The inheritance of resistance to the blossom weevil, Anthonomus rubi, in the cultivated strawberry, Fragaria×ananassa

The objective of this research was to determine which strawberry germplasm could be used to combine late flowering with reduced susceptibility to the strawberry blossom weevil, Anthonomus rubi. Seven cultivars and three breeding lines were compared for their susceptibility to A. rubi, by introducing adult weevils onto detached inflorescences. No consistent differences were detected but this contradicted observations from field trials. Five late‐flowering strawberry genotypes were crossed in a half‐diallel programme and progeny were tested for their susceptibility to A. rubi by using detached inflorescences and in a field trial where plants were exposed to a natural infestation. There was good correspondence between the results from the two methods and heritable differences were detected in both experiments, with the additive genetic variance being the more important. The cultivars ‘Idea’ and ‘Alice’ were identified as promising parents for reduced susceptibility while progeny from ‘Sophie’ were most likely to be susceptible.     

Late blight resistance in a diploid full-sib potato family

Late blight, caused by Phytophthora infestans (Mont.) de Bary, is the most destructive disease of potato worldwide. As this pathogen can rapidly overcome major race‐specific resistance genes, identifying the basis for enhanced quantitative resistance has become a crucial element for implementing advanced breeding strategies. A population of 230 full‐sib progeny derived from a cross between two diploid hybrid Solanum phureja × S. stenotomum clones was evaluated for foliage resistance against late blight in replicated trials at multiple locations in Pennsylvania between 1999 and 2002. In field experiments, plants were evaluated visually for per cent defoliation, and area under the disease progress curve (AUDPC) was determined. The two parents and three control cultivars (‘Atlantic’, ‘Kennebec’ and ‘Katahdin’) were included in all trials. In all three experiments, the presence of a significant number of clones exhibiting transgressive segregation were observed. There were significant differences among environments as well as among clones, and the clone × environment interaction was also significant. Stability analysis revealed that 37 clones made a significant contribution to the overall environment × clone interaction. Broad‐sense heritability for resistance, measured as AUDPC, was estimated as 0.67. The overall results indicate the presence in this potato family of a high level of field resistance against late blight. This segregating diploid family appears to be a good candidate for quantitative trait loci mapping to identify and characterize the genetic components of partial late blight resistance.     

Evaluation of resistance to sharka (Plum pox virus) of several Prunus rootstocks

Sharka (Plum pox virus, PPV) severely affects the production of most Prunus species in the areas affected by the disease. In this study, the resistance of 15 Prunus rootstocks to a Dideron type isolate of PPV was evaluated under controlled conditions in an insect‐proof greenhouse. After four cycles of study,‘GF677’ almond x peach hybrid,‘Myrobolan 29C plum and ‘L2’ cherry did not show any symptoms and were ELISA‐DASI and RT‐PCR negative. These were considered resistant to PPV. The rest of the rootstocks assayed showed symptoms of sharka (confirmed by ELISA‐DASI or RT‐PCR), although the level of susceptibility was different for each rootstock.‘GF305’ peach, ‘Puebla de Soto’ plum and ‘Real Fino’ apricot, were highly susceptible to PPV, showing strong sharka symptoms and being ELISA and RT‐PCR positive. ‘Marianna 2624’ plum,‘AC 9921‐07’ hybrid and ‘CP‐2’ plum showed susceptibility to PPV confirmed by ELISA‐DASI and RT‐PCR positives.‘Nemaguard’ and ‘Nemared’ hybrids, ‘Torinel’ plum and ‘STN2’ hybrid showed an intermediate susceptibility to PPV with slight sharka symptoms and were ELISA‐DASI and RT‐PCR positive. Whereas, ‘Montclar’ peach and ‘Evrica’ hybrid showed moderate resistance to PPV with slight sharka symptoms and were ELISA positive but RT‐PCR negative. The results open new possibilities in the search for different sources of resistance to PPV within Prunus.     

