Understanding the Sorghum–Colletotrichum sublineola Interactions for Enhanced Host Resistance

Improving sorghum resistance is a sustainable method to reduce yield losses due to anthracnose, a devastating disease caused by Colletotrichum sublineola. Elucidating the molecular mechanisms of sorghum–C. sublineola interactions would help identify biomarkers for rapid and efficient identification of novel sources for host-plant resistance improvement, understanding the pathogen virulence, and facilitating resistance breeding. Despite concerted efforts to identify resistance sources, the knowledge about sorghum–anthracnose interactions remains scanty. Hence, in this review, we presented an overview of the current knowledge on the mechanisms of sorghum-C. sublineola molecular interactions, sources of resistance for sorghum breeding, quantitative trait loci (QTL), and major (R-) resistance gene sequences as well as defense-related genes associated with anthracnose resistance. We summarized current knowledge about C. sublineola populations and its virulence. Illustration of the sorghum-C. sublineola interaction model based on the current understanding is also provided. We highlighted the importance of genomic resources of both organisms for integrated omics research to unravel the key molecular components underpinning compatible and incompatible sorghum–anthracnose interactions. Furthermore, sorghum-breeding strategy employing rapid sorghum germplasm screening, systems biology, and molecular tools is presented.

First Report of Colletotrichum sublineola causing Anthracnose on Sorghum bicolor in South Korea

Sorghum (Sorghum bicolor (L.) Moench) is one of the top five cereal crops in the world, but the cultivation area in Korea is estimated to be about 3,000 ha (MIFFAF, 2012). In August 2014, anthracnose symptoms on sorghum leaves were observed in two fields in Yecheon (36.62°, 128.41°) and Youngwol (37.20°, 128.49°), South Korea. Symptoms on leaves were brownish red irregular lesions with yellow and tan borders. Some darkened conidiomata and setae were observed on the lesions of infected leaves. Approximately 20% of sorghum plants (cv. Hwanggeumchal) were affected in each field with an area of about 0.1 ha. Fragments of diseased infected leaves were surface sterilized with 1% NaOCl for 30sec. The pieces were placed on water agar and incubated at 25°C for 7days. Two isolates were obtained through single sporing and cultured on synthetic nutrient poor agar at 25°C for 14days. Conidia (n=30) of YN1458 isolate were falcate and measured 22.0 to 32.7 × 4.2 to 6.4 µm. Brown to black setae (n=20) had 1-3 septa, with tapering acute apices and 53.7 to 95.2 × 4.7 to 7.8 µm in size. Appressoria (n=30) were dark brown, usually irregular and 10.5 to 16.9 × 8.6 to 13.6 µm in size. Colonies on PDA produced salmon spore masses in the center of the colony, and whitish grey to dark color in reverse. The morphological characteristics of two isolates were similar. Based on morphology, two isolates were tentatively identified as Colletotrichum graminicola species complex (Cannon et al. 2012; Crouch and Tomaso-Peterson 2012). To clarify taxonomic placement, DNA extracted from mycelia of the two isolates was PCR amplified and sequenced targeting internal transcribed spacer (ITS) regions of rDNA, actin (ACT), chitin synthase 1(CHS-1), and beta-tubulin (TUB) genes (Weir et al. 2012). The sequences of the above four loci of YN1458 and YN1728 were deposited in GenBank with accession numbers KT351801, KT351802 (ITS); KY769869, KY69870 (ACT); KY769871, KY769872 (CHS-1); and KY769873, KY769874 (TUB), respectively. The sequencing results of two isolates showed 99.6% (ITS), 99.6% (ACT of YN1458), 100% (ACT of YN1728), 100% (CHS-1), 100% (TUB of YN1458) and 99.8% (TUB of YN1728) similarity with C. sublineola CBS 131301 (JQ005771, JQ005834, JQ005792, and JQ005855) by BLASTn. Based on the morphological characteristics and multigene sequence analysis, the two isolates were identified as C. sublineola. Pathogenicity of two isolates was confirmed by spraying conidial suspensions (106 conidia/mL) on leaves of 3-week-old sorghum seedlings (cv. Hwanggeumchal) using a pot assay (5 plants per isolate). The same number of seedlings were sprayed with sterile distilled water and served as controls. All plants were maintained in a greenhouse at 25/32°C with natural light. After one week, symptoms similar to those in the field were observed on the leaves inoculated with the pathogen, but not on the control leaves. Colletotrichum sublineola was consistently re-isolated from the inoculated leaves showing anthracnose symptoms and the pathogen identity was confirmed by observing morphological characteristics. So far, C. graminicola was known as the only causal agent pathogen of sorghum anthracnose in South Korea (KSPP, 2009). To our knowledge, this is the first report of C. sublineola causing anthracnose on sorghum in South Korea. Although sorghum is a small-scale crop in South Korea, it is necessary to study the biological and pathogenic characteristics of C. sublineola for effective control of sorghum anthracnose.

