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Colletotrichum sublineola is a destructive fungal pathogen that causes anthracnose in sorghum. Senegalese sorghum germplasm is currently being considered as an option of sources for genetic resistance. In a recent study, Senegalese sorghum accessions were evaluated for response to a mixture of Texas isolates of C. sublineola at the 8-leaf stage in the greenhouse. As a comparison, 159 Senegalese sorghum accessions at the 1-leaf developmental stage were evaluated against a single Texas isolate of C. sublineola (FSP53) using an excised-leaf assay. A genome-wide association study (GWAS) was conducted based on the phenotypic data acquired to discover genetic variation associated with response to C. sublineola using 193,727 single nucleotide polymorphisms (SNPs) throughout the genome. Sorghum seedlings tended to be more resistant when compared with sorghum plants inoculated at the 8-leaf stage in the greenhouse in previous experiments. Based on the highest score evaluated in the 1-leaf developmental stage excised leaf assay for each accession, 16 accessions were labeled as susceptible. GWAS identified the SNP locus S01_72868925 that is associated with protein kinase domain // Leucine rich repeat N-terminal domain at a level of confidence that surpassed Bonferroni correction. Along with the SNP locus S01_72868925, other top SNP loci were also associated with genes that are known to play critical roles in plant defense or plant stress responses., (© 2022. The Author(s).)

Basal leaf angle distribution was surveyed in twenty-one Johnsongrass cultivars near the end of the vegetative stage. The angles increased from the top to the bottom leaves, and compared to cultivated grain sorghums, the average angle was larger in Johnsongrass. When basal leaf angle distribution data were correlated with pathogenicity test data from excised-leaf assays for three isolates of Colletotrichum sublineola, the results showed a weak positive correlation between basal leaf angle and pathogenicity level in Johnsongrass. In order to investigate a protective role of leaf thickness to C. sublineola, leaf thickness was measured in three sorghum cultivars and one Johnsongrass cultivar at the 8-leaf-stage. Leaf thickness near the apex, near the base, and half-way between the two points were measured in the top four leaves of each plant. Thickness of leaf blade and midrib were recorded separately. Using an excised-leaf-assay, the three points were inoculated with C. sublineola, and pathogenicity level was recorded 4-days-post-inoculation. Results showed strong negative correlations between leaf midrib thickness and pathogenicity level in sorghum and Johnsongrass but not in leaf blades.

Anthracnose, incited by Colletotrichum sublineola, is the most destructive foliar disease of sorghum and, under severe conditions, yield losses can exceed 80% on susceptible cultivars. The hyper-variable nature of the pathogen makes its management challenging despite the occurrence of several resistant sources. In this study, the genetic variability and pathogenicity of 140 isolates of C. sublineola, which were sequenced using restriction site-associated sequencing (RAD-Seq), resulted in 1244 quality SNPs. The genetic relationship based on the SNP data showed low to high genetic diversity based on isolates' origin. Isolates from Georgia and North Carolina were grouped into multiple clusters with some level of genetic relationships to each other. Even though some isolates from Texas formed a cluster, others clustered with isolates from Puerto Rico. The isolates from Puerto Rico showed scattered distribution, indicating the diverse nature of these isolates. A population structure and cluster analysis revealed that the genetic variation was stratified into eight populations and one admixture group. The virulence pattern of 30 sequenced isolates on 18 sorghum differential lines revealed 27 new pathotypes. SC748-5, SC112-14, and Brandes were resistant to all the tested isolates, while BTx623 was susceptible to all. Line TAM428 was susceptible to all the pathotypes, except for pathotype 26. Future use of the 18 differentials employed in this study, which contains cultivars/lines which have been used in the Americas, Asia, and Africa, could allow for better characterization of C. sublineola pathotypes at a global level, thus accelerating the development of sorghum lines with stable resistance to the anthracnose pathogen. [ABSTRACT FROM AUTHOR]

Sorghum is the fifth most important crop globally. Researching interactions between sorghum and fungal pathogens is essential to further elucidate plant defense mechanisms to biotic stress, which allows breeders to employ genetic resistance to disease. A variety of creative and useful inoculation and screening methods have been developed by sorghum pathologists to study major fungal diseases. As inoculation and screening methods can be keys for successfully conducting experiments, it is necessary to summarize the techniques developed by this research community. Among many fungal pathogens of sorghum, here we summarize inoculation and screening methods for five important fungal pathogens of sorghum: Claviceps africana, Colletotrichum sublineola, Sporisorium reilianum, Peronosclerospora sorghi and Macrophomina phaseolina. The methods described within will be useful for researchers who are interested in exploring sorghum-fungal pathogen interactions. Finally, we discuss the latest biotechnologies and methods for studying plant-fungal pathogen interactions and their applicability to sorghum pathology. [ABSTRACT FROM AUTHOR]

