Anthracnose Resistance Breeding Research Articles

Global transcriptome analysis reveals resistance genes in the early response of common bean (Phaseolus vulgaris L.) to Colletotrichum lindemuthianum

BackgroundDisease can drastically impair common bean (Phaseolus vulgaris L.) production. Anthracnose, caused by the fungal pathogen Colletotrichum lindemuthianum (Sacc. and Magnus) Briosi and Cavara, is one of the diseases that are widespread and cause serious economic loss in common bean.ResultsTranscriptome analysis of the early response of common bean to anthracnose was performed using two resistant genotypes, Hongyundou and Honghuayundou, and one susceptible genotype, Jingdou. A total of 9,825 differentially expressed genes (DEGs) responding to pathogen infection and anthracnose resistance were identified by differential expression analysis. By using weighted gene coexpression network analysis (WGCNA), 2,051 DEGs were found to be associated with two resistance-related modules. Among them, 463 DEGs related to anthracnose resistance were considered resistance-related candidate genes. Nineteen candidate genes were coexpressed with three resistance genes, Phvul.001G243600, Phvul.001G243700 and Phvul.001G243800. To further identify resistance genes, 46 candidate genes were selected for experimental validation using salicylic acid (SA) and methyl jasmonate (MeJA). The results indicated that 38 candidate genes that responded to SA/MeJA treatment may be involved in anthracnose resistance in common bean.ConclusionsThis study identified 38 resistance-related candidate genes involved in the early response of common bean, and 19 resistance-related candidate genes were coexpressed with anthracnose resistance genes. This study identified putative resistance genes for further resistance genetic investigation and provides an important reference for anthracnose resistance breeding in common bean.

QTL and PACE analyses identify candidate genes for anthracnose resistance in tomato.

Anthracnose, caused by the fungal pathogen Colletotrichum spp., is one of the most significant tomato diseases in the United States and worldwide. No commercial cultivars with anthracnose resistance are available, limiting resistant breeding. Cultivars with genetic resistance would significantly reduce crop losses, reduce the use of fungicides, and lessen the risks associated with chemical application. A recombinant inbred line (RIL) mapping population (N=243) has been made from a cross between the susceptible US28 cultivar and the resistant but semiwild and small-fruited 95L368 to identify quantitative trait loci (QTLs) associated with anthracnose resistance. The RIL population was phenotyped for resistance by inoculating ripe field-harvested tomato fruits with Colletotrichum coccodes for two seasons. In this study, we identified twenty QTLs underlying resistance, with a range of phenotypic variance of 4.5 to 17.2% using a skeletal linkage map and a GWAS. In addition, a QTLseq analysis was performed using deep sequencing of extreme bulks that validated QTL positions identified using traditional mapping and resolved candidate genes underlying various QTLs. We further validated AP2-like ethylene-responsive transcription factor, N-alpha-acetyltransferase (NatA), cytochrome P450, amidase family protein, tetratricopeptide repeat, bHLH transcription factor, and disease resistance protein RGA2-like using PCR allelic competitive extension (PACE) genotyping. PACE assays developed in this study will enable high-throughput screening for use in anthracnose resistance breeding in tomato.

Inheritance of Resistance to Anthracnose Fruit Rot Caused by Colletotrichum fioriniae in Highbush Blueberry

ABSTRACT Anthracnose fruit rot, caused by Colletotrichum spp. is a major disease of highbush blueberries. The inheritance of fruit rot resistance to C. fioriniae was investigated in crosses of parents with varying levels of susceptibility. Three cultivars with known resistance profiles (Bluecrop, Elliott, and Jersey) and progeny from 16 crosses of parents with varying levels of susceptibility were screened. Fruit of field-grown bushes was inoculated when immature, harvested when ripe, and rated for infection incidence after 5, 8, and 12 days of incubation at 100% RH and 22–23°C. Area under the disease progress curves (AUDPC) values were calculated for 2010 and 2011 and slightly higher disease pressure was observed in 2011. These values were then regressed against actual disease incidences of cultivars and predicted (midparent) values for cross families based on two previous studies in 2010 and 2011 and significant correlations with the proportion of fruit decayed and sporulation capacity were observed. These findings provide strong evidence that anthracnose resistance is heritable in highbush blueberries, which has important implications for anthracnose resistance breeding. Additionally, this research provides benchmark AUDPC values for evaluation of future breeding selections for their resistance to C. fioriniae.