Genes for wheat stem rust resistance postulated in German cultivars and their efficacy in seedling and adult‐plant field tests

Stem rust of wheat (caused by Puccinia graminis f.sp. tritici) gained high international attention in the last two decades, but does not occur regularly in Germany. Motivated by a regional epidemic in 2013, we analysed 15 spring and 82 winter wheat cultivars registered in Germany for their resistance to stem rust at the seedling stage and tested 79 of these winter wheat cultivars at the adult‐plant stage. A total of five seedling stem rust resistance genes were postulated: Sr38 occurred most frequently (n = 29), followed by Sr31 (n = 11) and Sr24 (n = 8). Sr7a and Sr8a occurred only in two spring wheat genotypes each. Four cultivars had effective seedling resistance to all races evaluated that could only be explained by postulating additional resistance genes (‘Hyland’, ‘Pilgrim PZO’, ‘Tybalt’) or unidentified gene(s) (‘Memory’). The three winter wheat cultivars (‘Hyland’ ‘Memory’ and ‘Pilgrim PZO’) were also highly resistant at the adult‐plant stage; ‘Tybalt’ was not tested. Resistance genes Sr24 and Sr31 highly protected winter wheat cultivars from stem rust at the adult‐plant stage in the field. Disease responses of cultivars carrying Sr38 varied. Mean field stem rust severity of cultivars without postulated seedling resistance genes ranged from 2.71% to 41.51%, nine of which were significantly less diseased than the most susceptible cultivar. This suggests adult‐plant resistance to stem rust may be present in German wheat cultivars.     

Assessment of partial resistance to powdery mildew in Chinese wheat varieties

Field trials in two cropping seasons and two locations in central China were conducted on 60 Chinese autumn‐sown wheat varieties to assess their partial resistance to powdery mildew. Mean levels of disease severity ranged from close to 0 to more than 90%. The method of inoculation and the location in which trials were conducted affected the relative performance of the varieties, but these effects were much smaller than the main effect of variety. The area under the disease progress curve was highly correlated with final disease severity, but both were poorly correlated with apparent infection rate. Disease severity was regressed against frequencies of virulence in the Blumeria graminis (syn. Erysiphe graminis) f sp. tritici populations in the trial plots. A vertical distance (D) from the mean mildew severity to the fitted line was calculated for each variety and was used to quantify partial resistance. Five of the 60 varieties, ‘Hx8541’, ‘E28547’, ‘Chuan1066’, ‘Zhe88pin6’ and ‘Lin5064’, consistently expressed relatively low levels of disease despite high frequencies of virulence in the pathogen and had consistently high D‐values. They may therefore have good levels of partial resistance.     

The Physiology and Stability of Leaf Carbon Isotope Discrimination as a Measure of Water‐Use Efficiency in Barley on the Canadian Prairies

Temporal and seasonal water deficit is one of the major factors limiting crop yield on the Canadian prairie. Selection for low carbon isotope discrimination (Δ¹³C) or high water‐use efficiency (WUE) can lead to improved yield in some environments. To understand better the physiology and WUE of barley under drought conditions on the Canadian prairie, 12 barley (Hordeum vulgare L.) genotypes with contrasting levels of leaf Δ¹³C were investigated for performance stability across locations and years in Alberta, Canada. Four of those genotypes (‘CDC Cowboy’, ‘Niobe’, ‘170011’ and ‘Kasota’) were also grown in the greenhouse under well‐watered and water‐deficit conditions to examine genotypic variations in leaf Δ¹³C, WUE, gas exchange parameters and specific leaf area (SLA). The water‐deficit treatment was imposed at the jointing stage for 10 days followed by re‐watering to pre‐deficit level. Genotypic ranking in leaf Δ¹³C was highly consistent, with ‘170011’, ‘CDC Cowboy’ and ‘W89001002003’ being the lowest and ‘Kasota’‘160049’ and ‘H93174006’ being the highest leaf Δ¹³C. Under field and greenhouse (well‐watered) conditions, leaf Δ¹³C was significantly correlated with stomatal conductance (g_s). Water deficit significantly increased WUE, with ‘CDC Cowboy’– a low leaf Δ¹³C genotype with significantly higher WUE and lower percentage decline in assimilation rate (A) and g_s than the other three genotypes (‘Niobe’, ‘170011’ and ‘Kasota’). We conclude that leaf Δ¹³C is a stable trait in the genotypes evaluated. Low leaf Δ¹³C of ‘CDC Cowboy’ was achieved by maintaining a high A and a low g_s, with comparable biomass and grain yield to genotypes showing a high g_s under field conditions; hence, selection for a low leaf Δ¹³C genotype such as ‘CDC Cowboy’ maybe important for maintaining productivity and yield stability under water‐limited conditions on the Canadian prairie.     

