The secret life of the Pyricularia fungi in the rhizosphere

Blast diseases caused by Pyricularia spp. are well-known as foliar diseases. Here, we found that Pyricularia grisea can infect the roots of crabgrass (Digitaria spp.) under natural conditions. Two strains, DRIS3-1 and DRIS3-4, were isolated from the roots. DRIS3-1 and DRIS3-4 had polymorphisms in their rDNA-internal transcribed spacer sequences, indicating genetic diversity in the root-infecting P. grisea. Amplicon sequencing revealed the presence of P. grisea in the roots of all the examined crabgrass plants. Interestingly, different haplotypes of P. grisea were detected in the leaves and roots of individual plants. When healthy crabgrass and rice were planted in the field, indigenous P. grisea was detected in both the rhizospheres. In the leaf inoculation experiment, the inoculated fungi did not reach the roots. However, when crabgrass seeds were contaminated with P. grisea, fungal DNA was detected in the roots. Furthermore, we observed transmission of the inoculated fungus to nearby healthy crabgrass roots. These findings indicated that P. grisea has a hidden underground infection cycle.