simulated grazing) on disease severity at plant maturity. In glasshouse experiments, stem canker severity increased from 4% to 24% in severely defoliated plants, but light defoliation had no effect compared with undefoliated control plants. Disease severity was increased with defoliation in all field experiments. Defoliation increased crown canker severity from 22.6% to 39.3% at Wagga Wagga and from 3.0% to 7.1% at Canberra and lodging from 9.6% to 11.9% at Naracoorte in the same set of cultivars assessed at each site. The increase in disease severity with defoliation was less in canola lines with moderate to high levels of stem canker resistance. Plants defoliated before stem elongation tended to develop less disease than those defoliated during the reproductive phase of plant growth. These findings suggest that the impact of grazing on L. maculans infection of canola crops can be minimised by sowing cultivars with a high level of stem canker resistance and grazing during the vegetative stage of plant growth prior to stem elongation. Further research is required to determine whether these management strategies are applicable in canola crops defoliated by grazing animals.

Association mapping of quantitative resistance for Leptosphaeria maculans in oilseed rape (Brassica napus L.)

Stem canker caused by the fungus Leptosphaeria maculans is a major disease of Brassica napus. Quantitative resistance factors appear to be important components for effective and durable control of this pathogen. Quantitative trait loci (QTL) for stem canker resistance have previously been identified in the Darmor variety. However, before these QTL can be used in marker-assisted selection (MAS) to breed resistant varieties, they must be validated in a wide range of genetic backgrounds. We used an association mapping approach to confirm the markers located within the QTL previously identified in Darmor and establish their usefulness in MAS. For this, we characterized the molecular diversity of an oilseed rape collection of 128 lines showing a large spectrum of responses to infection by L. maculans, using 72 pairs of primers for simple sequence repeat and other markers. We used different association mapping models which either do or do not take into account the population structure and/or family relatedness. In all, 61 marker alleles were found to be associated with resistance to stem canker. Some of these markers were associated with previously identified QTL, which confirms their usefulness in MAS. Markers located in regions not harbouring previously identified QTL were also associated with resistance, suggesting that new QTL or allelic variants are present in the collection. All of these markers associated with stem canker resistance will help identify accessions carrying desirable alleles and facilitate QTL introgression.

Development of SCAR marker linked to stem canker resistance gene in soybean

Stem canker caused by the fungus Diaporthe phaseolorum f. sp. meridionalis is a disease that limits soybean cultivation. Phenotypic evaluations aiming at disease resistance require labor-intensive processes, as for instance handling and transport of phytopathogens. The use of DNA markers in the selective procedures eases certain phases, besides being practical, safe and reliable. A RAPD fragment of 588pb was identified among bulks of resistant and susceptible plants in the cross BR92-15454 (R) x IAC-11 (S). Through co-segregation, the distance between the resistance locus and the fragment was estimated at 7.4 ± 2.1 cM, with a Lodmax. of 23.072 (first year) and at 6.0 ± 3.4 cM with a Lodmax. of 7.806 (second year). The fragment was converted into a SCAR marker and digested with enzyme Hinc II, which made the classification in homozygous resistant, heterozygous resistant and susceptible plants possible. This SCAR marker is suitable for use in the improvement program conducted in Jaboticabal.

Adaptability and stability of soybean genotypes evaluated using three backcrosses generations: BC4, BC5 and BC6

An adaptability and stability study by the traditional and Lin and Binns methods (1988) was carried out to assess the genotypic stem canker resistance of three backcross generations (BC4, BC5 and BC6) in three environments. The study consisted of genotype trials carried out in the towns of Florestal, Capinopolis and Rio Paranaiba, in Minas Gerais state during the 1997/98 growing season. The experiments assessed the recurrent (FT-Cristalina) and donor (Doko BC) parents and 13 derived lines, five from the BC6 generation (UFV95370A2156, UFV95-370A2142, UFV95-370A2133, UFV95-370A2121 and UFV95-370A2115); four from BC5 (UFV95-370A2022, UFV95-370A2021, UFV95-370A2020 and UFV95-370A2019); and four from BC4 (UFV95-370A667, UFV95-370A666, UFV95-370A65 and UFV95-370A661). The Lin and Binns (1988) method was more efficient in discriminating the genotypes. The UFV95-370A2021 line belonging to the BC5 generation showed the best genotypic performance among the studied genotypes, and also showed the second best yield mean in the experiments. The UFV95-370A665 line, belonging to the BC4 generation, had the poorest genotypic behavior and also presented the lowest mean yield in the experiments. As the backcross generation advanced, there was a simultaneous improvement in the genotypic performance of the individuals in these generations, most probably because of the greater recuperation of the recurrent parent in each generation. This suggests the need for at least five to six backcrosses to obtain genotypes as adapted as the recurrent parent.

