The potential of using Metarhizium anisopliae and Metarhizium humberi to control the Asian soybean rust caused by Phakopsora pachyrhizi

Phakopsora pachyrhizi causes Asian soybean rust, resulting in severe losses of soybean yield. This study assessed the biocontrol efficacy of cell-free culture filtrates and conidial suspensions of Metarhizium anisopliae (MABR-01) and M. humberi (MHBR-03) against P. pachyrhizi in vitro and in planta. A 50% concentration of culture filtrates of M. anisopliae and M. humberi inhibited the germination of P. pachyrhizi urediniospores by 85 and 96%, respectively, compared to the 50% potato-dextrose broth (PDB) control. The conidial suspensions caused no inhibition of P. pachyrhizi in vitro, but the conidial suspension of M. anisopliae controlled the rust disease in plants with a 51% efficiency. Soybean plants colonised by M. anisopliae MABR-01 and exposed to P. pachyrhizi showed less severe disease in the shoots compared to plants not colonised by M. anisopliae; the disease was reduced by 40%. Spraying plants with 50 and 75% culture filtrates of M. humberi and a 75% concentration of M. anisopliae significantly reduced rust disease, by an average of 86%. Plants pretreated with Metarhizium culture filtrates showed a 30% reduction in P. pachyrhizi colonisation based on qPCR quantification of the pathogen genomic DNA. These results indicate a high biotechnological and sustainable potential of these two Metarhizium species for control of Asian soybean rust.