Sources Of Genetic Resistance Research Articles

Identification of Ganoderma Disease Resistance Loci Using Natural Field Infection of an Oil Palm Multiparental Population.

Multi-parental populations are promising tools for identifying quantitative disease resistance loci. Stem rot caused by Ganoderma boninense is a major threat to palm oil production, with yield losses of up to 80% prompting premature replantation of palms. There is evidence of genetic resistance sources, but the genetic architecture of Ganoderma resistance has not yet been investigated. This study aimed to identify Ganoderma resistance loci using an oil palm multi-parental population derived from nine major founders of ongoing breeding programs. A total of 1200 palm trees of the multi-parental population was planted in plots naturally infected by Ganoderma, and their health status was assessed biannually over 25 yr. The data were treated as survival data, and modeled using the Cox regression model, including a spatial effect to take the spatial component in the spread of Ganoderma into account. Based on the genotypes of 757 palm trees out of the 1200 planted, and on pedigree information, resistance loci were identified using a random effect with identity-by-descent kinship matrices as covariance matrices in the Cox model. Four Ganoderma resistance loci were identified, two controlling the occurrence of the first Ganoderma symptoms, and two the death of palm trees, while favorable haplotypes were identified among a major gene pool for ongoing breeding programs. This study implemented an efficient and flexible QTL mapping approach, and generated unique valuable information for the selection of oil palm varieties resistant to Ganoderma disease.

Quantitative Trait Loci Associated with Anthracnose Resistance in Sorghum

Sorghum [Sorghum bicolor (L.) Moench] is highly susceptible to the fungal disease anthracnose, which is a major biotic constraint to forage and grain production. With an aim to develop durable resistance to anthracnose, two unique genetic sources of resistance in sorghum were selected to create genetic mapping populations and identify genomic regions that control anthracnose resistance. A series of quantitative trait loci (QTL) were identified that conferred variable levels of resistance, three of which controlled stable resistance across all environments, while two QTL were environment specific. A major anthracnose‐resistance QTL on chromosome 9 of the SC155‐14E genome was observed in all environments and controlled from 19 to 62% of the phenotypic variance. An additional major QTL on chromosome 5 of the SC414‐12E genome explained from 20 to 39% of the phenotypic variance and was observed in four of the six environments tested. Resequencing of the genomes of resistant cultivars SC155‐14E and SC414‐12E facilitated a preliminary survey of the coding regions of genes annotated as playing a role in host defense. The resequenced genomes of the resistant genotypes and the linkage mapping resources represent information relevant for more detailed molecular characterization of genes conditioning anthracnose resistance in this tropical cereal. The identification of QTL conferring anthracnose resistance and the identification of single‐nucleotide polymorphisms linked to these loci will provide the necessary molecular tools for marker‐assisted introgression of durable anthracnose resistance into elite sorghum inbreds.

Identification and Genetic Evaluation of a New Source of Pod Wart Resistance for Peanut (Arachis hypogaea L.)

Pod wart disease severely damages in‐shell peanut (Arachis hypogaea L.) production in areas in which peanut is routinely rotated with potato (Solanum tuberosum L.). It is caused by soil‐borne Streptomyces that cause unsightly scabs that render the affected pods unmarketable. Since all elite peanut cultivars are susceptible to pod wart, a new genetic source of resistance is required. Here, the identification and evaluation of a new genetic source of pod wart resistance is reported. The US mini‐core peanut collection was evaluated under field conditions, and the least susceptible genotypes were reevaluated in three locations. This led to the identification of two “landrace” genotypes that were significantly resistant to pod wart, with an average of 70% less infection than the control line. Two populations were prepared from crosses between these genotypes and an elite cultivar. Genetic analysis of segregating F2 populations showed that pod wart tolerance is polygenic with medium heritability estimations. No significant correlation was found between the incidence of pod wart and pod reticulation, indicating that the deep reticulation does not provide resistance. In addition, no significant correlation was found between the incidence of pod wart and pod yield, seed weight, or seed ratio. However, the incidence of disease was positively correlated with pod weight and pod length, indicating genetic linkage or a pleotropic effect on pod wart resistance. This study suggests that pod wart resistance can be selected in early generations of breeding pedigrees, but large populations are needed for optimal pyramiding of other pod‐ and yield‐related traits.

