Root Endophyte Colletotrichum tofieldiae Confers Plant Fitness Benefits that Are Phosphate Status Dependent.

A staggering diversity of endophytic fungi associate with healthy plants in nature, but it is usually unclear whether these represent stochastic encounters or provide host fitness benefits. Although most characterized species of the fungal genus Colletotrichum are destructive pathogens, we show here that C. tofieldiae (Ct) is an endemic endophyte in natural Arabidopsis thaliana populations in central Spain. Colonization by Ct initiates in roots but can also spread systemically into shoots. Ct transfers the macronutrient phosphorus to shoots, promotes plant growth, and increases fertility only under phosphorus-deficient conditions, a nutrient status that might have facilitated the transition from pathogenic to beneficial lifestyles. The host's phosphate starvation response (PSR) system controls Ct root colonization and is needed for plant growth promotion (PGP). PGP also requires PEN2-dependent indole glucosinolate metabolism, a component of innate immune responses, indicating a functional link between innate immunity and the PSR system during beneficial interactions with Ct.

Chitosan Sensitivity of Fungi Isolated from Mango (Mangifera indica L.) with Anthracnose.

In Mexico, the mango crop is affected by anthracnose caused by Colletotrichum species. In the search for environmentally friendly fungicides, chitosan has shown antifungal activity. Therefore, fungal isolates were obtained from plant tissue with anthracnose symptoms from the state of Guerrero in Mexico and identified with the ITS and ß-Tub2 genetic markers. Isolates of the Colletotrichum gloeosporioides complex were again identified with the markers ITS, Act, ß-Tub2, GADPH, CHS-1, CaM, and ApMat. Commercial chitosan (Aldrich, lot # STBF3282V) was characterized, and its antifungal activity was evaluated on the radial growth of the fungal isolates. The isolated anthracnose-causing species were C. chrysophilum, C. fructicola, C. siamense, and C. musae. Other fungi found were Alternaria sp., Alternaria tenuissima, Fusarium sp., Pestalotiopsis sp., Curvularia lunata, Diaporthe pseudomangiferae, and Epicoccum nigrum. Chitosan showed 78% deacetylation degree and a molecular weight of 32 kDa. Most of the Colletotrichum species and the other identified fungi were susceptible to 1 g L-1 chitosan. However, two C. fructicola isolates were less susceptible to chitosan. Although chitosan has antifungal activity, the interactions between species of the Colletotrichum gloeosporioides complex and their effect on chitosan susceptibility should be studied based on genomic changes with molecular evidence.

Phospholipid signaling pathway in Capsicum chinense suspension cells as a key response to consortium infection.

BACKGROUND: Mexico is considered the diversification center for chili species, but these crops are susceptible to infection by pathogens such as Colletotrichum spp., which causes anthracnose disease and postharvest decay in general. Studies have been carried out with isolated strains of Colletotrichum in Capsicum plants; however, under growing conditions, microorganisms generally interact with others, resulting in an increase or decrease of their ability to infect the roots of C. chinense seedlings and thus, cause disease. RESULTS: Morphological changes were evident 24 h after inoculation (hai) with the microbial consortium, which consisted primarily of C. ignotum. High levels of diacylglycerol pyrophosphate (DGPP) and phosphatidic acid (PA) were found around 6 hai. These metabolic changes could be correlated with high transcription levels of diacylglycerol-kinase (CchDGK1 and CchDG31) at 3, 6 and 12 hai and also to pathogen gene markers, such as CchPR1 and CchPR5. CONCLUSIONS: Our data constitute the first evidence for the phospholipids signalling events, specifically DGPP and PA participation in the phospholipase C/DGK (PI-PLC/DGK) pathway, in the response of Capsicum to the consortium, offering new insights on chilis' defense responses to damping-off diseases.

Different loci control resistance to different isolates of the same race of Colletotrichum lindemuthianum in common bean.