A Field Method for Evaluating the Potential Durability of New Resistance Sources: Application to the Leptosphaeria maculans-Brassica napus Pathosystem

ABSTRACT To increase the longevity of new resistance genes by avoiding a rapid change in pathogen populations, we established a new field method to determine, before the release of a resistant cultivar, whether and how rapidly the pathogen population is capable of responding to the selective pressure we impose. This method was applied to the Leptosphaeria maculans-Brassica napus pathosystem. The potential durability of two new major resistance genes introgressed into B. napus from the Brassica B genome was tested separately for each gene under field conditions for 4 years. Successive inoculations with residues of the resistant lines mixed with susceptible contaminated plant material recovered at harvest the previous year were performed in autumn. The Jlm1 resistance gene originating from B. juncea conferred complete resistance on the B. napus-B. juncea recombinant lines MX and MXS to inoculation of the cotyledons with a large diversity of L. maculans isolates. It also gave a high level of stem canker resistance in the field against natural populations of the pathogen. A similar level of resistance was obtained in the B. napus-B. nigra addition line LA4+, containing B. nigra chromosome 4 in a B. napus background. In the second year of the field experiment (i.e., the first in which residues from the resistant lines were included in the inoculation material), both MX and LA4+ maintained a high level of resistance. In the third and fourth years of the field experiment, the resistance of MX and MXS exposed to inoculum produced from their own residues broke down, but against fungal populations from susceptible B. napus or resistant B. nigra material remained effective. In contrast, LA4+ remained highly resistant to all sources of inoculum for the 4-year experiment.

Characterization of Stem Canker Resistance in ‘Hutcheson’ Soybean

Stem Canker caused by Diaporthe phaseolorum (Cooke & Ellis) Sacc. f. sp. meridionalis Morgan‐Jones (Dpm) can cause significant yield losses in soybean [Glycine max (L.) Merr.] in the southern USA. Some cultivars are known to be resistant to the disease; however, the parental donor, level, and inheritance of stem canker resistance for most of those cultivars is unknown. The objective of this study was to characterize stem canker resistance in ‘Hutcheson’, a widely used parent in breeding programs. To determine the inheritance of resistance and allelic relationships with known genes, Hutcheson was crossed with susceptible genotypes ‘Hartwig’ and J77‐339 and four lines carrying Rdcl, Rdc2, Rdc3, or Rdc4. Crosses were made in the field at Stoneville, MS, in 1993. The parents of Hutcheson and the six F2 populations were toothpick inoculated in the field in 1993 and 1994, respectively. In 1994, Hutcheson and 29 other soybean genotypes were inoculated with four isolates of Dpm and external lesion length was recorded as a measure of level of resistance. Genetic ratios from F2 populations indicate that resistance in Hutcheson is conferred by a single dominant gene that is allelic with Rdc4. ‘York’ was determined to be the parental donor of resistance carried by Hutcheson. Isolates differed in virulence. Hutcheson and several other genotypes were highly resistant to all isolates. Hutcheson should continue to be an excellent source of stem canker resistance.

