Characterization of Pythium isolates causing leaf rot of Basella alba in Brazil

Basella alba L. (Basellaceae family) belongs to a group of vegetable crops used in Brazilian regional cuisine. Herein, samples were collected from B. alba plants exhibiting symptoms of leaf rot in two fields in Central Brazil. Colonies with typical characteristics of Pythium species were obtained on selective medium from the field-collected leaf lesions. PCR amplification and sequencing of the Cox2 gene region were carried out aiming to refine the identification of the pathogen(s). Phylogenetic relationships among the isolates were assessed using reference isolates available in the GenBank. Eight isolates were obtained from leaf lesions, with seven identified as P. ultimum, one as P. aphanidermatum. The genomic region encompassing the Cox2 gene proved to be informative, enabling discrimination and identification of these Pythium species. Pathogenicity tests were carried under greenhouse conditions by inoculating B. alba leaves with mycelial discs of five Pythium isolates with and without injury. Detached B. alba leaves were also inoculated under laboratory conditions with the same five Pythium isolates. Lesion diameter was used for the evaluation of pathogenicity. P. aphanidermatum (Pyt 915 isolate) was the only statistically different from the rest of the the isolates (p = 0.05) as it induced smaller lesions in assays without injury. Therefore, there are unequivocal evidences of the infection of B. alba by a complex of Phytium species, with P. ultimum var. ultimum being the most prevalent one. To our knowledge, it is the first worldwide report of Pythium species causing leaf rot in B. alba, which may have relevant epidemiological role as an additional host of these two polyphagous soil-borne pathogens.