Genotypic response of faba bean to water stress

Forty resistant rice cultivars were studied for theinheritance of resistance to bacterial blight usingPhilippine races of Xanthomonas oryzae pv.oryzae (Xoo). Results showed that all thevarieties have at least two recessive genes forresistance. One of these genes governs resistance torace 1 (PXO61) while the other gene confers resistanceto race 6 (PXO99). In addition to the recessivegenes, nine of the varieties possess another dominantgene which also confers resistance to race 1.Allelism tests revealed that the recessive genesgoverning resistance to race 1 in 39 varieties areallelic to xa5 while the dominant genes in thenine varieties are allelic to Xa4. Therecessive gene conferring resistance to race 1 incultivar Sada Diga is inherited independently of xa5. Similarly, the recessive genes governingresistance to race 6 in all the varieties arenon-allelic to xa13. The allelic relationshipsof these genes with xa-24(t), a new recessivegene identified in cultivar DV86 which conveysresistance to race 6 are now being investigated.     

Genetic Analysis of Resistance to Brown Planthopper in Rice (Oryza sativa L.)

Seventeen rice cultivate resistant to brown planthoppers were genetically analyzed using the Bangladesh insect population. Seven cultivars were found to have a single dominant gene for resistance. These genes segregated independently of the recessive resistance gene bph-5. Tae dominant resistance gene of ‘Swarnalata’ was designated Bph-6. In ten cultivars, resistance is conferred by single recessive genes. In eight cultivars, the resistance genes are allelic to bph-5. However, the recessive genes o: two cultivars are non-allelic to bph-5. The recessive gene of T12 is designated bph-7.     

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.     

Genetic Analysis of Resistance to Green Leafhopper in Rice

The inheritance of resistance to green leafhopper, Nephotettix impicticeps Ichi, was studied in 11 cultivars of rice, Oryza saliva L. These resistant cultivars were crossed with the susceptible cultivar ‘TN1’. The materials consisted of F₁, F₂ and F₃ populations including parents which were assessed by the bulk screening test. It was found that resistance in the cultivars TR36′, UPR254-35-3′-2′, ‘Jhingasail’, ‘Govind’, ‘RP825-45-1-3’, ‘MRC603-303’, ‘RD4’, and ‘Irat104 ’ was conditioned by a single dominant gene, whereas resistance in ‘Ptb8’ IR9805-97-1′, and ‘BG367-7’ was controlled by one recessive gene. The test on the allelic relationships of the resistance genes in the test cultivars with the known genes Glb1 and Glb2 revealed that the single dominant gene that conveyed the resistance in ‘UPR254-35-3-2’ and ‘Jhingasail’ was allelic to Glh1 and segregated independently of Glh2. The resistance in ‘Govind’ and ‘RP82S-45-1-3’ was governed by the Glh2 gene which was independent of Glh1. The test cultivars ‘IR36’;. ‘MRC603-303’, ‘RD4’. and Irat104 ’ had a dominant gene for resistance which was nonallelic to Glb1 and Glb2. The recessive gene which conditioned the resistance in ‘Ptb8’, ‘IR9805-97-1’, and ‘BG367-1’ segregated independently of Glh1 and Glh2. Eleven trisomics in an ‘TR36’ background were crossed with ‘Java’, a cultivar susceptible to green leafhopper. The segregation pattern of the F₂ and backcross generations revealed that the Glb6 gene was located on chromosome 5.     

STS and microsatellite marker-assisted selection for bacterial blight resistance and waxy genes in rice, Oryza sativa L.

DNA marker-assisted selection was employed to select Xa-21 bacterial blight resistance and waxy (Wx) genes. Genotypes with both genes were selected from four F₂ populations involving indica × indica, indica × intermediate and japonica × indica crosses. With the assistance of PCR marker, 13 true breeding lines carrying Xa-21 were identified from F₂ generation of IRBB 21 × G 11353 cross. Similarly ten, eleven and fifty two plants having Xa-21 gene were isolated from G 3005-4-1 × IRBB 21, IRBB 21 × HJX 74 and IRBB 21 × SY 2crosses respectively. The lines with Wx gene in both homozygous and heterozygous state were also scored from the above crosses. Twenty plants having both Xa-21 and Wx loci in homozygous state were identified. DNA-based progeny testing was conducted to ensure the selection of homozygous lines for Xa-21 and Wx genes. Finally,twenty true breeding lines with high amylose content and Xa-21 gene were isolated from four crosses. These homozygous lines are phenotypically superior and resistant to Chinese race 5 of the bacterial blight pathogen. Fifty-six germplasm sources were surveyed for PCR polymorphism in order to facilitate future PCR-based marker assisted transfer of bacterial blight resistance genes xa-5, xa-13 and Xa-21 to any desired varietal background which will be useful for selection of parents in breeding programmes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Genetic Analysis of Resistance to Gall Midge (Orseolia oryzae Wood Mason) in Rice