Eyespot resistance in wheat ×Aegilops kotschyi backcross lines

In wheat, eyespot caused by PseudoCercosporella herpotrichoides, is one of the main foot‐rot diseases. Yield losses up to 40% occur in some years. Plant protection by fungicide application is possible, but a better way is through resistance breeding. Two resistance sources are currently used: Aegilops ventricosa and the old French variety ‘Cappelle Desprez’. A new source of resistance has been found in the accession AE120 of Ae. kotschyi from the Gatersleben gene bank with the genome constitution UUSvSv. This accession has been crossed and backcrossed twice to susceptible wheat varieties, and in each generation, plants with a relatively high level of resistance have been selected. From this material, lines have been developed and tested in F₆ to F₈. Finally, several lines could be classified as moderately resistant, such as the French variety ‘Cappelle Desprez’ after resistance determination during milk ripeness (DC75). No line reached the high resistance level achieved with Pch‐1 from Ae. ventricosa. The yield of these lines under infection conditions was higher compared with ‘Cappelle Desprez’. The line 6018‐96‐3 showed a high yield of 64.3 dt/ha compared with 59.6 dt/ha, on the average, in combination with the best expression of eyespot resistance in the adult growth stage over 3 years.     

Identification and mapping of adult‐plant stripe rust resistance in soft red winter wheat cultivar ‘USG 3555’

Little is known about the extent or diversity of resistance in soft red winter wheat (Triticum aestivum L.) to stripe rust, caused by the fungal pathogen Puccinia striiformis f.sp. tritici. The soft red winter (SRW) wheat cultivar ‘USG 3555’ has effective adult‐plant resistance to stripe rust, which was characterized in a population derived from ‘USG 3555’/‘Neuse’. The mapping population consisted of 99 recombinant inbred lines, which were evaluated for stripe rust infection type (IT) and severity to race PST‐100 in field trials in North Carolina in 2010 and 2011. Genome‐wide molecular‐marker screenings with 119 simple sequence repeats and 560 Diversity Arrays Technology (DArT) markers were employed to identify quantitative trait loci (QTL) for stripe rust resistance. QTL on chromosomes 1AS, 4BL and 7D of ‘USG 3555’ explained 12.8, 73.0 and 13.6% of the variation in stripe rust IT, and 13.5, 72.3 and 10.5% of the variation in stripe rust severity, respectively. Use of these and additional diagnostic markers for these QTL will facilitate the introgression of this source of stripe rust resistance into SRW wheat lines via marker‐assisted selection.     

A leaf rust resistance gene, different from Lr34, associated with leaf tip necrosis in wheat

The gene Lr34 has contributed to durable resistance to leaf rust caused by Puccinia triticina in wheat worldwide. The closely associated leaf tip necrosis is generally used as the gene's marker. Lr34 has been postulated in many Indian bread wheat cultivars including ‘C 306’, based on the associated leaf tip necrosis and a few other field and glasshouse observations. The present study showed monogenic control of adult‐plant resistance in ‘C 306’ to leaf rust pathotype 77‐5 (121R63‐1). The F₂ segregation in the crosses between ‘C 306’ and the two known carriers of Lr34, ‘Line 897’ and ‘Jupateco 73’‘R’ fitted a digenic ratio. The F₃ families derived from the susceptible F₂ segregants were true breeding for susceptibility, proving the absence of Lr34 in ‘C 306’. The cross between ‘Line 897’ and ‘Jupateco 73’‘R’ did not segregate for susceptibility. Resistance in the cross ‘Agra Local’ (susceptible) × ‘C 306’ was associated with leaf tip necrosis, showing that the leaf rust resistance gene in ‘C 306’ was associated with leaf tip necrosis, but was different from Lr34. This gene is being temporarily designated as Lr‘C 306’. Hence, leaf tip necrosis cannot be considered as an exclusive marker for selecting Lr34 in wheat improvement.     