Lindcove REC: Developing citrus varieties resistant to huanglongbing disease

News from the RECs Lindcove REC: Developing citrus varieties resistant to huanglongbing disease O f the many citrus varieties trialed at the Lindcove Research and Extension Center (REC), one has been a source of more than $14 million in licensing revenue to UC since 2006: the seedless, easy peel Tango mandarin, bred by Mikeal Roose, professor of genetics in the Department of Botany and Plant Sciences at UC Riverside, and UC Riverside researcher Tim Williams. Many millions of Tango trees have been planted worldwide, 4 million or so in California alone. Mikeal Roose, professor of genetics at UC Riverside, whose program bred the seedless Tango mandarin, which has been a huge commercial success. Now he’s working on a solution to the citrus industry’s greatest challenge — huanglongbing (HLB) disease. Elizabeth Grafton-Cardwell Eremocitrus crosses at Lindcove REC are potentially promising in the pursuit of a genetic source of resistance to HLB. The fruit is golfball size and inedible. 18 CALIFORNIA AGRICULTURE • VOLUME 71, NUMBER 1 Lonnie Duka Three trees with inedible fruit at Lindcove REC, on the eastern edge of the San Joaquin Valley near Visalia, have Roose’s attention now. They are 34-year-old trees, crosses of sweet orange (two) and Rangpur lime (one) with Eremocitrus glauca, the desert lime, a wild Australian citrus relative, which he had forgotten about until recently. It’s a long shot, but they could lead to an even bigger winner than Tango has been — a citrus variety resistant to huanglongbing (HLB) disease, the greatest threat to California’s citrus industry.

INVESTIGATION ON TOMATO SPOTTED WILT VIRUS INFECTING PEPPER PLANTS IN HUNGARY

In Hungary resurgence of Tomato spotted wilt virus (TSWV) frequently causesheavy crop losses in pepper production since the mid nineties. Management ofTSWV control was first directed against the thrips (using different insecticides orplastic traps), and against weeds as host plants of the virus and the thrips. Later onTsw resistance gene was introduced from Capsicum chinense PI 152225 and PI159236 into different types of pepper. In 2010 and 2011 sporadically, but in 2012more frequently a resistance breaking (RB) strain of TSWV on resistant peppercultivars was observed in the Szentes region (South-East Hungary). The presenceof a new resistance breaking strain was demonstrated by virological (test-plant,serological and RT-PCR) methods. Previously, the non-structural protein (NSs)encoded by small RNA (S RNA) of TSWV was verified as the avirulence factor forTsw resistance, therefore we analyzed the S RNA of the Hungarian RB and wildtype (WT) isolates and compared to previously analyzed TSWV strains with RBproperties from different geographical origins. Phylogenetic analysis demonstratedthat the different RB strains had the closest relationship with the local WT isolatesand there was no conserved mutation present in all the NSs genes of RB isolatesfrom different geographical origins. According to these results, it is concluded thatthe RB isolates evolved separately in geographic point of view and according to theRB mechanism. In order to find new genetic sources of resistance in Capsicumspecies 89 lines from Capsicum annuum, C. chinense, C. frutescens, C. chacoense,C. baccatum var. baccatum, C. baccatum var. pendulum and C. praetermissumwere tested with the Hungarian TSWV-RB isolate.

An overview of Vicia faba role in ecosystems sustainability and perspectives for its improvement

Faba bean (Vicia faba L.) that is grown world-wide as a protein source for food and feed also offers ecosystem services such as N2 fixation and enhancement of wild fauna diversity such as bee-pollinators and other beneficial organisms. Thus, it helps to mitigate the pollinator decline and it is recognized that also contributes to pest control not only in the crop itself but also in the neighbouring/associated crops by attracting predators and parasitoids. Despite this importance, the global faba bean cultivation area has declined due to its low yield performance and susceptibility to biotic and abiotic factors. To face these constraints, it is relevant to implement adequate breeding programs using the available germplasm and always taking in consideration the pollination and mating system of faba bean. The aim of this paper is 1) to review the role of faba bean in cropping systems; 2) to assess the applicability of different molecular markers to understand the genetic diversity of faba bean germplasm; 3) to analyse the role of pollination and the mixed-mating system underpinning breeding strategies; 4) to assess the natural enemies affecting faba bean and to analyse the genetic sources of resistance to them.