Response of the Set of Anthracnose Differentials to other Foliar and Panicle Diseases in Niger

This study was conducted to determine the reactions of 19 sorghum lines, of which 18 had been used for Colletotrichum sublineola pathotype determination to other foliar and panicle diseases in two agroecological zones in Niger. The anthracnose resistant check SC748-5 was infected with leaf blight, oval leaf spot, and zonate leaf spot but free of long smut, rough leaf spot, and target leaf spot. BTx623 and TAM428 which are susceptible to anthracnose were infected with leaf blight, long smut, oval leaf spot and zonate leaf spot. Across locations, all the lines tested were infected with leaf blight, caused by Exserohilum turcicum. PI570726, an accession from Sudan was infected with only leaf blight but free of all the other diseases observed in both locations. This work showed that some of the sorghum anthracnose differentials, especially PI570726 may possess genes for resistance to multiple sorghum diseases and can be utilized as parents in breeding programs in Niger.

Response to Anthracnose and Germination Rate of Colletotrichum Sublineola Acervuli of Greenhouse-Grown Sorghum

Colletotrichum sublineola, the causal agent of sorghum anthracnose, infects all above ground parts of the crop. The most pronounced phase of the disease is its foliar phase. In this study, 10 sorghum lines with checks were evaluated in the greenhouse for resistance against C. sublineola. Acervuli germination rate within infected leaves was also recorded. All the 10 sorghum lines along with checks BTX623, TAM428, and PI609251 were susceptible and as expected, SC748 was resistant. Variation among the lines for acervuli germination rate was observed; TAM428 and 1110248 recorded the highest percentage (98.3%) while PI609251 exhibited the lowest rate of acervuli germination (33.3%). Conidia produced from germinating acervuli are critical to the distribution and spread of the disease. However, conidia produced within the acervuli do not usually germinate due to the presence of self-inhibitor compounds. Thus, these self-inhibitors that may occur in the acervuli could explain the difference in levels of susceptibility among sorghum germplasm.

Effectiveness of Fungicides and Their Application Timing for the Management of Sorghum Foliar Anthracnose in the Mid-Atlantic United States

Sorghum anthracnose (Colletotrichum sublineola) reduces grain yield up to 50% but suggested management tactics have not yet been developed for the mid-Atlantic United States, where warm, wet conditions favor disease. Under factorial arrangement, five fungicides plus a nontreated control and four application timings were compared for foliar anthracnose control, yield, and profitability of fungicide use in grain sorghum over eight site-years in Virginia and North Carolina. Anthracnose severity was rated at the hard dough stage, and grain yield was determined at harvest. Every percent increase in disease severity resulted in yield losses of 27 to 85 kg/ha. Pyraclostrobin and pyraclostrobin plus fluxapyroxad reduced anthracnose (P < 0.01), and three applications resulted in less disease and greater yield compared with single applications (P < 0.01). However, three applications exceed the labeled maximum application for the fungicides and are not economical. Among single applications, boot or flowering timings reduced disease, and flowering applications resulted in the overall greatest yield. Results suggest that when disease onset occurs at or prior to boot, a single application of pyraclostrobin-containing fungicide at or just prior to flowering reduces anthracnose, protects yield, and increases income. However, when disease is absent or severity is low prior to flowering, fungicide application may not be profitable.