Additional Sources of Stem Canker Resistance in Soybean Plant Introductions

Stem canker caused by Diaporthe phaseolorum (Cooke & Ellis) Sacc. f. sp. meridionalis Morgan‐Jones can cause significant yield losses in soybean [Glycine max (L.) Merr.] in the southern USA. Many genotypes are known to be resistant to the disease; however, little is known about the inheritance of resistance and allelic relationships of genes conferring resistance. The objective of this study was to determine the inheritance of resistance in two germplasm accessions, PI 230976 and PI 398469, and define allelic relationships with the four described genes. Each PI was crossed with a susceptible genotype, J77‐339, and the four fines carrying Rdcl, Rdc2, Rdc3, or Rdc4. Crosses were made in the field at Stoneville, MS, in 1992, and F1 backcrosses to J77‐339 were made in 1993. Plants of parents and 10 F2 populations were toothpick inoculated in the field in 1993. The F2:3 and backcross populations were later evaluated in a greenhouse. The F2 populations derived from J77‐339 × PI 230976 and J77‐339 × PI 398469 approached a 3 resistant/1 susceptible ratio indicating that a single dominant gene for resistance is carried by each plant introduction. The F3 data confirmed this result. Backcross data substantiated complete dominance. Response of all F2 populations derived from crosses with genotypes carrying known genes suggested a 15:1 ratio indicating nonallelism. It is not known if both plant introductions carry the same gene. These sources of resistance may lessen genetic vulnerability should new virulent isolates overcome sources now used by breeders

Inheritance of Stem Canker Resistance in Soybean Cultivars Crockett and Dowling

Stem Canker caused by Diaporthe phaseolorum (Cooke & Ellis) Sacc. var. caulivora K. L. Athow & R. M. Caldwell (Dpc) has become a serious disease of soybean [Glycine max (L.) Merr.] throughout the southeastern USA. Sources of resistance have been identified and information on the inheritance of resistance is needed for each source. The objective of this work was to study the inheritance of resistance to stem canker in ‘Crockett’ and ‘Dowling’ soybean. Crockett, Dowling, and ‘Tracy‐M’ were crossed with each other and with two susceptible cultivars, Coker 338 and Johnston. Disease reactions of parent and progeny populations were studied in the greenhouse following artificial (toothpick) incoculation with a southern isolate of Dpc. Crosses between either Dowling or Crockett and the two susceptible parents resulted in segregating F2 populations that fit a 3 resistant : 1 susceptible ratio, and F2:3 families that fit a 1 resistant : 2 segregating : 1 susceptible ratio. The cross between Crockett and Dowling resulted in a 15:1 ratio in the F2 and a 7:8:1 ratio in the F3 generation. Crosses between Tracy‐M and Crockett or Dowling resulted in segregation ratios of 63:1 in the F2 and 37:26:1 and F3. These results suggest that Crockett and Dowling each have a major dominant gene, and that these two genes are different from RdC1 and RdC2 found in Tracy‐M. The gene symbols Rdc3 and Rdc4 are proposed to identify genes controlling resistance to stem canker in Crockett and Dowling, respectively.

Tomato susceptibility to Alternaria stem canker: Parameters involved in host-specific toxin-induced leaf necrosis

AAL-toxin causes severe necrosis in leaves of susceptible tomato cultivars at nanomolar concentrations. In resistant tomato cultivars harbouring the semi-dominant Alternaria stem canker resistance locus necrosis is also observed, however at much higher toxin concentrations. In both lines the percentage of the leaf area exhibiting necrosis is dependent on toxin concentration and on length of toxin exposure. However, at the same toxin concentration, periods of toxin exposure resulting in similar necrosis are much longer for the resistant than for the susceptible tomato. It was demonstrated that toxin uptake in the leaves does not imply toxin uptake in the cells since a discrepancy was observed between death of protoplasts, isolated from leaves cut for protoplast isolation immediately after incubation on AAL-toxin and necrosis in leaves when further incubated on water. However, when after exposure to AAL-toxin leaves were further incubated on water for 24 h before they were cut for protoplast isolation, a correlation was found between leaf necrosis and death of protoplasts. This suggests that further transport is needed in leaves after toxin uptake, bringing toxin to all the cells, that cannot occur in leaves cut for protoplast isolation. Light plays an important role in AAL-toxin induced necrosis and it was shown that length of light exposure controls necrosis development like toxin concentration and length of toxin exposure. The product of these 3 parameters can provide a good hint to predict the extent of leaf necrosis. The effect of light might be restricted to differentiated leaf tissue, since it was not observed in callus tissue.

