Inheritance of evolved thiocarbamate resistance in Rigid Ryegrass (Lolium rigidum) populations from Australia

Populations of rigid ryegrass (Lolium rigidum Gaudin) from southern Australia have evolved resistance to the thiocarbamate herbicide prosulfocarb. The inheritance of prosulfocarb resistance was explored by crossing R and S individuals. In all families within each cross, except 16.2, the response of the F1 were intermediate between the parents, suggesting that resistance is inherited as a single, partially dominant trait. For 16.2, the response of the F1 was more similar to the susceptible parent, suggesting resistance may be a recessive trait in this population. Segregation at the discriminating dose of 1200 g a.i. ha−1 prosulfocarb in populations 375-14 fitted the ratio (15:1) consistent with two independent dominant alleles; 198-15 fitted a ratio (13:3) for two independent alleles, one dominant and one recessive; and EP162 fitted a ratio (9:7) for two additive dominant alleles. In contrast segregation of population 16.2 fitted a (7:9) ratio consistent with two independent recessive alleles contributing to prosulfocarb resistance. Four different patterns of resistance to prosulfocarb were identified in different resistant populations, with inheritance as a dominant allele, dominant and recessive, additive dominant and as an independent recessive allele. This suggests there are several different mechanisms of prosulfocarb resistance present in L. rigidum.

Four Consistent Loci Confer Adult Plant Resistance to Leaf Rust in the Durum Wheat Lines Heller#1 and Dunkler

The durum wheat lines Heller#1 and Dunkler from the International Maize and Wheat Improvement Center Global Wheat Program showed moderate and stable adult plant resistance to leaf rust under high disease pressure over field environments in northwestern Mexico. Leaf rust phenotyping was performed on two recombinant inbred line (RIL) populations derived from crosses of Heller#1 and Dunkler with the susceptible parent Atred#2, conducted under artificially induced Puccinia triticina epidemics in 2013, 2014, 2015, and 2016. The Atred#2 × Heller#1 and Atred#2 × Dunkler populations were genotyped by single nucleotide polymorphism (SNP) platforms and diversity arrays technology markers, respectively. Four leaf rust resistance quantitative trait loci were detected simultaneously in the two RIL populations: Lr46, QLr.cim-2BC, QLr.cim-5BL, and QLr.cim-6BL based on phenotypic data across all four crop seasons. They explained 11.7 to 46.8%, 7.2 to 26.1%, 8.4 to 24.1%, and 12.4 to 28.5%, respectively, of the phenotypic variation for leaf rust resistance in Atred#2 × Heller#1 and 16.3 to 56.6%, 6.7 to 15.7%, 4.1 to 10.1%, and 5.1 to 20.2% of the variation in the Atred#2 × Dunkler population. Only the resistance allele of QLr.cim-2BC was from the susceptible parent Atred#2, and resistance alleles at other loci came from the resistant parents Heller#1 and Dunkler. The SNP markers closely linked to Lr46 and QLr.cim-2BC were converted to kompetitive allele specific PCR markers for use in marker-assisted selection to improve leaf rust resistance through crosses with Heller#1 and Dunkler sources.

Sunflower Resistance to Race G of Broomrape (Orobanche Cumana Wallr.) In the Russian Federation: the Development of the Lines and the Study of Inheritance

The objective of this work is the development of sunflower lines that are not affected by race G of broomrape, as well as the determination of genetic control of resistance. The testing of the resistance of VNIIMK’s collection accessions of a cultivated sunflower of various origins was carried out on an artificial background made from the seeds of race G of broomrape. 6 lines resistant to race G were developed by the method of inbreeding on the basis of the obtained unaffected forms. The genetic control of resistance of one of them was studied. The resistance was inherited monogeneuosly, with incomplete dominance. There was established the presence of a reciprocal effect and the dependence of the resistance characteristic on the genotype of a susceptible parent under crossbreeding with certain lines. The other 5 sunflower lines are in the process of a hybridological analysis in order to determine the genetic control of their resistance. The combination of different genes of resistance to the same race of broomrape in one sunflower genotype could contribute to the long-term resistance of the crop to the parasite. The results of the presented study are of high importance for breeders, since the gene that we studied provides a new source of resistance to race G, thereby ensuring the protection of sunflower from the spread of new Orobanche pathotypes.

