<i>Fusarium guttiforme</i> and <i>F. mexicanum</i> Causing Shoot Blight in Yerba Mate in Southern Brazil

ABSTRACT Yerba mate ( Ilex paraguariensis [St. Hil.]) is one of the main non‐timber forest products in the southern region of Brazil, playing a role of great social, economic and environmental importance. Commercial yerba mate consists of leaves and young branches of the plant, traditionally consumed as tea and chimarrão. Its extract is also used in food, cosmetics and cleaning products. In 2021 and 2022, shoot blight was observed in seedlings and adult plants of yerba mate in Paraná and Rio Grande do Sul states, Brazil. The disease is characterised by dark‐coloured necrotic lesions, which begin at the tip of the shoots and, in some cases, progress downwards along the shoot, causing its death. The objective of this work was to elucidate the aetiology of this disease. The causal agents of the disease were isolated from symptomatic shoots of yerba mate, and the isolates obtained were characterised using morphology and multilocus molecular phylogeny (RNA polymerase II subunit, elongation factor 1‐ɑ and β‐tubulin genes). Inoculation was carried out on detached leaves and shoots, under controlled conditions and on adult plants in the field. The isolates obtained proved pathogenic when inoculated, causing symptoms similar to those observed in the field. The multilocus molecular phylogeny identified two species: Fusarium guttiforme and F. mexicanum , with F. mexicanum being the most aggressive in pathogenicity tests. Fusarium cultures were reisolated, fulfilling Koch's postulates. This study represents the first report of F. guttiforme and F. mexicanum as causal agents of yerba mate shoot blight.