Mixed fungal strains challenge host resistance: insights into Magnaporthiopsis maydis pathogenicity in maize

Maize late wilt disease, caused by the fungus <i>Magnaporthiopsis maydis</i>, poses a significant threat to susceptible crops. Despite efforts to control it through resistant maize varieties, virulent fungal strains might overcome immunity. This study assessed Israeli <i>M. maydis</i> strains with weak, moderate, and highly pathogenic degrees in two open-air pot trials. Even weak pathogenic strains harmed susceptible cultivars (17% growth suppression and 33% death). In contrast, resistant cultivars were minimally affected, except when exposed to a highly aggressive isolate, resulting in a 5% growth suppression and 11% mortality at harvest. Unexpectedly, in a susceptible cultivar during sprouting, a mixed inoculum with the two more virulent isolates resulted in reduced disease (15%) compared to the highly aggressive strain alone (33%). At harvest (day 84), this pattern was reversed, and adding a weak virulent strain to this combination led to more severe growth (33%) and health (71%) disruption, accompanied by a higher level of <i>M. maydis</i> infection (371% compared to the aggressive strain alone). Similar interactions were found in other strain groups tested. Additionally, some subspecies groups specialize in growth suppression, while others in wilting, suggesting biotrophic/necrotrophic variations. The study revealed complex interactions in mixed populations, emphasizing the destructive potential of the pathogen to resistant cultivars. Understanding the role of maize age-related immunity in disease generation uncovers risks associated with this pathogen.