Genome-wide association analysis of anthracnose resistance in sorghum [Sorghum bicolor (L.) Moench

In warm-humid ago-ecologies of the world, sorghum [Sorghum bicolor (L.) Moench] production is severely affected by anthracnose disease caused by Colletotrichum sublineolum Henn. New sources of anthracnose resistance should be identified to introgress novel genes into susceptible varieties in resistance breeding programs. The objective of this study was to determine genome-wide association of Diversity Arrays Technology Sequencing (DArTseq) based single nucleotide polymorphisms (SNP) markers and anthracnose resistance genes in diverse sorghum populations for resistance breeding. Three hundred sixty-six sorghum populations were assessed for anthracnose resistance in three seasons in western Ethiopia using artificial inoculation. Data on anthracnose severity and the relative area under the disease progress curve were computed. Furthermore, the test populations were genotyped using SNP markers with DArTseq protocol. Population structure analysis and genome-wide association mapping were undertaken based on 11,643 SNPs with <10% missing data. The evaluated population was grouped into eight distinct genetic clusters. A total of eight significant (P < 0.001) marker-trait associations (MTAs) were detected, explaining 4.86-15.9% of the phenotypic variation for anthracnose resistance. Out of which the four markers were above the cutoff point. The significant MTAs in the assessed sorghum population are useful for marker-assisted selection (MAS) in anthracnose resistance breeding programs and for gene and quantitative trait loci (QTL) mapping.

Major Sorghum Production Constraints and Coping Mechanisms: The Case of Anthracnose (Colletotrichum sublineolum)

This paper attempts to review the major sorghum production constraints, the progress and perspective on sorghum anthracnose (Colletotrichum sublineolum) resistance breeding. The importance of anthracnose in sorghum production and breeding for resistance status and progress were also primly discovered. Sorghum is an ancient environment resilient crop and believed to be a future crop due to its important merits like tolerant to stresses, wide adaptability and low input requirement. Insects and disease are major biotic impediments to realizing the yield potential of the crop. Anthracnose disease caused by Colletotrichum sublineolum is the most important disease that severely affecting the crop in all sorghum producing regions of the world. Research results revealed that anthracnose resulted in 30-50% or greater yield losses. Several management strategies such as, cultural, chemical and using resistance varieties have been developed. Employing host-plant resistance is the most economical and environmentally friendly approach which can successfully control the disease. Breeding assisted with molecular markers plays a great role in resistance breeding programme as it makes easy to screen large number of genotypes at once. Recent advancement of molecular breeding and bio-informatics tools are playing a significant role in efficiencies and precisions of resistance breeding. QTLs or genomic area for resistance were identified using traditional molecular markers and recent research results revealed discoveries of specific gene and locus using high throughput markers like SNPs using GWAS approach. The discovery of genes/QTL associated with the resistance trait, using the high through put molecular markers like SNPs, facilitates the easiest way for gene pyramiding from different individual genotypes to a single variety, introgression into adapted elite cultivar through marker assisted and editing genes for elite landraces to develop durable resistance varieties. Transgenic approach is now a day becoming a powerful tool to utilize novel alien genes for crop improvement including anthracnose resistance breeding in sorghum.

Heterothallism among spatiotemporally diverse Colletotrichum lindemuthianum isolates and its implication in common bean anthracnose resistance breeding in the Northwestern Himalayan region

Heterothallism among spatiotemporally diverse Colletotrichum lindemuthianum isolates and its implication in common bean anthracnose resistance breeding in the Northwestern Himalayan region

Field-Relevant New Sources of Resistance to Anthracnose Caused by Colletotrichum truncatum in a Mungbean Mini-Core Collection.