Genealogical Analysis of the North-American Spring Wheat Varieties with Different Resistance to Pre-harvest Sprouting

Abstract A comparative analysis of genetic diversity of North American spring wheat varieties differing in resistance to pre-harvest sprouting was carried out. For identification of sources of resistance the genealogical profiles of 148 red-grained and 63 white-grained North-American spring wheat varieties with full pedigrees were calculated and estimates were made of pre-harvest sprouting. The cluster structure of the populations of red-grained and white-grained varieties was estimated. Analysis of variance revealed significant differences between the average contributions of landraces in the groups of resistant and susceptible varieties. Distribution of the putative sources of resistance in the clusters indicated that varieties having different genetic basis may have different sources of resistance. For red-grained varieties the genetic sources of resistance to pre-harvest sprouting are landraces Crimean, Hard Red Calcutta, and Iumillo, or Button, Kenya 9M-1A-3, and Kenya-U, or Red Egyptian and Kenya BF4-3B-10V1. Tracking of pedigrees showed these landraces contributed to the pedigrees, respectively, via Thatcher, Kenya-Farmer, and Kenya-58, which were likely donors of resistance for red-grained varieties. For white-grained varieties the sources of resistance were landraces Crimean, Hard Red Calcutta, Ostka Galicyjska, Iumillo, Akakomugi, Turco, Hybrid English, Rough Chaff White and Red King, and putative donors of resistance — Thatcher, RL2265, and Frontana. The genealogical profile of accession RL4137, the most important donor of resistance to pre-harvest sprouting in North American spring wheat breeding programmes, contains almost all identified sources of resistance.

Dissimilarity based on morphological characterization and evaluation of pollen viability and in vitro germination in Passiflora hybrids and backcrosses

Brazil is considered the main center of the genetic diversity and greatest worldwide producer of passion fruit. Passionfruit woodiness virus (Cowpea aphid born mosaic virus) is an important pathogenic agent of the passion fruit crop, causing severe losses in yield. Wild species of Passiflora are valuable sources of genetic resistance to this disease. Interspecific crosses are a promising method to incorporate resistance genes in commercial species (Passiflora edulis Sims.), despite problems with male sterility in developed hybrids. The objective of the present work was to study the dissimilarity based on morphological characteristics, and the viability and in vitro germination of pollen grains of progenitors, F1 hybrids, as well as progenies of the first generation of backcross BC1 [(P. edulis × P. cincinnata) × P. edulis]. Analysis of the severity of the viral disease showed that most genotypes were considered as susceptible to highly susceptible. The resistant hybrids did not show symptoms of foliar deformation. Cluster analysis showed two major groups; one represented by P. cincinnata and the second group by the hybrids and recurrent progenitor P. edulis. Morphological characterization of the hybrids showed phenotypic variation as to the color and shape of flowers, although the descriptors used were not able to show these differences. As expected, the progenitors (P. edulis and P. cincinnata) presented higher percentages of pollen viability, followed by the BC1-H45 progenies. The progenies of the BC1 will also be evaluated in the future with molecular markers aiming to accelerate the selection of hybrids with higher percentages of the genome of the recurrent parent.

English

Melon (Cucumis melo L.) is a cucurbitaceous of great appreciation worldwide. The intensive cultivation of melons has favored the increase of plant health problems in several producing regions. Among these problems, the root-knot nematode (Meloidogyne spp.) stands out. The development of genetically resistant cultivars is consolidated as an effective strategy for the management of these pathogens, it is then essential to screen cultivars and accessions for later identification of genetic sources of resistance. This study aimed to evaluates the reaction of melon genotypes to Meloidogyne enterolobii. The essay was conducted at the Sector of Vegetable Crops and Aromatic-Medicinal Plants of UNESP-FCAV Jaboticabal Campus, in greenhouse, from March to June, 2015. It was evaluated 18 melon genotypes, two commercial cultivars 'Fantasy' and 'Louis', and as susceptibility control, the tomato 'Santa Cruz Kada'. A completely randomized design was adopted, with 21 treatments and 7 repetitions. The total number of eggs and juveniles in the roots (TNEJ) and the reproduction factor (RF) were obtained in order to determine the reaction of each genotype evaluated. The accessions PI 414723, AC 29, and PI 124112 are resistant to M. enterolobii and are therefore promising for breeding programs. Key words: Cucumis melo, reproduction factor, plant breeding, root-knot nematode.