Tomato resistance to Alternaria stem canker: localization in host genotypes and functional expression compared to non-host resistance

The Alternaria stem canker resistance locus (Asc-locus), involved in resistance to the fungal pathogen Alternaria alternata f. sp. lycopersici and in insensitivity to host-specific toxins (AAL-toxins) produced by the pathogen, was genetically mapped on the tomato genome. Susceptibility and resistance were assayed by testing a segregating F2 population for sensitivity to AAL-toxins in leaf bioassays. Linkage was observed to phenotypic markers solanifolium and sunny, both on chromosome 3. For the Asc-locus, a distance of 18 centiMorgan to solanifolium was calculated, corresponding to position 93 on chromosome 3. This map position of the resistance locus turned out to be the same in three different resistant tomato accessions, one Dutch and two American, that are at least 40 years apart. AAL-toxin sensitivity in susceptible and resistant tomato genotypes was compared with AAL-toxin sensitivity in a non-host Nicotiana tabacum during different levels of plant cell development. In susceptible and resistant tomato genotypes, inhibitory effects were demonstrated at all levels, except for leaves of resistant genotypes. However, during pollen and root development, inhibitory effects on susceptible genotypes were larger than on resistant genotypes. In the non-host Nicotiana tabacum, hardly any effects of AAL-toxins were demonstrated. Apparently, a cellular target site is present in tomato, but not in Nicotiana tabacum. It was concluded that three levels of AAL-toxin sensitivity exist: (1) a susceptible host sensitivity, (2) a resistant host sensitivity, (3) a non-host sensitivity, and that the resistance mechanism operating in tomato is different from that operating in Nicotiana tabacum.

Field and Greenhouse Evaluations of Stem Canker Resistance in Soybean

Greenhouse screening using infested toothpicks was compared to field evaluation of soybean [Glycine max (L.) Merr.) breeding lines for resistance to stem canker disease, caused by southern strains ofDiaporthe phaseolorum (Cke. & E11. (Sacc.) var. caulivora Athow and Caldwell (Dpc). Field screenings are reliable indicators of resistance, but often the disease does not develop naturally in field screening nurseries. Thirty‐seven random F4:6 lines from the cross ‘Hutton’ (susceptible ) ✕ ‘Tracy M’ (resistant) were evaluated for their reaction to Dpc in the field (two locations, 2 yr) under natural infestation and infection conditions, and in the greenhouse (three experiments) with artificial inoculation using infested toothpicks. Our objectives were to compare field and greehouse screening and to determine the usefulness of greenhouse inoculation in predicting the yield and disease reaction of breeding lines when these lines were subjected to natural field infection conditions. Field screening based on symptoms and yield was highly effective in identifying resistant genotypes. Heritabilities for yield and disease ratings in the field were 87 and 92%, respectively. The toothpick inoculation procedure used in the greenhouse was effective with each of three Dpc isolates (different in geographic origin from the field location) in identifying the genotypes that showed highest levels of disease resistance in the field. Phenotypic correlations between greenhouse ratings and yield in the infested field ranged from −0.71 to −0.61. Results indicated that selection based on greenhouse screening can be an effective alternative to field screening when resistance is derived from Tracy M.

Inheritance of Reaction to Stem Canker in Soybean1

There are no known published reports concerning the inheritance of reaction to the disease stem canker [caused by the fungus Diaporthe phaseolorum (Cke. & Ell.) Sacc. var. caulivora Athow & Cald.] in soybean [Glycine max (L.) Merr.]. Because stem canker has become a serious soybean disease in the southern United States, and because a source of resistance has been identified, information on the inheritance of reaction to this disease is needed. The objective of this study was to determine the inheritance of resistance to stem canker found in the soybean ‘Tracy‐ M.’ Crosses were made between Tracy‐M and a very susceptible line, J77‐339. The disease reactions of parental, F1, F2 and F3 populations from that cross to stem canker were studied under field conditions to determine the genetic basis for resistance. All F1 plants were resistant and the F2 generation fit a 15 resistant:1 susceptible ratio (✕2 probability = 0.75 to 0.90). The 199 F, families fit a ratio of 7 resistant:8 segregating:! susceptible (X2 probability = 0.25 to 0.50). These results indicate that stem canker resistance in Tracy‐M is controlled by two major dominant genes.