Anthracnose is a prevalent disease of mungbean in Asian countries and Sub-Saharan Africa. It is caused by multiple Colletotrichum species. The high levels of anthracnose resistance in mungbean have not been studied in depth in India, but genetic resistance is desired. In this study, we identified the causal agent of mungbean anthracnose in two regions of India as Colletotrichum truncatum through morphological and molecular methods. A set of 296 mungbean mini-core accessions developed by WorldVeg was screened under a natural disease pressure from July to September (kharif season) in 2016, 2017, and 2018 in Hyderabad (a hot spot for anthracnose) to identify anthracnose resistance. Based on disease severity scores, 22 accessions were consistently anthracnose resistant under the categories of immune, highly resistant, and resistant with scores ranging from ≥1.0 to ≤3.0 during the period of study. Furthermore, based on the agronomic performance, anthracnose resistance in Hyderabad, and other desirable traits, a subset of 74 mungbean accessions was selected from 296 mini-core accessions. These accessions were evaluated under natural disease pressure from July to September in 2018 and 2019 in Palampur (another hot spot for anthracnose) to determine the variation in anthracnose resistance. Out of the 74 accessions, two accessions were resistant in 2018; in 2019, one was immune, nine were highly resistant, and 15 were resistant. Combined analysis of variance of 65 accessions common in Hyderabad and Palampur revealed highly significant effects of environment, genotype (accessions), and genotype × environment interaction on the disease severity. The combined GGE biplot analysis of data across years and locations confirmed that the seven accessions MC-24, MC-51, MC-75, MC-127, MC-207, MC-208, and MC-292 were resistant during 2016 to 2018 in Hyderabad, and only in 2019 in Palampur, and the same accessions were moderately resistant in 2018 in Palampur. The seven resistant accessions identified from both test locations could be used as potential donors in the anthracnose resistance breeding program.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Genetic inheritance and identification of germplasm sources for anthracnose resistance in soybean [Glycine max (L.) Merr.

Soybean anthracnose caused by Colletotrichum truncatum (Schw.) Andrus & W.D. Moore is a devastating disease in Central and Southern India. Chemical control of this disease is neither economical nor eco-friendly and genetic resistance is considered to be an efficient means for its management. Therefore, in the current study, a total of 225 soybean [Glycine max (L.) Merr.] germplasm accessions were evaluated for anthracnose resistance which resulted in the identification of five genotypes viz., EC 538828, EC 34372, EC 457254, AKSS 67 and Karune as highly resistant. Genetics of anthracnose resistance in three F2 populations derived from EC 34372 × JS 95-60, EC 457254 × JS 95-60 and AKSS 67 × JS 95-60 revealed that the resistance in all the three resistant parents was governed by two major genes interacting in complementary fashion. The χ2 test for goodness of fit revealed that F2 plants in each of the three populations segregated in 9 resistant: 7 susceptible ratio. This is the first report on genetics of anthracnose resistance in soybean and will aid soybean breeders to develop a strategic anthracnose resistance breeding program and to map the genes governing resistance. Research findings from this study indicated the potential role of exotic germplasm in Indian soybean improvement against anthracnose disease.

Assessment of farmers’ perceptions of production constraints, and their trait preferences of sorghum in western Ethiopia: implications for anthracnose resistance breeding

ABSTRACTSorghum, Sorghum bicolor (L.) Moench is an important food security crop widely grown by smallholder farmers in sub-Saharan Africa (SSA), including Ethiopia. In SSA, the potential of sorghum production and productivity has not been realised due to an array of constraints. Colletotrichum sublineolum disease is one of the main biotic constraints causing significant yield losses. The objectives of this study were to assess farmers’ perception, preferences and constraints to sorghum production and productivity in western Ethiopia, and to identify key drivers for anthracnose resistance breeding. A participatory rural appraisal study was undertaken in six selected districts in the East Wellega and West Shewa Administrative Zones in Ethiopia. Data were collected through structured questionnaires involving 165 respondent farmers. Further focus group discussions were held with 180 farmers and development agents. Sorghum is the third most preferred cereal crop after Eragrostis tef and Zea mays in western Ethiopia. About 79% of interviewed farmers cultivated sorghum during the study season. Farmers perceived that sorghum production is constrained by anthracnose disease, bird attack, loose smut and covered smut diseases. The most important farmer-preferred traits in sorghum varieties in the study areas were anthracnose resistance and tolerance to bird attack. Breeding sorghum varieties with the farmer-preferred traits is an important consideration to enhance productivity and adoption of improved sorghum cultivars in western Ethiopia.