Variation in Susceptibility to Potato Psyllid, Bactericera cockerelli (Hemiptera: Triozidae), among Solanum verrucosum Germplasm Accessions

Potato psyllid, Bactericera cockerelli (Sulc) (Hemiptera: Triozidae), is a key pest of potato and the vector of “Candidatus Liberibacter solanacearum,” the pathogen associated with zebra chip disease. Development of potato cultivars with genetic resistance to potato psyllid would enable cost-effective control of this pest with reduced use of insecticides. To facilitate the development of resistant cultivars, the objective of our study was to screen germplasm accessions of Solanum verrucosum for resistance to potato psyllid. The susceptibility of S. verrucosum germplasm accessions to potato psyllid was highly variable in choice prescreening assays and no-choice performance assays. Compared with the susceptible potato cultivar, ‘Russet Burbank,’ several S. verrucosum populations exhibited strong resistance to potato psyllid. The S. verrucosum accession, PI 195170 was highly resistant to potato psyllid, and is a potential source of genetic resistance for the development of resistant potato cultivars.

Bananas adrift in time – a case study in the Solomons

Diseases, pests and environmental constraints pose a major threat to the sustainability of banana production globally. To address these challenges, the discovery and study of new sources of genetic resistance and adaptability are required, along with the general conservation of diversity. The Solomon Islands, located in the south-western Pacific region near Papua New Guinea, are a major center of banana diversity. Some collections had been made by nationals of some of the diversity present but little was known internationally of the rich genetic resource present. Two separate visits to the Solomon Islands characterized banana collections, documented and collected germplasm, recommended conservation strategies and provided training in cultivar characterization. A remarkable range of genetic diversity was found, including: many AA and AAA cooking types somewhat like those present in Papua New Guinea; nine Fei cultivars in relatively common usage, and two undescribed wild species as well as five AAB Pacific Plantains and four ABB cooking bananas belonging to the Kalapua subgroup. About six of the unique cultivars were successfully collected and established in the regional in vitro germplasm collection of SPC in Suva, Fiji. Nine Solomon Islanders were trained in the finer points of characterizing banana cultivars. Further collecting and study/evaluation of this rich diversity will promote its appreciation and potential utilization for meeting the challenges and opportunities ahead. Future studies could also determine the spread of the Awawe species and variability of morphological traits in the population. Community-based conservation could promote awareness of dietary diversity for better nutrition, via using the Fei bananas described in this paper. Establishing a virus-free regional field collection could help in comprehensively characterizing and evaluating regional Musa genetic resources. Existing sites could embrace the broader unique diversity of the Solomon Islands, and facilitate sharing this diversity in conjunction with a regional virus-tested in vitro collection.

Collecting banana diversity in eastern Indonesia

Major diseases, including Fusarium wilt tropical race 4, threaten banana production systems worldwide. New sources of genetic resistance are considered necessary in the fight against such diseases. The triangular region of Indonesia taking in Sulawesi, the Maluku Islands and Lesser Sunda Islands was prioritized by the Global Musa Genetic Resources Network, MusaNet for exploration and collecting. It is just east of the Wallace Line, which is recognized as a transition zone for flora in southeast Asia, and had been little explored. Bioversity International funded a team of scientists from Indonesia and Australia to make collecting missions in the triangle in October 2012 and February 2013. Suckers and seeds of 35 promising new accessions were collected. About 90% of these are either wild species or diploid cultivars of more direct use to breeding programs. These were morphologically characterized during the collecting missions and included a set of photographs recommended by Bioversitys Taxonomic Advisory Group. Cigar leaf samples were also collected and sent as fresh samples to the International Banana Genotyping Centre in the Czech Republic. Ploidy and DNA (SSR) genotyping determinations from these samples have been invaluable in quickly interpreting and better appreciating what has been discovered. The new accessions have been grown on at Solok field collection, West Sumatra and will be made available by Indonesia to the international community, including breeding programs, for evaluation and utilization. Information on wild Eumusa prompts a rethinking of the phytogeography of Musa acuminata. The variation within the Australimusa species M. lolodensis highlights the need for broader study of this Musa section. French Plantain-like edible AAs and prospects for the generation of African plantains in the region were identified. The mission indicated existence of local edible ABs in eastern Indonesia in association with balbisiana hybrids origins in the region. Further explorations in the region should add to Musa diversity knowledge.