KEY MESSAGE: Linkage and genome-wide association analyses using high-throughput SNP genotyping revealed different loci controlling resistance to different isolates of race 65 of Colletotrichum lindemuthianum in common bean. Development of varieties with durable resistance to anthracnose is a major challenge in common bean breeding programs because of the extensive virulence diversity of Colletotrichum lindemuthianum fungus. We used linkage and genome-wide association analyses to tap the genomic regions associated with resistance to different isolates of race 65. Linkage mapping was done using an F2 population derived from the cross between the Mesoamerican common beans BRS Estilo x Ouro Vermelho, inoculated with two different isolates of race 65. Association genetics relied on a diversity common bean panel containing 189 common bean accessions inoculated with five different isolates of race 65 as an attempt to validate the linkage analysis findings and, eventually, identify other genomic regions associated with resistance to race 65. The F2 population and diversity panel were genotyped with the BARCBean6K_3 Illumina BeadChip containing 5398 SNP markers. Both linkage and genome-wide association analyses identified different loci controlling resistance to different isolates of race 65 on linkage group Pv04. Genome-wide association analysis also detected loci on Pv05, Pv10 and Pv11 associated with resistance to race 65. These findings indicate that resistance to race 65 can be overcome by the virulence diversity among different isolates of the same race and could lead to the loss of resistance after cultivar release. We identified 25 resistant common bean cultivars to all five isolates of race 65 in the diversity panel. The accessions should be useful to develop cultivars combining different resistance genes that favor durable resistance to anthracnose in common bean.

Conyza bonariensis as an alternative host for Colletotrichum species in Argentina.

AIMS: This study investigated the diversity of Colletotrichum isolates recovered from Conyza bonariensis leaves through the use of morphological characteristics, growth rate, carbon sources utilization and phylogenetic analysis. METHODS AND RESULTS: In all, 30 Colletotrichum isolates recovered from C. bonariensis leaves showing symptoms of disease were included in the present study. Based on the analysis of morphology and sequences, the isolates were distributed into six Colletotrichum species complexes. The concatenated alignment of GAPDH and ITS sequences showed that 20 out of 30 isolates were included in four species complexes which comprise the most important pathogens causing anthracnose in soybean or anthracnose and stalk rot in maize: C. truncatum, C. orchidearum, C. gloeosporioides and C. graminicola. The remaining 10 isolates were included in the C. boninense and C. destructivum species complexes or could not be assigned to any complex with the available information. CONCLUSION: Weeds belonging to genus Conyza are host to soybean and maize potential pathogenic species of Colletotrichum and could have a role as inoculum reservoir for cross contamination in the agroecosystem. SIGNIFICANCE AND IMPACT OF THE STUDY: The combined use of morphological, kinetics and physiological parameters of growth and phylogenetic analysis in Colletotrichum isolates from Conyza leaves allowed the detection of species complexes previously not identified in Argentina.

Quantitative Detection and Monitoring of Colletotrichum siamense in Rubber Trees Using Real-Time PCR.

Colletotrichum siamense is one of the most important pathogens of rubber trees in Asia. The proper detection and quantification of C. siamense populations in rubber trees are of importance for monitoring the epidemics of the disease. In this study, we developed an internal transcribed spacer-based real-time PCR method to efficiently detect C. siamense infecting rubber trees, which reliably detected as little as 100 fg of genomic DNA, 100 copies of target DNA, and 20 conidia. The real-time PCR protocol recognized all C. siamense isolates collected from three provinces in China, whereas no amplification was observed with the rubber tree and its other pathogens. Detection and quantification of C. siamense were performed in artificially and naturally infected rubber leaves. We could still detect C. siamense in plant mixes, of which only 0.0001% of the tissue was infected. An accumulation of C. siamense DNA was observed during the whole infection process at all three leaf phenological stages, suggesting that the real-time PCR method can be used to monitor C. siamense development in rubber trees. Finally, the method allowed the detection of C. siamense in naturally infected and symptomless leaves of rubber trees in the fields. Compared with earlier detection methods, the real-time PCR method is more specific and more sensitive, and it will be of great use for studies aiming to gain a better understanding of the epidemiology of Colletotrichum leaf disease, as well as the prediction of disease risk and proposals to control it.

Identification and characterization of Colletotrichum destructivum causing anthracnose on sunflower.

Sunflower is one of the most economically important oil crops. Recently, sunflower anthracnose caused by Colletotrichum destructivum was reported and suggested to be a potential threat to the quality of oil and edible seeds derived from sunflower in the field and even on the ornamentals in the residential gardens. Colletotrichum destructivum, as the causal agent of sunflower anthracnose, has been rarely studied. In this study, the vegetative growth and sporulation of this fungal species were investigated by assessing the requirements of nutrition and other environmental conditions, such as temperature, ambient pH, and lightness regime. Additionally, the sensitivity of C. destructivum to several fungicides was assessed. The results will provide a baseline for better understanding of the biology and etiology of C. destructivum. This study will be the first reference for a sustainable management strategy according to the occurrence and prevalence of the sunflower anthracnose.

