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Thielaviopsis sp. as the causal agent of Black Rot of Ilex paraguariensis in Argentina.

Myrian Rybak: Instituto Nacional de Tecnología Agropecuaria

<div>In 2014, high plant mortality was observed in the main commercial nursery of yerba mate, <em>Ilex paraguariensis</em> (Hill), located northwest of the Corrientes province of Argentina. Symptoms were observed as damaged, necrotic juvenile roots and a conspicuous dieback of twigs of younger plants, which later died by the thousands. In 2017, the same symptoms were observed on adult trees in which, in addition to the branches dieback, mature leaves showed dark black apical necrosis. Microscopically, a very thin, semi-transparent mycelium was observed. The septate hyphae were hyaline at first and difficult to observe. Two types of conidia, endospores and chlamydospores, were observed. Chlamydospores were produced in chains, two to eight spores per chain which separated at maturity and measured 25-60 X 8-11 µm. Endoconidia were more abundant than chlamydospores. Endoconidia had rounded ends, varied in size, but were smaller than chlamydospores. Based on morphological characteristics and morphometrics, the fungus was identified as <em>Thielaviopsis </em>sp<em>.</em> To obtain pure cultures, chlamydospores were germinated on PDA. Pathogenicity tests conducted on young healthy plants produced the same symptoms as in the field. The same fungus was reisolated from diseased tissue, thus fulfilling Koch’s postulates. Molecular characterization will assist in identifying this pathogen to the species level.</div>