Identification of resistant sources and inheritance of Fusarium wilt resistance in garden pea (Pisum sativum ssp. hortense)

Wilt caused by Fusarium oxysporum Schlechtend. Fr. f.sp. pisi is a devastating disease of garden pea in India and worldwide. The pathogen being soil-borne fungus is quite difficult to manage by chemical methods and the development of resistant varieties seems to be the only alternative. In the present study 34 garden pea genotypes collected from different sources were evaluated during winter 2012–13 and 2013–14 under wilt sick plot as well as artificial controlled conditions to identify genetic sources of resistance against Fusarium wilt. Three genotypes (GP-6, GP-55 and GP-942) were found to be highly resistant, four genotypes (GP-17, GP-48, GP-473, GP-941) were resistant and identified as new donor source. All the popular cultivated varieties (Arkel, Pusa Pragati, AP-3, VRP-6, VL-7, VL-10 and Arka Ajit) were highly susceptible to wilt. The inheritance of Fusarium resistance gene(s) was also studied using F1, F2, B1 and B2 progenies of three crosses, viz., Pusa Pragati × GP-55, Pusa Pragati × GP-6 and Arkel × GP-17. Chi-square analysis was performed to determine the Mendelian segregation ratios of resistance and susceptibility among the inoculated progenies. The F1 and backcross progenies to the resistant parents segregated in the ratio of 1:0, while the F2 progenies segregated in the ratio of 3:1. The backcross progenies to the susceptible parent segregated in the ratio of 1:1. The results obtained revealed that the resistance to Fusarium wilt in garden pea is governed by monogenic dominant gene. The resistance gene can easily be introgressed by backcross breeding in desired genotype.

Assessment of sorghum germplasm from Burkina Faso and South Africa to identify new sources of resistance to grain mold and anthracnose

Sorghum is an important worldwide crop whose yield can be significantly reduced by anthracnose (Colletotrichum sublineola) and grain mold diseases (multiple fungi). The identification of new genetic sources of resistance to both diseases is imperative for the development of new sorghum varieties. To this end, a total of 80 exotic germplasm accessions from Burkina Faso (BFA) and South Africa (ZAF) were evaluated for anthracnose and grain mold resistance during two planting periods in 2012 at the USDA-ARS experimental farms in Isabela, Puerto Rico. Twelve accessions were resistant to anthracnose during both evaluations of which 10 are originally from BFA. The anthracnose resistant accessions identified herein had a hypersensitive reaction characterized by lesions having red and purple color. Likewise, 9 accessions exhibited grain mold resistance after being inoculated with a mixture of a conidial suspension of Fusarium thapsinum, Fusarium semitectum, and Curvularia lunata during both periods. Eight of these accessions (PI 586182, PI 586186, PI 647705, PI 647706, PI 647707, PI 647708, PI 647710, and PI 647712) originated from BFA, while one (PI 61666) is from ZAF. The PI 586186 was the only accession that exhibited resistance to both anthracnose and grain mold. The grain mold resistant accession PI 61666 has a panicle shape that resembles a standard United States commercial type sorghum and is also photoperiod insensitive. The results presented herein indicate that the BFA germplasm could be an important source for anthracnose and grain mold resistance genes. The integration of these anthracnose and grain mold resistant germplasm into sorghum breeding programs should aid in expanding the genetic diversity and in the development of new resistant varieties.

Comparative transcriptomics of Central Asian Vitis vinifera accessions reveals distinct defense strategies against powdery mildew.