Specific and sensitive detection of the guava fruit anthracnose pathogen (Colletotrichum gloeosporioides) by loop-mediated isothermal amplification (LAMP) assay.

Anthracnose of guava, caused by the fungus Colletotrichum gloeosporioides, is a major factor limiting worldwide guava production. Timely and accurate detection of the pathogen is important in developing a disease management strategy. Herein, a loop-mediated isothermal amplification (LAMP) assay for the specific and sensitive detection of C. gloeosporioides was developed using primers targeting the ß-tubulin 2 (TUB2) gene. The optimal reaction conditions were 64 °C for 60 min. The specificity of the method was tested against C. gloeosporioides isolates, Colletotrichum spp. isolates, and isolates of other genera. Positive results were obtained only in the presence of C. gloeosporioides, whereas no cross-reaction was observed for other species. The detection limit of the LAMP assay was 10 fg of genomic DNA in a 25 µL reaction. The LAMP assay successfully detected C. gloeosporioides in guava fruit collected in the field. The results indicate that the developed LAMP assay is a simple, cost-effective, rapid, highly sensitive, and specific tool for the diagnosis of guava anthracnose caused by C. gloeosporioides and could be useful for disease management.

Colletotrichum keratitis: A rare yet important fungal infection of human eyes.

BACKGROUND: Colletotrichum is a rare cause of human infection. Previous reports about Colletotrichum keratitis were limited, and most diagnoses from past reports were based on morphological distinction, which could have led to underestimation of the prevalence of Colletotrichum species. OBJECTIVE: We reported phylogenetic analysis, clinical feature and treatment outcome of molecularly diagnosed Colletotrichum keratitis in our hospital. PATIENTS/METHODS: We recruited 65 patients with culture-proven filamentous fungal keratitis between January 1, 2015 and December 30, 2018. Through molecular sequencing including internal transcribed spacer (ITS) and multi-locus phylogenetic analysis of fungal DNA, seven patients were verified as infected with Colletotrichum species, and their medical records were reviewed to determine the clinical characteristics. RESULTS: Six of seven patients had predisposing factors including trauma (5) and immunosuppressive status (1). Six isolates were initially misidentified as other fungi through morphological identification. ITS sequencing identified the isolates belonged to two species complex (SC): C. truncatum and C. gloeosporioides; multi-locus phylogenetic analysis enabled species identification including C. tropicale (3), C. fructicola (2), C. truncatum (1) and C. fusiforme (1). Five patients with C. gloeosporioides SC responded well to medical treatment and two patients with C truncatum SC underwent evisceration because of either no visual potential or intractable pain. CONCLUSIONS: The molecular approach provides accurate diagnosis and raises epidemiological awareness of Colletotrichum keratitis. Through multi-locus phylogenetic analysis, we report the human infections caused by C. tropicale, C. fructicola and C. fusiforme. We also highlight the different clinical outcomes between C. gloeosporioides SC and C. truncatum SC.

Molecular Identification of Endophytic Fungi and Their Pathogenicity Evaluation Against Dendrobium nobile and Dendrobium officinale.

: Dendrobium are tropical orchid plants that host diverse endophytic fungi. The role of these fungi is not currently well understood in Dendrobium plants. We morphologically and molecularly identified these fungal endophytes, and created an efficient system for evaluating the pathogenicity and symptoms of endophytic fungi on Dendrobium nobile and Dendrobium officinale though in vitro co-culturing. ReThe colony morphological traits of Dendrobium myco-endophytes (DMEs) were recorded for their identification. Molecular identification revealed the presence of Colletotrichum tropicicola, Fusarium keratoplasticum, Fusarium oxysporum, Fusarium solani, and Trichoderma longibrachiatum. The pathogenicity results revealed that T. longibrachiatum produced the least pathogenic effects against D. nobile protocorms. In seedlings, T. longibrachiatum showed the least pathogenic effects against D. officinale seedlings after seven days. C. tropicicola produced highly pathogenic effects against both Dendrobium seedlings. The results of histological examination of infected tissues revealed that F. keratoplasticum and T. longibrachiatum fulfill Koch's postulates for the existence of endophytes inside the living tissues. The DMEs are cross-transmitted inside the host plant cells, playing an important role in plant host development, resistance, and alkaloids stimulation.

Effectiveness of Cymbopogon citratus Oil Encapsulated in Chitosan on Colletotrichum gloeosporioides Isolated from Capsicum annuum.