The inheritance of resistance to rice gall midge (Ranchi biotype) was studied in 12 resistant cultivars by crossing with susceptible cultivars. By the study of F₁, F₂, F₃, B₁ and B₂ generations, it was found that resistance was governed by a single dominant gene in ‘Surekha’, ‘Phalguna’, ‘Rajendra Dhan 202’, ‘IET 7918’‘IET 6187’, ‘BG 404-1’; by duplicate dominant genes in ‘W 1263’, ‘RPW 6-17’ and ‘WGL 48684’ and a monogenic recessive gene in ‘OB 677’ and ‘BKNBR 1008-21’. The allelism test of the resistant genes in the test cultivars with already known genes Gm1 and Gm2 was carried out. A single dominant gene that conveyed the resistance in ‘RPW 6–17’, ‘IET 7918’ and ‘IET 6187’ was allelic to Gm1 and segregated independently of Gm2. The resistance in ‘Phalguna’, ‘Rajendra Dhan 202’, ‘W 1263’ and ‘RPW 6–17’, ‘IR 36’ and ‘WGL 48684’ was governed by Gm2 gene which was independent of Gm1. Two additional genes were identified and designated as Gm3 and gm4. Three test cultivars ‘BG 404-1’, ‘W 1263’ and ‘WGL 48684’ were found to have Gm3 gene for resistance which was non-allelic and segregated independently of Gm1 and Gm2. Thus the cultivars ‘W 1263’ and ‘WGL 48684’ had two resistance genes Gw2 and Gm3 together. The cultivar ‘RPW 6–17’ also had two resistance genes Gm1 and Gm2 together. The recessive gene gm4 which conditioned the resistance in ‘OB 677’ and ‘BKNBR 1008-21’ was nonallelic to and segregated independently of Gm1, Gm2 and Gm3 genes. Linkage studies of the resistance gene with pigment characters were carried out in ‘Purple gora/IR 36’ cross. The resistance gene Gm2 was found to be linked with the genes governing the pigmentation in node, apiculus and stigma with crossover values of 15.78, 31.57 and 35.78 % respectively. By the trisomic analysis, it was found that the Gm2 gene was located on chromosome 3.     

Evaluation of common wheat cultivars for tan spot resistance and chromosomal location of a resistance gene in the cultivar 'Salamouni'

A total of 50 wheat (Triticum aestivum L.) cultivars were evaluated for resistance to tan spot, using Pyrenophora tritici‐repentis race 1 and race 5 isolates. The cultivars ‘Salamouni’, ‘Red Chief’, ‘Dashen’, ‘Empire’ and ‘Armada’ were resistant to isolate ASC1a (race 1), whereas 76% of the cultivars were susceptible. Chi‐squared analysis of the F₂ segregation data of hybrids between 20 monosomic lines of the wheat cultivar ‘Chinese Spring’ and the resistant cultivar ‘Salamouni’ revealed that tan spot resistance in ‘Salamouni’ was controlled by a single recessive gene located on chromosome 3A. This gene is designated tsn4. The resistant cultivars identified in this study are recommended for use in breeding programmes to improve tan spot resistance in common wheat.     

Pyramiding of Xa7 and Xa21 for the improvement of disease resistance to bacterial blight in hybrid rice

‘Minghui 63’ is a restorer line widely used in hybrid rice production in China for the last two decades. This line and its derived hybrids, including ‘Shanyou 63’, are susceptible to bacterial blight (BB), caused by Xanthomonas oryzae pv. oryzae (Xoo). To improve the bacterial blight resistance of hybrid rice, two resistance genes Xa21 and Xa7, have been introgressed into ‘Minghui 63’ by marker‐assisted selection and conventional backcrossing, respectively. The single resistance gene‐introgressed lines, Minghui 63 (Xa21) and Minghui 63 (Xa7) had higher levels of resistance to bacterial blight than their derived hybrids, Shanyou 63 (Xa21) or Shanyou 63 (Xa7). Both Xa21 and Xa7 showed incomplete dominance in the heterozygous background of rice hybrids by infection with GX325 and KS‐1‐21. The improved restorer lines, with the homozygous genotypes, Xa21Xa21 or Xa7Xa7, were more resistant than their hybrids with the heterozygous genotypes Xa21xa21 or Xa7xa7. To further enhance the bacterial blight resistance of ‘Minghui 63’ and its hybrids, Xa21 and Xa7 were pyramided into the same background using molecular marker‐aided selection. The restorer lines developed with the resistance genes Xa21 and Xa7, and their derived hybrids were evaluated for resistance after inoculation with 10 isolates of pathogens from China, Japan and the Philippines, and showed a higher level of resistance to BB than the restorer lines and derived hybrids having only one of the resistance genes. The pyramided double resistance lines and their derived hybrids have the same high level of resistance to BB. These results clearly indicate that pyramiding of dominant genes is a useful approach for improving BB resistance in hybrid rice.     