Analysis of leaf microbiome composition of near-isogenic maize lines differing in broad-spectrum disease resistance

SummaryPlant genotype strongly affects disease resistance, and also influences the composition of the leaf microbiome. However, these processes have not been studied and linked in the microevolutionary context of breeding for improved disease resistance. We hypothesized that broad-spectrum disease resistance alleles also affect colonization by non-pathogenic symbionts.Quantitative trait loci (QTL) conferring resistance to multiple fungal pathogens were introgressed into a disease-susceptible maize inbred line. Bacterial and fungal leaf microbiomes of the resulting near-isogenic lines were compared to the microbiome of the disease-susceptible parent line at two timepoints in multiple fields.Introgression of QTL from disease-resistant lines strongly shifted the relative abundance of diverse fungal and bacterial taxa in both 3-week-old and 7-week-old plants. Nevertheless, the effects on overall community structure and diversity were minor and varied among fields and years. Contrary to our expectations, host genotype effects were not any stronger in fields with high disease pressure than in uninfected fields, and microbiome succession over time was similar in heavily infected plants and uninfected plants.These results show that introgressed QTL can greatly improve broad-spectrum disease resistance while having only limited and inconsistent pleiotropic effects on the leaf microbiome in maize.

Inheritance of Resistance to Pepper huasteco yellow vein virus in Capsicum annuum L.

Pepper huasteco yellow vein virus (PHYVV) is a major disease in pepper (Capsicum annuum) that causes quantitative and qualitative losses to the crop in Central America and part of North America. To date, no resistant cultivars are available, and breeding is hampered by the lack of knowledge of the inheritance of this trait. Sources of resistance to PHYVV have been identified in the wild peppers of Mexico. The objectives of this study were to determine the grade of dominance, to analyze the maternal influence, and to estimate the number of genes involved in this resistant trait to PHYVV in the resistant wild pepper accession UAS12. Three susceptible parent lines—‘Anaheim’ (Ana), ‘Ancho Gigante’ (AG), and ‘Yolo Wonder’ (YW)—were crossed with resistant UAS12 accession to develop F1 (reciprocal), F2, and BC1 progenies in three families. Plants from this study were inoculated with PHYVV through Bemisia tabaci, evaluated phenotypically, and the segregation of disease scores was studied. A single recessive gene was found to control resistance to PHYVV in the resistant UAS12 accession, although segregation patterns suggested that other minor genes could participate in the expression of this resistant trait. No proof was found for maternal inheritance of PHYVV resistance. The gene symbol phv is proposed for PHYVV resistance in UAS12 accession in pepper. These results provide useful information for the design of pepper breeding programs in the introgression of this trait into commercial pepper backgrounds.

Inheritance and Allelic Relationship Among Downy Mildew Resistance Genes in Pearl Millet

Pearl millet downy mildew (DM), caused by Sclerospora graminicola, is of serious economic concern to pearl millet farmers in the major crop-growing areas of the world. To study the inheritance and allelic relationship among genes governing resistance to this disease, three DM-resistant pearl millet lines (834B, IP 18294-P1, and IP 18298-P1) and one susceptible line (81B) were selected on the basis of disease reaction under greenhouse conditions against two isolates of S. graminicola (Sg 526-1 and Sg 542-1). Three resistant parents were crossed with the susceptible parent to generate F1, F2, and backcross BC1P1 (susceptible parent × F1) and BC1P2 (resistant parent × F1) generations for inheritance study. To carry out a test for allelism, the three resistant parents were crossed with each other to generate F1 and F2 generations. The different generations of these crosses were screened for disease reaction against two isolates (Sg 526-1 and Sg 542-1) by artificial inoculation under greenhouse conditions. The segregation pattern of resistance in the F2 and corresponding backcross generations revealed that resistance to DM is controlled by a single dominant gene in 834B and IP 18294-P1 and by two dominant genes in IP 18298-P1. A test for allelism inferred that a single dominant gene for resistance in 834B is nonallelic to that which governs resistance in IP 18294-1, whereas one of the two dominant genes for DM resistance in IP 18298-P1 against the test isolates is allelic to the gene for DM resistance in 834B and a second gene is allelic to the resistance gene present in IP 18294-P1.

Inheritance of Resistance to Papaya Ringspot Virus-Watermelon Strain in Watermelon

Sources of resistance to the watermelon strain of papaya ringspot virus-watermelon strain (PRSV-W) have been identified within the watermelon (Citrullus lanatus) germplasm collection. Inheritance of the resistance to PRSV-W was studied in three Citrullus amarus (formerly C. lanatus var. citroides) PI accessions: PI 244017, PI 244019, and PI 485583. Three susceptible parent lines, ‘Allsweet’, ‘Calhoun Gray’, and ‘New Hampshire Midget’, were crossed with resistant PI accessions to develop F1, F2, and BC1 progenies in six families. A single recessive gene was found to control the resistance to PRSV-W in all three resistant PI accessions. Allelism tests indicated that the three PI accessions carry the same resistance allele to PRSV-W. The gene symbol ‘prv’ is proposed for PRSV-W resistance in PI 244017, PI 244019, and PI 485583 in watermelon.