Genotype analysis of the wheat semidwarf Rht‐B1b and Rht‐D1b ancestral lineage

In wheat, semidwarfism resulting from reduced height (Rht)‐B1b and Rht‐D1b was integral to the ‘green revolution’. The principal donors of these alleles are ‘Norin 10’, ‘Seu Seun 27’ and ‘Suwon 92’ that, according to historical records, inherited semidwarfism from the Japanese landrace ‘Daruma’. The objective of this study was to examine the origins of Rht‐B1b and Rht‐D1b by growing multiple seed bank sources of cultivars comprising the historical pedigrees of the principal donor lines and scoring Rht‐1 genotype and plant height. This revealed that ‘Norin 10’ and ‘Suwon 92’ sources contained Rht‐B1b and Rht‐D1b, but the ‘Seu Seun 27’ source did not contain a semidwarf allele. Neither Rht‐B1b nor Rht‐D1b could be definitively traced back to ‘Daruma’, and both ‘Daruma’ sources contained only Rht‐B1b. However, ‘Daruma’ remains the most likely donor of Rht‐B1b and Rht‐D1b. We suggest that the disparity between historical pedigrees and Rht‐1 genotypes occurs because the genetic make‐up of seed bank sources differs from that of the cultivars actually used in the pedigrees. Some evidence also suggests that an alternative Rht‐D1b donor may exist.     

Alleles at a quantitative trait locus for stem solidness in wheat affect temporal patterns of pith expression and level of resistance to the wheat stem sawfly

The stem solidness trait in wheat has been the most effective mechanism for management of the wheat stem sawfly (WSS) for six decades. However, recent results have shown that in certain genotypes, the degree of stem solidness is not a useful indicator of WSS resistance. A morphological characterization of solidness expression indicated that in the genotype ‘Conan’, very solid pith undergoes rapid retraction during stem maturation, resulting in significantly less solidness at maturity. In other solid‐stemmed genotypes, including the standard WSS‐resistant cultivar ‘Choteau’, dense pith in the stem remains nearly unchanged throughout plant development. In cage trials, ‘Conan’ plants were less preferred for oviposition by the WSS when paired with ‘Choteau’ plants. Field bioassays using near‐isogenic lines differing for alleles at Qss.msub‐3BL showed that the Conan allele provides higher levels of early stem solidness and rapid pith retraction during stem maturation. These results suggest that the traditional approach for increasing WSS resistance by selecting for increasing stem solidness needs to be modified to consider temporal variations in pith expression associated with alleles at Qss.msub‐3BL.     

Pyramiding of insect- and disease-resistance genes into an elite indica, cytoplasm male sterile restorer line of rice,'Minghui 63'

The wild‐rice‐derived dominant gene Xa21 conferring multi‐race resistance to bacterial blight and a fused Bt gene cry1Ab/cry1Ac conferring resistance to lepidopteran insects were individually introduced into the same genetic background of an elite indica cytoplasm male sterile (CMS) restorer line ‘Minghui 63′. The line showed the desirable insect‐ and disease‐resistant phenotypes. To maximize the effect, the two genes were also pyramided into the same recipient plant of ‘Minghui 63’ by marker‐assisted selection. After being subjected to natural infestation of leaf‐folders and yellow stem borers and inoculation of Xoo strain mixtures, the pyramiding line and its derived hybrids showed high levels of resistance against both insect damage and disease. Furthermore, data from field trials demonstrated that the hybrids made by crossing this pyramiding line with the CMS lines ‘Zhenshan 97A’ and ‘Maxie A’ retained a similar level of yield under conditions without chemical spray, indicating that the pyramiding genes have a yield‐stabilizing effect on the recipient line and its hybrids.     

Genetic analysis of resistance in barley to barley yellow dwarf virus

The inheritance of resistance to barley yellow dwarf virus (BYDV) was studied in the selected 24 spring and winter barley cultivars that showed a high or intermediate resistance level in 1994‐97 field infection tests. The polymerase chain reaction diagnostic markers YLM and Ylp were used to identify the resistance gene Yd2. The presence of the Yd2 gene was detected with both markers in all the resistant spring barley cultivars and lines from the CIMMYT/ICARDA BYDV nurseries. The results of field tests and genetic analyses in winter barley corresponded with marker analyses only when the Ylp marker was used. Genes non‐allelic with Yd2 were detected by genetic analyses and the Ylp marker in moderately resistant spring barley cultivars ‘Malvaz’, ‘Atribut’ and ‘Madras’, and in the winter barley cultivars ‘Perry’ and ‘Sigra’. Significant levels of resistance to BYDV were obtained by combining the resistance gene Yd2 with genes detected in moderately resistant cultivars. The utilization of analysed resistance sources in barley breeding is discussed.     