Grape powdery mildew (PM), caused by the biotrophic ascomycete Erysiphe necator, is a devastating fungal disease that affects most Vitis vinifera cultivars. We have previously identified a panel of V. vinifera accessions from Central Asia with partial resistance to PM that possess a Ren1-like local haplotype. In this study, we show that in addition to the typical Ren1-associated late post-penetration resistance, these accessions display a range of different levels of disease development suggesting that alternative alleles or additional genes contribute to determining the outcome of the interaction with the pathogen. To identify potential Ren1-dependent transcriptional responses and functions associated with the different levels of resistance, we sequenced and analyzed the transcriptomes of these Central Asian accessions at two time points of PM infection. Transcriptomes were compared to identify constitutive differences and PM-inducible responses that may underlie their disease resistant phenotype. Responses to E. necator in all resistant accessions were characterized by an early up-regulation of 13 genes, most encoding putative defense functions, and a late down-regulation of 32 genes, enriched in transcriptional regulators and protein kinases. Potential Ren1-dependent responses included a hotspot of co-regulated genes on chromosome 18. We also identified 81 genes whose expression levels and dynamics correlated with the phenotypic differences between the most resistant accessions ‘Karadzhandahal’, DVIT3351.27, and O34-16 and the other genotypes. This study provides a first exploration of the functions associated with varying levels of partial resistance to PM in V. vinifera accessions that can be exploited as sources of genetic resistance in grape breeding programs.

Sources of resistance to eastern filbert blight in hazelnuts from the Republic of Georgia

In North America, the native hazelnut Corylus americana harbors an endemic fungus, Anisogramma anomala, which causes the disease eastern filbert blight (EFB). Although C. americana is tolerant of EFB, the economically important European hazelnut, C. avellana, is highly susceptible to the disease. This susceptibility greatly limits commercial European hazelnut production in North America. The breeding and subsequent utilization of resistant plants are considered the most effective means of control and efforts are underway to identify diverse sources of genetic resistance. In this study, 1374 C. avellana seedlings, spanning 47 seed lots collected in the Republic of Georgia and 3 in Azerbaijan, were inoculated with the EFB pathogen and evaluated for disease response in New Jersey, USA. After 5 years, plant responses were rated on a scale of 0–5, in which 0 represents no signs or symptoms of EFB and 5 represents all stems showing cankers. Cankers were found on over 94% of the seedlings, with the large majority being highly susceptible (91.4%=rating 4 and 5). However, 79 plants from 34 seed lots remained free of signs or symptoms of EFB (rating 0). In most cases, only one or two resistant seedlings were in each of these seed lots, making it difficult to infer any genetic control of resistance. In contrast, seed lots from ‘Gulshishvela’ and ‘Kharistvala’ yielded significant numbers of resistant plants, which may indicate that dominant genes for resistance are present. Overall, these new plants will add to the genetic resources available for breeding new EFB-resistant cultivars.

Genome-Enabled Phylogeographic Investigation of the Quarantine Pathogen Ralstonia solanacearum Race 3 Biovar 2 and Screening for Sources of Resistance Against Its Core Effectors.

Phylogeographic studies inform about routes of pathogen dissemination and are instrumental for improving import/export controls. Genomes of 17 isolates of the bacterial wilt and potato brown rot pathogen Ralstonia solanacearum race 3 biovar 2 (R3bv2), a Select Agent in the United States, were thus analyzed to get insight into the phylogeography of this pathogen. Thirteen of fourteen isolates from Europe, Africa, and Asia were found to belong to a single clonal lineage while isolates from South America were genetically diverse and tended to carry ancestral alleles at the analyzed genomic loci consistent with a South American origin of R3bv2. The R3bv2 isolates share a core repertoire of 31 type III-secreted effector genes representing excellent candidates to be targeted with resistance genes in breeding programs to develop durable disease resistance. Toward this goal, 27 R3bv2 effectors were tested in eggplant, tomato, pepper, tobacco, and lettuce for induction of a hypersensitive-like response indicative of recognition by cognate resistance receptors. Fifteen effectors, eight of them core effectors, triggered a response in one or more plant species. These genotypes may harbor resistance genes that could be identified and mapped, cloned, and expressed in tomato or potato, for which sources of genetic resistance to R3bv2 are extremely limited.