Transfer of Co-1 Gene Locus for Anthracnose Disease Resistance to Fresh Bean (Phaseolus vulgaris L.) Through Hybridization and Molecular Marker-Assisted Selection (MAS)

Anthracnose disease, caused by the fungal pathogen Colletotricum lindemuthianum brings about great yield losses in the Blacksea Region of Turkey. The present study is carried out to start the resistance breeding program against anthracnose disease. Five pathogenic strains (2175, 3071, 3303, 3321, 4071) identified previously from Blacksea Region were inoculated on seven foreign anthracnose resistant varieties of fresh bean (MDRK, PM, Kaboon, Widusa, Machinac, Isles and Chinook) as well as on nine breeding lines (T23, TK57, TK1, C31,C28, T7, T26, TK15 and T21) which were developed as Ay?e kad?n type by pure-line selection, a method used for self-pollinated crops. Results indicated that while all the foreign varieties were resistant to these pathogen strains, the pure-lines were all susceptible. Thus foreign varieties and pure-lines were selected as parents to perform 63 combinations of genetic cross with the intention to start anthracnose resistance breeding program, only 18 of which produced seed. Through self fertilization of 360 F1 plants an F2 population of 4365 plants were obtained which were phenotyped for resistance both by inoculation with five pathogen strains as well as using resistance gene (Co-1) linked molecular marker (SEactMcca). Since Ayse Kadin type lines are of Andean origin and Co-1 locus controls resistance against Andean pathogens additional confirmation of Co-1 presence in the resistant F2 lines is of great importance. Marker screening results indicated the presence of Co-1 gene in 6 out of 18 hybrid lines. Confirmation of resistance trait inheritance on 6 F2 lines both by inoculations as well as molecular marker screening for Co-1 gene have given us the opportunity to continue with the backcross studies with high confidence. With this study anthracnose resistance breeding has been initiated in Turkey for the first time and molecular marker assisted selection has been integrated into the breeding program.

Novel source of resistance and differential reactions on chilli fruit infected by Colletotrichum capsici

Pre- and post-harvest anthracnose fruit rot is a main disease of chilli (Capsicum spp.). Among Colletotrichum species causing anthracnose, C. capsici is the most common in India. A total of 41 Capsicum genotypes were screened for anthracnose resistance under field conditions. Bhut Jolokia, PBC-380 and IC-383072 were found symptomless and some marginally cultivated lines and other crosses were observed to be resistant. In vitro inoculation of the selected set of genotypes with C. capsici revealed that 9 days after inoculation was appropriate to record observations on disease resistance. Screening the 41 genotypes for resistance to anthracnose under in vitro conditions revealed four symptomless and 11 highly resistant lines. Comparing field and in vitro evaluations, nine lines (BS-35, BS-20, BS-28, Punjab Lal, Bhut Jolokia, Taiwan-2, IC-383072, Pant C-1 and Lankamura Collection) showed consistent resistance in field as well as in vitro to the disease. The study on differential reactions on fruits of 16 genotypes inoculated by the two C. capsici isolates (Ccf-Varanasi and Ccc2-Raichur) clearly showed that these isolates are two different pathotypes. A detailed analysis on prevalent strains in the target region would be needed to initiate resistance breeding. Nevertheless, newly identified resistant sources offer better choices for the ongoing anthracnose resistance breeding program.