Gene expression was analyzed at 0- and 24-h post-inoculation of two inbred sorghum cultivars known to differ in response to inoculation with Colletotrichum sublineola , the fungal pathogen that causes anthracnose. QL3 is reported to have quantitative resistance, while Theis is susceptible to most pathotypes of the pathogen; RNASeq identified over 3,000 specific genes in both cultivars as showing significant changes in expression following inoculation; in all but one gene, the changes in QL3 and Thies were in the same direction. Many other genes showed significant changes in only one of the two cultivars. Overall, more genes were downregulated than upregulated. Differences in changes in expression levels of a few genes suggested potential roles for the difference in disease response between QL3 and Theis, but did not identify known resistance genes. Gene ontology (GO) and pathway enrichment analysis identified upregulation of 23 transcription factor encoding genes as well as genes involved in the production of secondary metabolites, which are part of a typical host defense reaction. [ABSTRACT FROM AUTHOR]

Key message: SNPs identify prospective genes related to response to Colletotrichum sublineola (anthracnose) in the sorghum association panel lines. Sorghum association panel (SAP) lines were scored over several years for response to Colletotrichum sublineola, the causal agent of the disease anthracnose. Known resistant and susceptible lines were included each year to verify successful inoculation. Over 79,000 single-nucleotide polymorphic (SNP) loci from a publicly available genotype by sequencing dataset available for the SAP lines were used with TASSEL association mapping software to identify chromosomal locations associated with differences in disease response. When the top-scoring SNPs were mapped to the published sorghum genome, in each case, the nearest annotated gene has precedence for a role in host defense. [ABSTRACT FROM AUTHOR]

Three isolates of Colletotrichum sublineolum P. Henn were inoculated on leaf blades and midribs of twenty-six johnsongrass cultivars with two sorghum cultivars, BTx 623 and SC 748-5 by using an excised leaf method. Phenotypic host responses from no response to severe infection, were observed. Some johnsongrass cultivars and BTx 623 showed different responses between the two tissues. Expression of six defense response related genes were measured using Real-time qRT-PCR. Leaf blade and midrib showed different patterns in BTx 623 inoculated with C. sublineolum ; high expression of CHS8 and TLP were measured in midrib at 24 and 48 hpi respectively. • Some cultivars appeared to differ in symptoms on leaf blades and midribs in natural infections. • The majority of the 26 cultivars tested showed little difference in the relative scores, but several were significant. • Two johnsongrass and two sorghum inbreds were tested for defense response based on levels of mRNA for 6 genes. • Sorghum BTx623 differed in both symptoms and induction of defense responses in the two tissues. [ABSTRACT FROM AUTHOR]

Four isolates of Colletotrichum sublineola were inoculated on excised leaf blades and rhizomes of nine Johnsongrass accessions and leaf blades of two sorghum accessions, BTx623 (+) and SC748-5 (−). Successful acervuli formations were observed in seven Johnsongrass accession- C. sublineola isolate combinations. Although infection leading to pathogen reproduction was rarely observed, this is the first study that confirmed Johnsongrass rhizomes can be colonized by C. sublineola. • Johnsongrass rhizome tissue could be infected by Colletotrichum sublineola. • Acervulus formation was rare, even in positive cultivar by isolate inoculations. • Johnsongrass cultivar SH1247 was infected by all 4 isolates tested. • Three Johnsongrass cultivars were positive only with one isolate. [ABSTRACT FROM AUTHOR]

The anthracnose pathogen Colletotrichum sublineola and its sorghum host provide an excellent system to examine concepts associated with gene-for-gene resistance. In this study, an excised-leaf spot inoculation assay method was used to evaluate host-pathogen interactions across several sorghum cultivars that serve as host differentials for defining races of the pathogen. Host cultivars tested included sorghum cultivars BTx398, RTx2536, Theis and johnsongrass cultivar SH1152. The cultivars were inoculated at varying conidia concentrations both singly and in pairs with conidia from strains of C. sublineola that differ in ability to incite the disease anthracnose. The hypothesis tested was that host defense triggered by effectors produced by an avirulent strain will limit infection when simultaneously inoculated with a strain that otherwise would lead to disease. Observations revealed significant changes in disease ratings based on conidial concentrations of the C. sublineola isolates used. However, disease rating scores for combined inocula in the excised-leaf spot inoculation assays led to scores either between the scores generated by single isolates or near the score of the more virulent isolate. • Sorghum plants were co-inoculated with strains of C. sublineola expected to cause different disease responses. • It was hypothesized that an avirulent strain will trigger host defense to limit disease by the virulent strain. • Although variations were observed, the excised-leaf spot inoculation assays did not support that hypothesis. • Significant changes in disease ratings were observed based on conidial concentrations of the C. sublineola isolates used. [ABSTRACT FROM AUTHOR]