Survival of Colletotrichum truncatum as Microsclerotia in Soil

Anthracnose has become one of the main threats to soybean production and is considered the most important disease in some soybean-producing areas. Colletotrichum truncatum is the species most commonly associated with anthracnose and produces microsclerotia. However, the role of microsclerotia in the epidemiology of soybean anthracnose disease has not yet been described. The aim of this study was to determine whether C. truncatum microsclerotia can survive and maintain pathogenicity for a period of up to 246 days, corresponding to the off-season period of soybean cultivation in Brazil. Therefore, microsclerotia of two pathogenic isolates of C. truncatum (CMES1059 and LFN0297) were produced and placed in polyester bags, which were kept under field conditions either on the soil surface under maize straw or buried 8-cm deep. The bags were collected monthly for a period of up to 246 days to assess the viability of microsclerotia based on their germination and typical colony growth. The logistic regression model was used for data analysis considering viable and nonviable microsclerotia. In addition, periodic sowing of soybean was done in the soil infested with LFN0297 microsclerotia to test pathogenicity up to 246 days after soil infestation. C. truncatum microsclerotia survived from 92 to 246 days in the field soil, with the highest recovery of viable microsclerotia at 153 days. C. truncatum was reisolated from soybean plants sown in infested soil at 245 days postinoculation. The isolates from the last microsclerotia sampling from the field (246 days) and those obtained from a plant at the last sowing date (245 days) had the same genotypic profile for 12 microsatellite loci as the isolates used to perform the experiments. C. truncatum microsclerotia in soil may serve as the primary inoculum for soybean anthracnose.

Identification and Comparison of Colletotrichum Secreted Effector Candidates Reveal Two Independent Lineages Pathogenic to Soybean

Colletotrichum is one of the most important plant pathogenic genus of fungi due to its scientific and economic impact. A wide range of hosts can be infected by Colletotrichum spp., which causes losses in crops of major importance worldwide, such as soybean. Soybean anthracnose is mainly caused by C. truncatum, but other species have been identified at an increasing rate during the last decade, becoming one of the most important limiting factors to soybean production in several regions. To gain a better understanding of the evolutionary origin of soybean anthracnose, we compared the repertoire of effector candidates of four Colletotrichum species pathogenic to soybean and eight species not pathogenic. Our results show that the four species infecting soybean belong to two lineages and do not share any effector candidates. These results strongly suggest that two Colletotrichum lineages have acquired the capability to infect soybean independently. This study also provides, for each lineage, a set of candidate effectors encoding genes that may have important roles in pathogenicity towards soybean offering a new resource useful for further research on soybean anthracnose management.

Soybean anthracnose caused by Colletotrichum species: Current status and future prospects

Soybean (Glycine max) is one of the most important cultivated plants worldwide as a source of protein-rich foods and animal feeds. Anthracnose, caused by different lineages of the hemibiotrophic fungus Colletotrichum, is one of the main limiting factors to soybean production. Losses due to anthracnose have been neglected, but their impact may threaten up to 50% of the grain production.

TAXONOMY

While C. truncatum is considered the main species associated with soybean anthracnose, recently other species have been reported as pathogenic on this host. Until now, it has not been clear whether the association of new Colletotrichum species with the disease is related to emerging species or whether it is due to the undergoing changes in the taxonomy of the genus.

DISEASE SYMPTOMS

Typical anthracnose symptoms are pre- and postemergence damping-off; dark, depressed, and irregular spots on cotyledons, stems, petioles, and pods; and necrotic laminar veins on leaves that can result in premature defoliation. Symptoms may evolve to pod rot, immature opening of pods, and premature germination of grains.

CHALLENGES

As accurate species identification of the causal agent is decisive for disease control and prevention, in this work we review the taxonomic designation of Colletotrichum isolated from soybean to understand which lineages are pathogenic on this host. We also present a comprehensive literature review of soybean anthracnose, focusing on distribution, symptomatology, epidemiology, disease management, identification, and diagnosis. We consider the knowledge emerging from population studies and comparative genomics of Colletotrichum spp. associated with soybean providing future perspectives in the identification of molecular factors involved in the pathogenicity process.

USEFUL WEBSITE

Updates on Colletotrichum can be found at http://www.colletotrichum.org/. All available Colletotrichum genomes on GenBank can be viewed at http://www.colletotrichum.org/genomics/.