고추 유전자원의 탄저병(Colletotrichum acutatum) 저항성 평가

농업유전자원센터에 보존하고 있는 896점의 고추(C. annuum 430점, C. baccatum 219점, C. chacoense 14점, C. frutescens 70점, C. pubescens 2점, 미동정 8점)에 C. acutatum을 접종 후 <TEX>$28^{\circ}C$</TEX>에서 14일간 배양 후 탄저병 발병 정도를 평가하였다. 무상처 분무 접종에서 총 49점이 C. acutatum에 저항성을 나타내었으며, 그 중 9점은 고도의 저항성을 나타내었다. 고도의 저항성을 나타내는 자원들은 C. baccatum 4점, C. chacoense 1점, C. frutescens 4점이었다. 무상처 분무 접종에서 탄저병 저항성을 나타내는 자원 중 42점을 상처 접종법으로 탄저병균을 접종하였다. 그 중 5점은 접종 5일 후에도 병반의 크기가 3%미만으로 나타나 저항성을 나타내었으며, 이들은 모두 C. baccatum이었다. 저항성을 보이는 5점은 탄저병 저항성 육성을 위한 자원으로 이용가능 할 것이다. Resistance of pepper (Capsicum spp.) to anthracnose (Colletotrichum acutatum) was evaluated during regeneration of Capsicum spp. in National Agrobiodiversity Center. Disease severity of 896 pepper accessions (430 accessions of C. annuum, 219 accessions of C. baccatum, 14 accessions of C. chacoense, 153 accessions of C. chinense, 70 accessions of C. frutescens, 2 accessions of C. pubescens, and unidentified 8 accessions) was investigated at 14 days after inoculation in <TEX>$28^{\circ}C$</TEX> humid chamber. Forty nine accessions of pepper germplasm were resistant to C. acutatum. Among them, nine accessions were highly resistant to C. acutatum without wounding spray inoculation. Four accessions belonged to the species C. baccatum, one accession to C. chacoense, and four accessions to C. frutescens. Forty two resistant candidate accessions were inoculated with pin-prick wounding using a syringe needle. Five accessions were resistant as a less than 3% of disease severity to C. acutatum with wounding inoculation 5 days after inoculation. All resistant accessions were C. baccatum. These five pepper germplasm might be used as breeding resources for the anthracnose resistance breeding program.

Linkage mapping of the Phg-1 and Co-1 4 genes for resistance to angular leaf spot and anthracnose in the common bean cultivar AND 277

The Andean common bean AND 277 has the Co-1 4 and the Phg-1 alleles that confer resistance to 21 and eight races, respectively, of the anthracnose (ANT) and angular leaf spot (ALS) pathogens. Because of its broad resistance spectrum, Co-1 4 is one of the main genes used in ANT resistance breeding. Additionally, Phg-1 is used for resistance to ALS. In this study, we elucidate the inheritance of the resistance of AND 277 to both pathogens using F2 populations from the AND 277 × Rudá and AND 277 × Ouro Negro crosses and F2:3 families from the AND 277 × Ouro Negro cross. Rudá and Ouro Negro are susceptible to all of the above races of both pathogens. Co-segregation analysis revealed that a single dominant gene in AND 277 confers resistance to races 65, 73, and 2047 of the ANT and to race 63-23 of the ALS pathogens. Co-1 4 and Phg-1 are tightly linked (0.0 cM) on linkage group Pv01. Through synteny mapping between common bean and soybean we also identified two new molecular markers, CV542014450 and TGA1.1570, tagging the Co-1 4 and Phg-1 loci. These markers are linked at 0.7 and 1.3 cM, respectively, from the Co-1 4 /Phg-1 locus in coupling phase. The analysis of allele segregation in the BAT 93/Jalo EEP558 and California Dark Red Kidney/Yolano recombinant populations revealed that CV542014450 and TGA1.1570 segregated in the expected 1:1 ratio. Due to the physical linkage in cis configuration, Co-1 4 and Phg-1 are inherited together and can be monitored indirectly with the CV542014450 and TGA1.1570 markers. These results illustrate the rapid discovery of new markers through synteny mapping. These markers will reduce the time and costs associated with the pyramiding of these two disease resistance genes.