Resistance to Charcoal Rot Identified in Ancestral Soybean Germplasm

ABSTRACTCharcoal rot, caused by the fungal pathogen Macrophomina phaseolina, is an economically important disease on soybean and other crops including maize (Zea mays L.), sorghum [Sorghum bicolor (L.) Moench], and sunflower (Helianthus annuus L.). Without effective cultural or chemical options to control charcoal rot in soybean [Glycine max (L.) Merr.], finding sources of genetic resistance is of high interest. In this study, 70 ancestral soybean genotypes were screened for resistance to M. phaseolina using a cut‐stem inoculation technique under semi‐controlled greenhouse conditions. Lesion progression on the stems in the first experiment was measured 7 to 15 d after inoculation. Three follow‐up experiments were conducted to select and confirm the genotypes with the strongest partial resistance. Two experiments evaluated lesion lengths and the third experiment evaluated seedling survival. In the two experiments measuring lesion lengths, PI 548302 (42 and 38 mm) and PI 548414 (36 and 52 mm) had significantly shorter lesion lengths than the moderately resistant genotype, DT97‐4290 (58 and 87 mm). In the fourth experiment, percentage survival of PI 548414 (88%), PI 548302 (81%), and PI 548178 (66%) were significantly higher than survival of DT97‐4290 (32%). These three genotypes may be useful as parents for developing soybean cultivars with charcoal rot resistance.

Incidence of 'Candidatus Liberibacter asiaticus'-Infected Plants Among Citrandarins as Rootstock and Scion Under Field Conditions.

Huanglongbing (HLB), caused by the bacterium 'Candidatus Liberibacter' spp., is currently one of the most serious diseases of citrus plants and has caused substantial economic losses. Thus far, there is no source of genetic resistance to HLB in the genus Citrus or its relatives. However, several studies have reported Poncirus trifoliata and some of its hybrids to be more tolerant to the disease. The main objective of this study was to report differences in the incidence of 'Ca. L. asiaticus' infection in citrandarin plants, hybrids from Sunki mandarin (Citrus sunki (Hayata) hort. ex Tanaka), and trifoliate orange Rubidoux (P. trifoliata (L.) Raf.)), after conducting an extensive survey under field conditions. These hybrid plants were established for approximately 7 years in an area with a high incidence of 'Ca. L. asiaticus'-infected plants. We selected two experimental areas (area A and area B), located approximately 10 m apart. Area A consists of Pera sweet orange (C. sinensis (L.) Osb.) grafted onto 56 different citrandarin rootstocks. Area B consists of citrandarin scions grafted onto Rangpur lime (C. limonia Osb.) rootstock. Bacteria in the leaves and roots were detected using real-time quantitative polymerase chain reaction. The incidence of 'Ca. L. asiaticus'-infected plants was 92% in area A and 14% in area B. Because infected plants occurred in both areas, we examined whether the P. trifoliata hybrid rootstock influenced HLB development and also determined the distribution of 'Ca. L. asiaticus' in Citrus tree tissues. Although this survey does not present evidence regarding the resistance of P. trifoliata and its hybrids in relation to bacteria or psyllids, future investigation, mainly using the most promising hybrids for response to 'Ca. L. asiaticus', will help us to understand the probable mechanism of defense or identifying compounds in P. trifoliata and its hybrids that are very important as strategy to combat HLB. Details of these results are presented and discussed in this article.

Resistance Genes of Oryza sativa for Protection Against Xanthomonas oryzae pv. oryzae, the Causative Agent of Bacterial Leaf Blight

Rice (Oryza sativa) is a major staple crop around the world, particularly in developing regions such as Africa and Asia. Attack of the rice crop by pathogens has the capacity to greatly diminish crop yield with major agricultural, economical and human consequences. Xanthomonas orzyza pv. oryzae (Xoo), the causative agent of bacterial leaf blight, has the ability to cause substantial crop loss, up to 80% worldwide. Genetic sources of resistance have long been considered the most sustainable method of disease control, both from an environmental and agricultural perspective. This review considers the discovery, evolution, characterization and utilization of resistance genes (R genes) in rice against Xoo. R genes for Xoo are plentiful and exist at multiple loci throughout the rice genome due to multiple gene duplication and transposition events. The presence of multiple gene loci makes gene pyramiding and resistance stacking accessible and fruitful applications in breeding for resistance. Biotechnological approaches to take advantage of R gene function have also begun to take root. However, a more global and concerted effort to study the multiple varieties and climates in which rice is grown is required in order to truly attain food security and find sustainable resistance.