One of the principal etiological agents associated with losses in horticultural crops is the fungus Colletotrichum sp. This study aimed to evaluate the in vitro effectiveness of the essential oil (EO) from Cymbopogon citratus in chitosan supports for the control of Colletotrichum gloeosporioides isolated from sweet pepper plants. METHODS: The extraction and phytochemical analysis of the EO of C. citratus were performed along with its encapsulation in chitosan-agar in order to compare it with other techniques and determine its effect on C. gloeosporioides. RESULTS: The EO from the citral chemotype (58%) encapsulated in the chitosan-agar, with an 83% encapsulation efficiency in mass percentage, resulted in the total inhibition of mycelial growth at a minimum inhibitory concentration of 1370 ppm. This concentration was effective in controlling the disease under greenhouse conditions. The effectivity of the capsules containing EO was superior to that of other controls using EO evaluated in vitro. The capsules demonstrated an effective period of 51 days, with an additional 30 days of effectiveness after a reinfection cycle, thus providing similar results to the control with Trichoderma sp. CONCLUSIONS: Chitosan capsules present a promising strategy in the use of C. citratus EO on C. gloeosporioides, and they are highly effective and stable under in vitro and field conditions.

[Spatial distribution of anthracnose (Colletotrichum gloeosporioides Penz) in avocado in the State of Mexico, Mexico]. / Distribución espacial de antracnosis (Colletotrichum gloeosporioides Penz) en aguacate en el Estado de México, México.

Persea americana is a species of great nutritional and economic importance for Mexico, however, like any other agricultural crop, it is affected by pests and diseases that limit its worldwide commercialization. The phytopathogenic fungus Colletotrichum gloeosporioides is the causative agent of anthracnose in avocado and manifests itself in the early stages of fruit development as well as in post-harvest and storage, under conditions of high relative humidity (80%) and at temperatures from 20°C, causing losses economic up to 20% of production. Applying geostatistical methods the present study aims to define the spatial distribution of anthracnose in Hass avocado fruits in four municipalities of the State of Mexico during the period from January to June 2017. The results show that the distribution of anthracnose was adjusted to gaussian and exponential models in most, the infestation maps made through the kriging show more than one center of aggregation of the disease, based on it the infested surface was estimated, finding an infestation of more than 50% in the first samples and up to 98% in the samplings belonging to the month of June in all the areas studied.

The repertoire of effector candidates in Colletotrichum lindemuthianum reveals important information about Colletotrichum genus lifestyle.

The fungus Colletotrichum lindemuthianum is the causal agent of anthracnose in the common bean (Phaseolus vulgaris), and anthracnose is one of the most devastating diseases of this plant species. However, little is known about the proteins that are essential for the fungus-plant interactions. Knowledge of the fungus' arsenal of effector proteins is of great importance for understanding this pathosystem. In this work, we analyzed for the first time the arsenal of Colletotrichum lindemuthianum effector candidates (ClECs) and compared them with effector proteins from other species of the genus Colletotrichum, providing a valuable resource for studying the infection mechanisms of these pathogens in their hosts. Isolates of two physiological races (83.501 and 89 A2 2-3) of C. lindemuthianum were used to predict 353 and 349 ClECs, respectively. Of these ClECs, 63% were found to be rich in cysteine, have repetitive sequences of amino acids, and/or possess nuclear localization sequences. Several conserved domains were found between the ClECs. We also applied the effector prediction to nine species in the genus Colletotrichum, and the results ranged from 247 predicted effectors in Colletotrichum graminicola to 446 in Colletotrichum orbiculare. Twelve conserved domains were predicted in the effector candidates of all analyzed species of Colletotrichum. An expression analysis of the eight genes encoding the effector candidates in C. lindemuthianum revealed their induction during the biotrophic phase of the fungus on the bean.

Fungal Keratitis Caused by Colletotrichum dematium: Case Study and Review.

Colletotrichum species are known as important pathogens of plants with an impact on crop production. Some of these species are also known as a cause of rare ophthalmic infections in humans. A case of keratitis caused by Colletotrichum dematium after corneal trauma in a 56-year-old woman is presented. Infection was diagnosed based on positive microscopy and culture. The fungal isolate was identified by morphological characteristics and DNA sequencing of the ITS rDNA region, ß-tubulin (tub2) and glyceraldehyde-3-phosphate dehydrogenase (gapdh) genes. The patient responded well to topical therapy with amphotericin B combined with intravenous amphotericin B but improvement was associated with the corneal collagen cross-linking. The review of the literature revealed another 13 cases of C. dematium keratitis, all but one patient having at least one keratitis risk factor in their history. Almost all patients (n = 12) were treated with topical polyene antibiotics (natamycin or amphotericin B), improvement and cure were achieved in eight of them.