Genetic analysis of resistance to green leafhopper, Nephotettix virescens (Distant), in three varieties of rice

The genetics of resistance to green leafhopper, Nephotettix virescens (Distant), in rice varieties ‘IR36’ and ‘Maddai Karuppan’ and breeding line ‘IR20965‐11‐3‐3’ was studied. The reactions of F₁ hybrids, F₂ populations and F₃ lines from the crosses of test varieties with the susceptible variety ‘TN1’ revealed that resistance in ‘IR36’ and ‘Maddai Karuppan’, is governed by single recessive genes while resistance in ‘IR20965‐11‐3‐3’ is controlled by a single dominant gene. Allele tests with the known genes for resistance to green leafhopper revealed that the recessive gene of ‘IR36’ is different from and inherited independently of Glh1, Glh2, Glh3, Glh4, Glh5, Glh8 and Glh9t. This gene is designated as glh10t. The recessive gene of ‘Maddai Karuppan’ and the dominant gene of ‘IR20965‐11‐3‐3’ are also non‐allelic to Glh1, Glh2, Glh3, Glh4, Glh5 and Glh8t. Thus, the dominant gene of IR20965‐11‐3‐3 is designated as Glh11t. The allelic relationships of the recessive gene of ‘Maddai Karuppan’ with glh8 and glh10t should be investigated.     

Isolation and linkage mapping of disease-resistance-like sequences from various rice cultivars, containing different recognition specificities

Degenerated oligonucleotide primers identified from the nucleotide‐binding sites of known disease resistance (R) genes were used from rice cultivars harbouring different recognition specificities to amplify and clone homologous sequences of R genes. A total of 68 non‐redundant clones, which showed various degrees of sequence homology to R genes, were obtained from 18 rice cultivars. These clones had a high degree of sequence diversity both in the nucleotides and in the predicted amino acids, and were classified into five groups using clustal analysis. Fifteen of the 68 clones were mapped to 17 loci on chromosomes 3, 5, 11 and 12 in the rice molecular linkage map. The loci of the mapped clones correlated with the locations of known rice R genes for blast resistance and bacterial blight resistance on chromosomes 11 and 12. Other mapped loci occurred in cluster on chromosome 3, and correlated with the position of a quantitative trait locus for bacterial blight resistance. The mapping of the R gene homologues may aid the identification and isolation of R gene candidates.     

Identification of two new genes conferring resistance to rice blast in the Chinese native cultivar 'Maowangu'

The Chinese native rice cultivar ‘Maowangu’ expresses a high level of resistance to many races of rice blast (Pyricularia grisea) collected from North China and Japan. ‘Maowangu’ was crossed with 10 Japanese differential cultivars and the susceptible Chinese cultivar ‘Lijiangxintuanheigu’ (LTH). Allelism tests were conducted in the F₂ populations with rice blast races. The resistance of ‘Maowangu’ was governed by two dominant genes which were non-allelic to the resistance genes at seven loci: Pi-a, Pi-i, Pi-k, Pi-z, Pi-ta, Pi-b, and Pi-t. To identify the two resistance genes, two F₃ lines of ‘Shin 2/Maowangu’ segregating 3R:1S were selected for linkage tests in 1994. One was linked to marker genes C and Amp-3 on chromosome 6 with recombination frequencies of 35.8 ± 6.4% and 42.1 ± 6.2%, respectively, and the other to Amp-1 on chromosome 2 with a recombination frequency of 37.6 ± 6.0%. To confirm these results, two F₃ lines of ‘LTH/Maowangu’ were selected for linkage tests in 1995. The one was linked to Amp-3, and other was linked to Amp-1, with recombination frequencies of 36.9 ± 3.1% and 34.3 ± 3.2%, respectively. The two genes on chromosomes 6 and 2 were designated Pi13(t) and Pi14(t), respectively.     