'Francolí', a late flowering almond cultivar re-classified as self-compatible

To verify the compatibility behaviour of the almond cultivar ‘Francolí’ and to clarify its S genotype a combination of pollination tests, stylar ribonuclease and allele specific PCR analysis was used. ‘Francolí’ was released from IRTA's breeding programme in 1994, having been putatively raised from the cross ‘Cristomorto’ (S₁S₂) × ‘Gabaix’ (S₁₀S₂₅). This cultivar was also reported to be self‐incompatible but revealing only one S band in the zymograms after S‐RNases analysis. ‘Francolí’ sets nuts after test crossing with two S₁S₂₅ cultivars, having a different genotype from that earlier reported. ‘Francolí’ was also observed to be self‐compatible after selfing flowers in the field and in the laboratory. ‘Francolí’ was re‐assigned the S₁S_f genotype after test crossing, stylar ribonuclease and PCR data analysis. After microsatellite analysis, the self‐compatible ‘Tuono’ (S₁S_f) cultivar is suggested as the male parent of ‘Francolí’ instead of the earlier reported ‘Gabaix’.     

Genetic control of crossability of triticale with rye

Limited genetic knowledge is available regarding crossability between hexaploid triticale (2n= 6x= 42, 21″, AABBRR, amphiploid Triticum turgidum L.‐Secale cereale L.) and rye (2n= 14, 7″, RR). Our objectives were to determine (1) the crossability between triticales and rye and (2) the inheritance of crossability between F₂ progeny from intertriticale crosses and rye. First, ‘8F/Corgo’, a hexaploid triticale, was crossed as a female with two landrace ryes, ‘Gimonde’ and, ‘Vila Pouca’ and two derived north European cultivars, ‘Pluto’ and ‘Breno’. These crosses produced 21.7, 20.9, 5.9, and 5.6%, seed‐set or crossability, respectively, showing that the landrace ryes produced higher seed‐set than the cultivars. Second, ‘Gimonde’ rye was crossed as a male with four triticales for 3 years. The control cross, ‘Chinese Spring’ wheat × rye, produced 80‐90% seed‐set. Of the four triticales, ‘Beagle’ produced 35.7‐56.8% seed‐set. The other three triticales produced less than 20% seed‐set, showing that the triticales differ in crossability with ‘Gimonde’ rye. Third, six FiS from intertriticale crosses (‘8F/Corgo’בBeagle’, ‘Beagle’בCachirulo’, ‘Lasko’בBeagle’, ‘8F/Corgo’בCachirulo’, ‘Lasko’בCachirulo’, ‘Lasko’ב8F/Corgo’) were crossed to ‘Gimonde’ rye. Results indicated that lower crossability trait was partially dominant in the two F₁S from crosses involving ‘Beagle’(high crossability) with‘8F/Corgo’ and ‘Cachirulo’(low crossability) and completely dominant in the ‘Beagle’בLasko’ cross, as it happens in wheat. Fourth, segregants in four F₂ populations (‘Lasko’בBeagle’, ‘8F/Corgo’בBeagle’, ‘Lasko’ב8F/Corgo’, and‘8F/Corgo’בCachirulo’) were crossed with rye. Segregation for crossability was observed, although distinct segregation classes were blurred by environmental and perhaps other factors, such as self‐incompatibility alleles in rye. Segregation patterns showed that ‘Beagle’, with high crossability to rye, carries either Kr1 or Kr2. The three triticales with low crossability with rye were most likely homozygous for Kr1 and Kr2. Therefore, it is likely that the Kr loci from A and B genomes acting in wheat also play a role in triticale × rye crosses.     

Chromosomal location of Fusarium head blight resistance genes and analysis of the relationship between resistance to head blight and brown foot rot

In order to identify chromosomes involved in resistance to Fusarium head blight, a set of 21 substitution lines of Triticum macha (resistant) chromosomes into ‘Hobbit’'sib’(susceptible) were evaluated in trials over 2 years. For the first year's trial, all plants were inoculated on the same day with a conidial suspension of F. culmorum. For the second trial, individual plants were inoculated precisely at mid anthesis of each plant over a period of 2 weeks. The disease level was assessed by visual scoring, relative ear weight and F. culmorumn‐specfic quantitative polymerase chain reaction. The results showed that T. macha chromosomes 1B, 4A and 7A conferred good overall resistance, suggesting that they carry important genes for resistance. In two additional trials, T. macha and ‘Hobbit’'sib’ were evaluated for resistance to brown foot rot. The results showed that T. macha was more susceptible than ‘Hobbit’‘sib’, indicating that stem base disease response is not correlated with head blight resistance in these cultivars.     

