Alternaria species associated with cladode brown spot in cactus prickly pear (Nopalea cochenillifera)

Abstract

Brown spot is the most important disease for the culture of cactus prickly pear (Nopalea cochenillifera (L) Salm-Dyck; Cactaceae family), and in many countries, Alternaria spp. are commonly associated with the disease. In Brazil, only three species of Alternaria are associated with brown spot: A. alternata, A. tenuissima, and A. longipes. However, other Alternaria spp. may be involved in the etiology of the disease. This study aimed to identify Alternaria species associated with prickly pear brown spot in Northeast Brazil. Cladodes presenting symptoms of brown spot were collected from three counties in the state of Alagoas and three in the state of Pernambuco. Isolation, obtaining monosporic cultures, and pathogenicity tests were performed. For correct taxonomic positioning of the isolates, morphological and molecular analyses were performed using the internal transcribed spacer (ITS) region and the translation elongation factor 1-alpha (tef1), Alternaria major allergen (Alt a1), glyceraldehyde-3-phosphate dehydrogenase (gapdh), and second largest subunit of RNA polymerase (rpb2). Fourteen pathogenic isolates were obtained, and according to the morphological and molecular analyses, three species of Alternaria were identified: A. gossypina, A. jacinthicola, and A. tomato, all belonging to the Alternata section. A. gossypina, A. jacinthicola, and A. tomato are described for the first time in Nopalea cochenillifera worldwide.