In vitro antifungal activity of dimethyl trisulfide against Colletotrichum gloeosporioides from mango.

Colletotrichum gloeosporioides, one of the main agents of mango anthracnose, causes latent infections in unripe mango, and leads to huge economic losses during storage and transport. Dimethyl trisulfide (DMTS), one of the main volatile compounds produced by some microorganisms or plants, has shown antifungal activity against some phytopathogens in previous studies, but its effects on C. gloeosporioides and mechanisms of action have not been well characterized. In fumigation trials of conidia and mycelia of C. gloeosporioides for 2, 4, 6, 8, or 10 h, at a concentration of 100 µL/L of air space in vitro, DMTS caused serious damage to the integrity of plasma membranes, which significantly reduced the survival rate of spores, and resulted in abnormal hyphal morphology. Moreover, DMTS caused deterioration of subcellular structures of conidia and mycelia, such as cell walls, plasma membranes, Golgi bodies, and mitochondria, and contributed to leakage of protoplasm, thus promoting vacuole formation. In addition, to better understand the molecular mechanisms of the antifungal activity, the global gene expression profiles of isolate C. gloeosporioides TD3 treated in vitro with DMTS at a concentration of 100 µL/L of air for 0 h (Control), 1 h, or 3 h were investigated by RNA sequencing (RNA-seq), and over 62 Gb clean reads were generated from nine samples. Similar expressional patterns for nine differentially expressed genes (DEGs) in both RNA-seq and qRT-PCR assays showed the reliability of the RNA-seq data. In comparison to the non-treated control groups, we found DMTS suppressed expression of ß-1, 3-D-glucan, chitin, sterol biosynthesis-related genes, and membrane protein-related genes. These genes related to the formation of fungal cell walls and plasma membranes might be associated with the toxicity of DMTS against C. gloeosporioides. This is the first study demonstrating antifungal activity of DMTS against C. gloeosporioides on mango by direct damage of conidia and hyphae, thus providing a novel tool for postharvest control of mango anthracnose.

[Isolation and identification of endophytic fungi from Chelidonium majus and their antifungal activity].

In order to find new source of antifungal agents, eleven cultivable endophytic fungi were isolated from the roots,stems and leaves of Chelidonium majus by traditional method. Seven of them were identified as Colletotrichum(L1, L2, L3, S1, S3, S4, S5), and three of them were identified as Fusarium(R1,R2,R3) by morphological features and molecular biological technology. The antifungal activity test showed that all the tested fungi displayed some inhibitory activity against five common plant pathogens(C. gloeosporioides, Curvularia lunata, Pyricularia oryza, Alternaria alternate and A. brassicae), and their inhibition rate of some test items were over 60%. Among them, R1, S2, S3 and S4 were more potent than others. This study enriches the understanding of endophytes from Ch. majus and provides a basis for the study of new microbial fungicides.

Postharvest control of anthracnose lesions and its causative agent, Colletotrichum musae by some oils.

Anthracnose of banana is incited by Colletotrichum  musae. It is recognized as one the most destructive diseases of mature and immature banana fruits, resulting in huge economic losses all over the world. Present research deals with screening some oils both in vitro and in vivo for their antifungal activity against C.musae. Clove oil (0.1µl/ml) completely arrested the conidial germination and mycelial growth of C. musae. Fenugreek and almond oil exhibited significant inhibition of mycelial growth, 61% and 57% at a concentration of 2µl/ml. However, olive oil was least inhibitory on the test fungi. Clove oil also a showed marked reduction in anthracnose lesions on banana fruits, thereby suggesting disease control. Scanning electron microscopy revealed severely damaged mycelium and conidia. FTIR studies show the presence of important bands representing phenols, terpenes, aldehydes, and ketones. Based on our findings; clove, fenugreek and almond oil demonstrated fungicidal and fungistatic activities against anthracnose pathogen. Hence, these oils can be considered as potential alternatives to chemical treatments.

Detection of latent infections caused by Colletotrichum sp. in olive fruit.