Identification of marker alleles linked to fire blight resistance QTLs in apple genotypes

Molecular marker analysis can be an effective tool when searching for new fire blight resistance donors. It can speed up the breeding process as well, even though many of the available markers linked to fire blight resistance QTLs have not yet been tested by screening a large number of cultivars. The aim of this study was to search for alternate sources of the three major QTLs of fire blight resistance; FBF7, FB_MR5 and FB_E, as well as to test the efficiency of some markers linked to minor QTLs. Altogether, nine primer pairs were used on 77 genotypes including new Hungarian cultivars and old apple cultivars from the Carpathian basin. Several marker alleles of FB resistance QTLs have been detected in the screened genotypes, most importantly the alleles coupling with FB_MR5 in the old cultivars ‘Kéresi muskotály’, ‘Szabadkai szercsika’ and ‘Batul’. We propose these cultivars as the first available resistance donors of FB_MR5 instead of the crabapple Malus × robusta 5. The results also bring new information regarding the resistance alleles of new Hungarian cultivars and selections.     

Molecular mapping and identification of candidate gene conferring organophosphate-sensitive reaction in sorghum (Sorghum bicolor)

Some sorghum cultivars are sensitive to organophosphate insecticides, which cause red to purple injury spots and, in severe cases, leaf death. Sensitivity to organophosphates is conditioned by a single locus on chromosome 5. We constructed a high-density genetic map around the locus, termed osr, using DNA markers, and delimited osr to a 377-kb region on the short arm of chromosome 5. Sequence analysis of this region predicted 19 gene candidates. Four of the candidates are homologous to a disease resistance NBS-LRR gene. mRNA-Seq analysis of gene expression and SNPs from two resistant cultivars (‘JN43’ and ‘Greenleaf’) and one sensitive cultivar (‘Nakei MS3B’) suggest that one or more members of this NBS-LRR gene family are osr. Our results suggested that the recessive allele of osr results in sensitivity to OP insecticides.     

Multiplex PCR genotyping for five bacterial blight resistance genes applied to marker‐assisted selection in rice (Oryza sativa)

Bacterial blight (BB) is the most economically damaging disease of rice in Asia and other parts of the world. In this study, a multiplex PCR genotyping method was developed to simultaneously identify genotypes of five BB resistance genes, Xa4, xa5, Xa7, xa13 and Xa21. The resistance R alleles were amplified using five functional markers (FMs) to generate amplicons of 217, 103, 179, 381 and 595 bp in IRBB66. Amplicons of 198, 107, 87, 391 and 467 bp corresponded to susceptible alleles in Taiwanese japonica rice cultivars. In backcross breeding programmes, the multiplex PCR assay was integrated into selection from a population using BB resistance donor IRBB66 crossed to rice cultivar ‘Tainung82’. Two plants with homozygosity for Xa4, xa5, Xa7, xa13 and Xa21 were selected from 1100 BC₂F₂ plants. In addition, the five BB resistance genes were also accurately identified in F₂ populations. This multiplex PCR method provides a rapid and efficient method for detecting various BB resistance genes, which will assist in pyramiding genes to improve durability of BB resistance in Taiwanese elite rice cultivars.     

Inheritance of resistance to bacterial blight disease of rice

Summary Inheritance of resistance to the Punjab isolate of Xanthomonas campestris pv. oryzae of bacterial blight disease of rice was studied in seven breeding lines resistant to the disease. The results revealed that resistance in breeding lines PAU 122-73-1-4-1, PAU 164-102-1-2-1-1-1, KJT 24, IR 5657-33-2-1-2 and IR 22082-41-2-2 was controlled by single dominant genes allelic to the dominant gene which confers resistance to the Punjab isolate in Patong 32. Resistance to the Punjab isolate in breeding lines IET 7172 and RP 2151-40-1 was found to be controlled by single recessive resistance genes allelic to one of the recessive resistance genes present in BJ 1. The two genes are independently inherited and are being used to develop bacterial blight resistant varieties.     

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.     

Linkage relationships of some genes for disease and insect resistance and semidwarf stature in rice

Summary Two-way classification of 400 F₃ families from the rice cross IR2153-159-1 x Babawee for plant stature and for resistance to brown planthopper, green leafhopper, and bacterial blight indicated that Glh 3 (dominant gene for resistance to green leafhopper) and bph 4 (recessive gene for resistance to brown planthopper) are linked with a map distance of 34 units. The bph 4 gene also appears to be linked with sd ₁ (recessive gene for semidwarf stature) although the linkage is less strong. However, bph 4 and Xa 4 (dominant gene for bacterial blight resistance) are inherited independently of each other. No segregation for susceptibility was observed among F₃ families of crosses between varieties having Bph 3 and bph 4 genes for resistance to brown planthopper. Apparently, Bph 3 and bph 4 are either allelic or closely linked.     