Inheritance studies on Fusarium wilt resistance in long duration pigeonpea [Cajanus cajan (L.) Millsp.]

Fusarium wilt (FW) is a major fungal disease of pigeonpea causing huge economic losses annually and breeding of FW resistant cultivars is essential as other control measures such as fungicides are expensive and harmful to environment. To understand the inheritance of FW resistance, nine populations each of F1, F2 and BCSF1 (F1 × susceptible parent) derived from crossing between three adopted but susceptible long duration pigeonpea cultivars (MA-6, MAL-13 and MAL-18) with three FW resistant lines (BSMR-846, BWR-23 and BDN-2029). All F1 plants were resistant to FW indicating the dominance of resistance. The ÷2 test for goodness-of-fit showed F2 segregation ratio of 13:3 (one dominant and one recessive), 15:1 (two dominant genes) and 3:1 (one dominant gene) in BSMR-846, BWR-23 and BDN-2029, respectively. The information generated on the genetics of FW resistance will be helpful in development of high yielding and stable wilt resistant, long duration pigeonpea varieties.

Transfer of Downy Mildew Resistance from Wild Basil (Ocimum americanum) to Sweet Basil (O. basilicum)

Sweet basil (Ocimum basilicum) is susceptible to downy mildew caused by the oomycete foliar pathogen Peronospora belbahrii. No resistant varieties of sweet basil are commercially available. Here, we report on the transfer of resistance gene Pb1 from the highly resistant tetraploid wild basil O. americanum var. americanum (PI 500945, 2n = 4x = 48) to the tetraploid susceptible O. basilicum ‘Sweet basil’ (2n = 4x = 48). F1 progeny plants derived from the interspecific hybridization PI 500945 × Sweet basil were resistant, indicating that the gene controlling resistance (Pb1) is dominant, but sterile due to the genetic distance between the parents. Despite their sterility, F1 plants were pollinated with the susceptible parent and 115 first backcross generation to the susceptible parent (BCs1) embryos were rescued in vitro. The emerging BCs1 plants segregated, upon inoculation, 5:1 resistant/susceptible, suggesting that resistance in F1 was controlled by a pair of dominant genes (Pb1A and Pb1A’). Thirty-one partially fertile BCs1 plants were self-pollinated to obtain BCs1-F2 or were backcrossed to Sweet basil to obtain the second backcross generation to the susceptible parent (BCs2). In total, 1 BCs1-F2 and 22 BCs2 progenies were obtained. The BCs1-F2 progeny segregated 35:1 resistant/susceptible, as expected from a tetraploid parent with two dominant resistant genes. The 22 BCs2 progenies segregated 1:1 resistant/susceptible (for a BCs1 parent that carried one dominant gene for resistance) or 5:1 (for a BCs1 parent that carried two dominant genes for resistance) at a ratio of 4:1. The data suggest that a pair of dominant genes (Pb1A and Pb1A’) residing on a two homeologous chromosomes is responsible for resistance of PI 500945 against P. belbahrii.

Mapping Resistance to Alternaria cucumerina in Cucumis melo

Infection with Alternaria cucumerina causes Alternaria leaf blight (ALB), a disease characterized by lesion formation on leaves, leading to substantial yield and quality losses in Cucumis melo (melon). Although fungicides are effective against ALB, reduction in the frequency of application would be economically and environmentally beneficial. Resistant melon lines have been identified but the genetic basis of this resistance has not been determined. A saturated melon genetic map was constructed with markers developed through genotyping by sequencing of a recombinant inbred line population (F6 to F10; n = 82) derived from single-seed descent of a F2 population from a cross between the ALB-resistant parent MR-1 and the ALB-susceptible parent Ananas Yokneum. The population was evaluated for A. cucumerina resistance with an augmented block greenhouse study using inoculation with the wounded-leaf method. Multiple quantitative trait loci (QTL) mapping identified two QTL that explained 33.9% of variation in lesion area. Several candidate genes within range of these QTL were identified using the C. melo v3.5 genome. Markers linked to these QTL will be used to accelerate efforts to breed melon cultivars resistant to ALB.