Inheritance of resistance to the 'Kanpur' race of Phytophthora drechsleri in pigeonpea

Phytophthora drechsleri causes stem blight, which is one of the most serious diseases of pigeonpea. Eight races of this fungus have been identified, but the inheritance of resistance to all these races is not clear except for race P2. This study examined the inheritance of resistance to race ‘Kanpur’ (KPR) of P. drechsleri in eight crosses involving four resistant parents, viz.‘KPBR 80‐2‐1′, ‘KPBR 80‐2‐2′, ‘Hy 3C and ‘BDN 1′, and two susceptible parents, viz.‘Bahar’ and ‘PDA 10′. The reactions of the parental lines, and their F₁, F₂ and backcross generations were studied in an infected plot. In the F₁ generation of all crosses, a susceptible reaction was observed that indicated dominance of susceptibility over resistance. The segregation pattern in F₂ indicated that two homozygous recessive genes (pdr₁pdr₁pdr₂pdr₂) were responsible for imparting resistance in the parents, ‘KPBR 80‐2‐1’ and ‘KPBR 80‐2‐2′, and that a single homozygous recessive gene (pdrpdr) was responsible for resistance in the parents ‘Hy 3C and ‘BDN 1′. Therefore, ‘KPBR 80‐2‐1’ and ‘KPBR 80‐2‐2’ with two genes for resistance are better donors because the resistance transferred from them will be more durable compared with ‘Hy3C and ‘BDN1’ with only one gene for resistance.     

Powdery mildew resistance in Czech and Slovak barley cultivars

Fifteen powdery mildew resistance genes and the gene MlaN81 derived from ‘Nepal 81’were found in 76 Czech and Slovak spring and winter barley cultivars when tested for reaction to a set of powdery mildew isolates. Nine cultivars (‘Donum’, ‘Expres’, ‘Jubilant’, ‘Orbit’, ‘Primus’, ‘Progres’, ‘Stabil’, ‘Vladan’ and ‘Zlatan’) are composed of lines with different resistance genes. The Mlat gene is present in nine cultivars and was transferred from the Anatolian landrace ‘A‐516′. The resistances derived from ‘KM‐1192’and ‘CI 7672’were identical and designated Ml(Kr). Five winter barley cultivars possess the Ml(Bw) resistance. The winter barley line ‘KM‐2099’carries the mlo gene. The parental cultivar ‘Palestine 10’was also tested in which the genes Mlk1, MlLa were identified. The German cultivar ‘Salome’, a parent of seven cultivars tested, probably carries the gene MlLa in addition to mlo and Mla7. The gene mlo6 may be present in the cultivar ‘Heris’. Most of the results were confirmed by the pedigrees of the cultivars.     

The linkage between the stem rust resistance gene Sr2 and pseudo-black chaff in wheat can be broken

Recessively inherited gene Sr2 has provided the basis of durable resistance to stem rust (caused by Puccinia graminis tritici) in wheat (Triticum aestivum L.) worldwide. The associated earhead and stem melanism or ‘pseudo‐black chaff’ is generally used as a marker for this gene. Sr2 has been postulated in many wheat cultivars of India including ‘Lok 1’, based on associated pseudo‐black chaff in adult plants, and leaf chlorosis in seedlings. However, dominant inheritance of the resistance factor operating in ‘Lok 1’, and a 13 : 3 (resistant : susceptible) F₂ segregation in the ‘Sr2‐line’ (‘Chinese Spring’⁶ × ‘Hope’ 3B) × ‘Lok 1’ cross confirmed that Sr2 was absent in ‘Lok 1’. Susceptible plants with a pseudo‐black chaff phenotype were observed in F₂ populations of ‘Agra Local’ (susceptible) × ‘Lok 1’, and the ‘Sr2‐line’ × ‘Lok 1’ crosses. Most of the F₃ families derived from the susceptible F₂ segregants with pseudo‐black chaff phenotypes were true breeding for the expression of pseudo‐black chaff with susceptibility to stem rust. Thus, linkage of pseudo‐black chaff with Sr2 in wheat can be broken, and hence, caution may be exercised in using pseudo‐black chaff as a marker for selecting Sr2 in breeding programmes.