Molecular and phenotypic characterization of Chinese wheat (Triticum aestivum) cultivars for resistance to Fusarium head blight

To identify excellent cultivars resistant to Fusarium head blight (FHB), 104 wheat cultivars were tested by single-flower inoculation using two prevalent pathogens from 2018 to 2020. Agronomic traits were also investigated. Six FHB-resistance quantitative trait loci (QTL), Fhb1, Fhb2, Fhb4, Fhb5, Fhb7 and Qfhb.crc-2D, have been assessed using previously reported DNA markers. A diagnostic marker has been used for Fhb1, and indicative markers linked to the other QTL were used. Results showed that (i) 12 (11.5%) cultivars were resistant to two pathogens in 3 years; among them, ‘Shengxuan 6’, ‘Wanhongbian 759’, ‘Yunong 903’ and ‘Yunong 901’ had good agronomic traits. (ii) Among cultivars with one resistance QTL, the severities of cultivars carrying Fhb1 and Qfhb.crc-2D were 2.2 and 2.8, respectively, whereas those of cultivars with Fhb2 or Fhb7 were 3.6. Among cultivars with two resistance QTL, the severities of cultivars with Fhb1 + Fhb4, Fhb1 + Fhb7 and Fhb4 + Fhb5 were 2.2, 3.0 and 3.6, respectively. The severity of five cultivars possessing three or four resistance QTL was below 2.5. Fhb1 and Qfhb.crc-2D showed better resistance effects than other resistance QTL.     

Dominant gene for common bean resistance to common bacterial blight caused by <Emphasis Type="Italic">Xanthomonas</Emphasis><Emphasis Type="Italic">axonopodis</Emphasis> pv. <Emphasis Type="Italic">phaseoli</Emphasis>

The common bacterial blight pathogen [Xanthomonas axonopodis pv. phaseoli (Xap)] is a limiting factor for common bean (Phaseolus vulgaris L.) production worldwide and resistance to the pathogen in most commercial cultivars is inadequate. Variability in virulence of the bacterial pathogen has been observed in strains isolated from Puerto Rico and Central America. A few common bean lines show a differential reaction when inoculated with different Xap strains, indicating the presence of pathogenic races. In order to study the inheritance of resistance to common bacterial blight in common bean, a breeding line that showed a differential foliar reaction to Xap strains was selected and was crossed with a susceptible parent. The inheritance of resistance to one of the selected Xap races was determined by analysis of segregation patterns in the F₁, F₂, F₃ and F₄ generations from the cross between the resistant parent PR0313-58 and the susceptible parent ‘Rosada Nativa’. The F₁, F₂ and F₃ generations were tested under greenhouse conditions. Resistant and susceptible F_3:4 sister lines were tested in the field. The statistical analysis of all generations followed the model for a dominant resistance gene. The resistant phenotype was found to co-segregate with the SCAR SAP6 marker, located on LG 10. These results fit the hypothesis that resistance is controlled by a single dominant gene. The symbol proposed for the resistance gene is Xap-1 and for the bacterial race, XapV1.     

Heritability of resistance to foliar late blight in a diploid hybrid potato population of Solanum phureja×Solanum stenotomum

The emergence of new races of Phytophthora infestans has necessitated the search for additional sources of potato germplasm with resistance to late blight. This study examined 281 clones, derived from 72 families of a diploid random-mated hybrid population of Solanum phureja×Solanum stenotomum. The clones were evaluated in a replicated field trial for 2 years with the control cultivar ‘Atlantic’ in Pennsylvania, USA. The P. infestans US-8 A2 mating type culture was used to inoculate spreader rows of susceptible S. tuberosum cv.‘Russet Burbank’. Percent defoliation caused by the late blight fungus was estimated visually in each plot three times near the end of the growing season. Area under the disease progress curve (AUDPC) was estimated and showed that late blight was more severe in 1997 than in 1996. Genetic differences among clones and significant, but small, clone–environment interaction were detected for AUDPC. Broad-sense and narrow-sense heritability estimates, over years, were 0.79 ± 0.05 (P = 0.05) and 0.78 ± 0.29, respectively. Seventy-five percent of the diploid clones had a significantly lower mean AUDPC than Atlantic. These results support the idea that this diploid population is worthy of use in breeding for late blight resistance in tetraploid potato cultivars.