Pathogenic and genetic variability among Colletotrichum gloeosporioides isolates from different yam hosts in the agroecological zones in Nigeria

AbstractAnthracnose, caused by Colletotrichum gloeosporioides Penz., is the most severe foliar disease of water yam (Dioscorea alata) worldwide. Population genetic analyses can yield useful insights into the evolutionary potential of C. gloeosporioides and thus lead to the development of appropriate disease management strategies. The genetic structure of C. gloeosporioides populations from yam and non‐yam hosts in three agroecological zones of Nigeria was investigated. Microsatellite‐primed polymerase chain reaction (MP‐PCR), virulence phenotyping using five putative D. alata differentials, cross‐inoculation tests, and the presence/absence of a Glomerella teleomorph in yam fields were used to infer the evolutionary potential of C. gloeosporioides on yam. We observed high genotypic diversity (GD = 0.99 to 1.00) for populations from all hosts and agroecological zones, with multiple pathogen genotypes in individual anthracnose lesions. Genetic differentiation was low among pathogen populations from different hosts (GST = 0.10, θ = 0.034), and agroecological zones (GST = 0.04, θ = 0.018), indicating limited host differentiation and significant gene flow. No evidence was found for the existence of C. gloeosporioides f. sp. alatae reported in previous studies. The fungus was recovered from several non‐yam host species commonly found in yam fields but non‐yam isolates caused only mild to moderate symptoms on yam. Eighteen C. gloeosporioides virulence phenotypes were identified among 217 isolates but there was a weak correlation (r = 0.02, P = 0.40) between virulence phenotype and MP‐PCR haplotype. Consistent with the above findings, we observed for the first time the Glomerella teleomorph on anthracnose‐infected yam plants in Nigeria, indicating that sexual recombination might play an important role in anthracnose epidemics on yam. The implications of these findings for C. gloeosporioides evolutionary potential and anthracnose resistance breeding are discussed.

Inheritance of Resistance toColletotrichum acutatuminFragaria×ananassa

Anthracnose, caused by Colletotrichum acutatum, is a major disease of the octoploid cultivated strawberry, Fragaria × ananassa The inheritance of high and intermediate level plant resistances to C. acutatum, pathogenicity group 2, was investigated in an 8 × 8 factorial design. A single dominant gene (Rca2) controlled the high-level resistance, although minor genes may also contribute to resistance in cultivars such as Belrubi. The intermediate level of resistance was quantitative and controlled by minor genes. Analysis of 26 genotypes and cultivars from Fragaria spp. showed that the dominant gene was not rare in the germ plasm of F. × ananassa and that anthracnose resistance was also present in other species of Fragaria. These findings have important implications for anthracnose resistance breeding.

Classification and inheritance of genetic resistance to anthracnose in sorghum

Anthracnose is a major disease of sorghum [ Sorghum bicolor (L.) Moench]. Breeding for stable host plant resistance to this disease has been difficult due to the variable nature of the pathogen and an incomplete understanding of the host/pathogen interaction. To develop new lines with possibly more durable forms of resistance, different sources of genetic resistance must be identified and characterized. The objectives of this study were (1) to determine if different sources with anthracnose resistance possess different genes for resistance, (2) to determine the inheritance of anthracnose resistance in the groups identified in Objective 1, and (3) to identify which sources provide resistance across environments. Populations created from hybridizing resistant by resistant lines were evaluated to determine if segregation for resistance occurred within a family. The presence of segregation (susceptible plants) within a population indicated that the parents have different resistance genes. In the 11 germplasms evaluated, five different sources of resistance were identified. Segregation ratios in resistant × susceptible F 2 populations were consistent with the expectations of simply inherited traits and resistance was dominant in some lines and recessive in others. Evaluation of the sources of resistance across environment indicated that one source (SC748-5) provided resistance in all evaluation environments. Methods for the use of this germplasm in anthracnose resistance breeding are recommended.

