Bisifusarium lunatum causing cladode soft rot in pear cactus (Nopalea cochenillifera L. Salm-Dyck) in Brazil.

ABSTRACT

Cactus pear var. miúda (Nopalea cochenillifera L. Salm-Dyck) is an important crop for the Northeast region of Brazil, composing one of the main sources of animal feed. By April 2021, cladode rot caused death of several cactus pear plants in a production area located in Itaporanga, Paraíba state, Brazil (7°21'55.35" S and 38°11'38.68" W). The infected cladodes showed brown circular necrotic spots, and soft rot with perforations that extended throughout the cladode, followed by tipping over and death of the infected plants. The incidence of the disease ranged from 10 to 30% of the plants. Bisifusarium strains were isolated and cultured on potato dextrose agar (PDA) and syntetic-nutrient-poor-agar (SNA). The colonies showed purple color on PDA. On SNA, macroconidia (n = 100) were abundant, hyaline, slightly falcate, three-septate, measuring 11.0-23.1 x 2.3-4.1 µm. Microconidia (n = 100) were oval, generally aseptate, measuring 4.1-8.7 x 2.3-3.0 µm. Conidiogenic cells formed into short monophialides. Chlamydospores were not observed. According to these morphological features, the pathogen was initially identified as Bisifusarium lunatum (Gryzenhoutm et al. 2017). For further confirmation of the identification, the partial sequences of translation elongation factor 1-alpha (TEF1-α) and the second largest subunit of RNA polymerase II (RPB2) genes were sequenced for a representative isolate (CMA 34 GenBank accession no TEF1-α OR536502; and RPB2 OR553509) and compared to other Bisifusarium species from GenBank database. Subsequently, it was subjected to a phylogenetic analysis of maximum likelihood including previously published sequences. According to BLAST searches, the TEF1-α and RPB2 sequences were 99% (637/640 nt) and 100% (312/312 nt) similar to B. lunatum (COUFAL0213 TEF1-α (MK640219), and RPB2 (MK301291)), respectively. The isolate was also clustered in a clade containing the ex-type of B. lunatum with 100% support (SH-aLRT and UFboot), being confidently assigned to this species. The pathogenicity test was performed after Medeiros et al. (2015), by using healthy two months old cactus pear seedlings (n = 10) cultivated in a greenhouse. Sterile toothpicks were distributed over colonies of the representative isolate grown on PDA at 25 ± 2 °C for seven days. Seedling cladodes were stuck with the toothpicks, moistened with sterile water and covered with transparent plastic bags for 24h, thus simulating a humid chamber. Following three months, all control plants (stuck with sterile toothpicks) remained healthy, while those inoculated with the representative isolate exhibited rot symptoms. This test was performed twice. B. lunatum was reisolated from symptomatic cladodes and identified as previously described, thus fulfilling the Koch's postulates. To our best knowledge, this is the first report of B. lunatum causing soft rot on N. cochenillifera in Brazil. Besides N. cochenillifera, this species was also reported on Opuntia ficus-indica in India (Gryzenhoutm et al., 2017), which raises concern regarding its ability to infect other forage sources for cattle feed in Brazilian semiarid regions. The present study highlights that the precise identification of B. lunatum is a key factor to adjust control strategies and management of the disease to prevent the spread of this disease to prevent its spread to other crops.