Differences Among Fungal Microbiomes Associated with Tar Spot of Corn in Ecuador, Guatemala, and Indiana, USA

Tar spot of corn, caused by the obligate fungal pathogen Phyllachora maydis, was previously found to be associated with Monographella maydis (syn. Microdochium maydis) and Coniothyrium phyllachorae in Mexico. While M. maydis was once believed to cause fisheye lesions and C. phyllachorae was described as a mycoparasite of P. maydis, their presence and roles in tar spot remain uncertain. This study hypothesizes that the tar spot-associated fungal microbiome varies geographically and includes fungal species with inconsistent associations with the disease. To test this, tar spot-infected leaves were collected in Guatemala, Ecuador, and the USA. This study shows that the distribution and abundance of these fungi exhibit a geographical pattern and are not consistently associated with tar spot lesions in all locations. We confirmed the presence of Microdochium in some parts of Ecuador and Guatemala but not in the USA. The role of Microdochium remains unclear and whether it can be a primary pathogen needs to be tested by additional research, as its occurrence and abundance depend more on geographical locations rather than the presence of fisheye lesions. Coniothyrium was detected at low abundance in samples from Ecuador and Guatemala but was absent in the USA. Additionally, we identified the likely mycoparasite Paraphaeosphaeria as highly prevalent across samples from all three countries. Microbiome diversity analysis grouped tar spot samples into four distinct clusters: two dominated by P. maydis, one primarily associated with Microdochium, and a fourth characterized by a combination of P. maydis and Paraphaeosphaeria.