English

Water yam (Dioscorea alata L.) is the most widely cultivated yam species globally. The major limitation to the profitable and sustainable production of D. alata is its susceptibility to anthracnose disease. The availability of resistant varieties could potentially form the cornerstone of an integrated management strategy for yam anthracnose; however, anthracnose resistance breeding is hampered by the dearth of knowledge on pathogen identity and diversity. Four forms of Colletotrichum are now known to be associated with foliar anthracnose of yam: the slow-growing grey (SGG), the fast-growing salmon (FGS), the fast-growing olive (FGO), and the fast-growing grey (FGG) forms. The close phylogenetic relationship of the first three forms to reference isolates of Colletotrichum gloeosporioides, and the fact that only strains of these forms have been observed to induce typical anthracnose symptoms on D. alata, recently confirmed that C. gloeosporioides is the causal agent of yam anthracnose disease. The FGG form possibly represents a distinct, endophytic, species as indicated by morphological, biological and molecular criteria. Previous research emphasized epidemiology and control but limited progress was made in understanding yam anthracnose disease based on this classical approach. Molecular approaches have started to unravel the systematics and ecology of Colletotrichum strains associated with yam anthracnose, as well the population biology of C. gloeosporioides on yam. Sexual recombination is a likely mechanism contributing to the high genetic diversity of C. gloeosporioides in yam-based cropping systems. Studies have been initiated to understand the mechanisms that generate genetic variation in C. gloeosporioides, and to gain some insight into the biochemistry of the interactions between the pathogen and yam. Our thesis in this article is that integrating traditional and molecular approaches to understanding C. gloeosporioides systematics, epidemiology and population genetics will lead to a much better understanding of yam anthracnose disease, and thus to the development of effective and sustainable control measures. Research successes and challenges are discussed, as well as their implications for future studies on pathogen evolutionary potential, anthracnose resistance breeding, and the deployment of resistance genes. Key words : Anthracnose, Colletotrichum gloeosporioides, Dioscorea spp., molecular markers, molecular systematics, population biology, resistance breeding, yam. African Journal of Biotechnology Vol. 2 (12), pp. 486-496, December 2003

Phytoplasma host range and symptom expression in the pasture legume Stylosanthes

A field trial of 23 stylo taxa was assessed for susceptibility to stylosanthes little leaf (SLL) disease. The trial included Stylosanthes scabra cvv. Seca, Siran, and Fitzroy, S. guianensis var. guianensis cv. Cook, S. hamata cvv. Amiga and Verano and “ Stylosanthes seabrana” cvv. Unica and Primar, as well as seven unreleased cultivars from a CSIRO anthracnose resistance breeding program, and four unreleased introduced lines. Plants that had SLL symptoms were sampled on six occasions from March to August 1999 and tested for phytoplasmas, the presumed causal organism of this disease, using the polymerase chain reaction (PCR). All taxa were found to be susceptible to SLL disease except S. hamata cv. Verano and “ S. seabrana” cv. Unica. Varieties were ranked on disease incidence, with S. scabra varieties bred for anthracnose resistance having the highest incidence of the disease. These varieties also had significantly higher SLL incidence than that of the original introductions from which they were derived. Before testing, symptom profiles were recorded noting the presence of chlorosis, spike elongation, reddening of leaf tips and floral parts, and reduced leaf size (little leaf). Statistical analysis of the association of these characteristics with SLL disease showed that little leaf was the best disease indicator and that chlorosis and excessive reddening of leaf tips and floral parts were also associated with the disease. The presence of floral abnormalities was also recorded in the symptom profiles. At no time were normal flowers found on samples that had SLL, making this symptom a good indicator of SLL. However, not all plants with floral symptoms tested positive for phytoplasmas by PCR, probably because these symptoms are difficult to diagnose visually. Virescence and phyllody were the most common floral abnormalities and were found to be associated specifically with the phytoplasmas pigeon pea little leaf and sweet potato little leaf variant V4, respectively. An additional floral abnormality, termed abortion, was also observed in association with this disease.