Fusarium pseudocircinatum causing stunting and malformation of sunflower plants in Brazil.

ABSTRACT

Sunflower (Helianthus annuus L.) is among the main oleaginous crops used in Brazil. During January, 2017, at CCA/UFPB laboratory and greenhouses (Areia/Brazil, 6°58'12″ S; 35°42'15″ W), we observed various sunflower seeds (cultivar Olisun 3, 2017-2018 crop) highly infested with Fusarium. Those seeds were from crops in the municipality of Alagoinha -PB/Brazil (06º57'00'' S; 35º32'42'' W), supplied by Empresa Brasileira de Pesquisa Agropecuária/EMBRAPA. The emerged seedlings from these seeds were also contaminated, with 5% to 26% of them exhibiting stunting and malformation. Fusarium strains were isolated from symptomatic plants, and a single spore was used to grow pure colonies on potato-dextrose-agar (PDA) and synthetic-nutrient-poor-agar (SNA) media. Mycelia of PDA colonies were floccous and dense varying from yellow to orange. Fungal colonies developed aerial mycelium, producing orange pigments. On SNA, hyaline macroconidia, measuring 2.9-4.1 x 32.4-65.0 µm, slightly falcate with three to six septa. Oval microconidia, measuring 2.4-3.6 x 5.1-9.0 µm, were abundant in false heads forming on monophyalides. Chlamydospores were absent. Sterile hyphae were rarely formed. Colectively, the morphological features corresponded to species that belong to the Fusarium fujikuroi species complex (Leslie & Summerell, 2006). To assure the species identity, we sequenced the elongation factor 1α region of two representative isolates (i.e., F2 and F3, GenBank access numbers MZ666934 and MZ666935, respectively) and compared them to the other Fusarium species found at Fusarium-ID and GenBank databases. Subsequently, we performed a maximum likelihood phylogenetic analysis including previously published sequences (Nicolli et al., 2020). Both isolates exhibited 100% similarity with Fusarium pseudocircinatum (MN386745), and clustered with its ex-type at 100% bootstrap values. The isolates were then grown on PDA amended with manitol to adjust the osmotic pressure to -1.0 Mpa, at 25 ± 2 ° C, for seven days (Sousa et al., 2008). A total of 100 disinfested sunflower seeds (cultivar Olisun 3, 2018-2019 crop) were distributed over the colonies and 48h later they were sown on sterile substrate maintained inside a greenhouse. About 30 days after inoculation, the emerged plants exhibited symptoms of stunting and malformation (60%) compared to controls, which were healthy. F. pseudocircinatum was reisolated from the symptomatic plants, completing Koch's postulates and identified based on above morphological and molecular biological methods. This test was performed twice. Fusarium pseudocircinatum is a broadly distributed and ecologicaly diverse species that infects several wild and cultivated plants. For instance, it was reported on seeds of the wild 'Peroba Rosa' (Aspidosperma polyneuron Muell. Arg.) in Brazil (Mazarotto et al. 2020). Infection of sunflowers may cause plant stand failures, thus resulting in yield and economic losses for Brazilian growers. The correct identification of any pathogen, especialy a generalist one such as F. pseudocircinatum, is crucial to develop eficient management strategies. To our best knowledge, this is the first report of F. pseudocircinatum causing stunting and malformation of sunflower plants in Brazil.