AIMS: To set up a practical method to detect latent infections of Colletotrichum sp., the causal agent of olive anthracnose, on olives before the onset of disease symptoms. METHODS AND RESULTS: Freezing, sodium hydroxide (NaOH), ethanol and ethylene treatments were evaluated to detect latent infections on inoculated and naturally infected olive fruit by Colletotrichum sp. as non-hazardous alternatives to paraquat. Treatments were conducted using fruit of cultivars Arbequina and Hojiblanca. The disease incidence and T50 were calculated. Dipping in NaOH 0·05% solution and the paraquat method were the most effective treatments on both inoculated and naturally infected fruit, although the value of T50 was lower for the NaOH method than for the paraquat method in one of the experiments. Subsequently, the dipping time in NaOH 0·05% was evaluated. Longer dipping times in NaOH 0·05% were better than shorter ones in cultivar Arbequina, with 72 h being the most effective in cultivar Hojiblanca. CONCLUSIONS: NaOH solution is a practical method to detect latent infections of Colletotrichum sp. on immature olive fruit. SIGNIFICANCE AND IMPACT OF THE STUDY: This study is relevant because we set up a viable, non-hazardous alternative to paraquat to detect latent infections of Colletotrichum sp. using NaOH. The use of NaOH is a simple and eco-friendly tool that allows the determination of the level of latent infections by Colletotrichum in olives. Therefore, our method will be useful in decision-making processes for disease management before the appearance of the first visible symptoms.

Comparative analysis of the mitochondrial genome of the fungus Colletotrichum lindemuthianum, the causal agent of anthracnose in common beans.

Fungi of the genus Colletotrichum are economically important and are used as models in plant-pathogen interaction studies. In this study, the complete mitochondrial genomes of two Colletotrichum lindemuthianum isolates were sequenced and compared with the mitochondrial genomes of seven species of Colletotrichum. The mitochondrial genome of C. lindemuthianum is a typical circular molecule 37,446 bp (isolate 89 A2 2-3) and 37,440 bp (isolate 83.501) in length. The difference of six nucleotides between the two genomes is the result of a deletion in the ribosomal protein S3 (rps3) gene in the 83.501 isolate. In addition, substitution of adenine for guanine within the rps3 gene in the mitochondrial genome of the 83.501 isolate was observed. Compared to the previously sequenced C. lindemuthianum mitochondrial genome, an exon no annotated in the cytochrome c oxidase I (cox1) gene and a non-conserved open reading frame (ncORF) were observed. The size of the mitochondrial genomes of the seven species of Colletotrichum was highly variable, being attributed mainly to the ncORF, ranging from one to 10 and also from introns ranging from one to 11 and which encode a total of up to nine homing endonucleases. This paper reports for the first time by means of transcriptome that then ncORFs are transcribed in Colletotrichum spp. Phylogeny data revealed that core mitochondrial genes could be used as an alternative in phylogenetic relationship studies in Colletotrichum spp. This work contributes to the genetic and biological knowledge of Colletotrichum spp., which is of great economic and scientific importance.

Genotypic and Pathogenic Diversity of Colletotrichum sublineola Isolates from Sorghum (Sorghum bicolor) and Johnsongrass (S. halepense) in the Southeastern United States.

Anthracnose caused by Colletotrichum sublineola is an important disease of cultivated sorghum (Sorghum bicolor) worldwide. Anthracnose is also common on the ubiquitous wild sorghum relative Johnsongrass (S. halepense). Analysis of repetitive molecular fingerprinting markers revealed that isolates of C. sublineola from both hosts in the southeastern United States were genotypically diverse, with relatively few haplotypes found in more than one location. With few exceptions, isolates recovered from S. bicolor belonged to a population that was genetically distinct from the population recovered from S. halepense. Twenty-three isolates from cultivated sorghum were all pathogenic to at least one of 13 heritage inbred lines of S. bicolor. In all, 4 of 10 isolates from S. halepense were also pathogenic to one or more of the lines, while the rest caused no disease in greenhouse assays. The four pathogenic isolates from S. halepense were less aggressive, on average, than isolates from S. bicolor, although the ranges overlapped. Pathogenicity tests involving 15 representative pathogenic isolates from S. bicolor and S. halepense on eight heritage inbred lines of S. bicolor identified 12 races. The combined results of this study demonstrated that C. sublineola comprises two separate host-associated subpopulations in the field, even though some isolates from S. halepense were able to cause disease on S. bicolor under ideal greenhouse conditions. Nonetheless, the apparent existence of infrequent cross-infection events in the field, indicated by molecular fingerprinting, suggests that Johnsongrass has the potential to serve as a refuge and an incubator for genetic diversity in C. sublineola, which can complicate efforts to develop and deploy resistant sweet sorghum varieties in the region.