First report of anthracnose on welsh onion (Allium fistulosum) in Brazil caused by Colletotrichum theobromicola and c. truncatum

Abstract

Welsh onion (Allium fistulosum L.) is cultivated in temperate and subtropical regions worldwide. Brown, necrotic, anthracnose-like lesions that extended the length of Welsh onion leaves were first observed in the village Paraná do Supiá (60°45′34.8″ W; 3°24′17.11″ S), Manacapuru district, in 2008 and in the village Nossa Senhora de Fátima (60°17′35.2″ W; 3°15′51.5″ S), Iranduba district, in 2014. Both villages are situated in Amazonas state, Brazil. Aggregated orange spores and acervuli collected directly from the necrotic lesions were plated on potato dextrose agar (PDA) and incubated at 25°C. Single spore cultures were deposited in the Microorganisms Culture Collection of the National Institute of Amazonian Research, Manaus, Brazil (INPA 1809 and INPA 2743). For morphological characterization, the strains were grown on SNA. The isolate INPA 1809 formed a colony varying from light to dark gray. Microscopically, conidia were cylindrical to subcylindrical, aseptate, measuring (n = 20) 10.8 to 17.4 × 4.0 to 4.6 μm. Appressoria were solitary, dark brown, irregular, sometimes lobate, measuring 6.3 to 13.6 × 4.0 to 5.6 μm. The isolate INPA 2743 formed light gray colonies. Conidia were hyaline, aseptate, slightly curved with tapered tips and truncate bases, measuring (n = 20) 16 to 22 × 2.5 to 3.5 μm. Appressoria were solitary or in groups, light brown, lobate or rounded, measuring from 9.0 to 19.0 × 4.3 to 5.0 μm. Total DNA was extracted and fragments of actin (ACT), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and chitin synthase (CHS) genes were amplified, sequenced, and deposited in GenBank. Accession numbers of sequences from isolate INPA 1809 were KX702276 (ACT), KX702274 (GADPH), and KX702272 (CHS). Likewise, accession numbers deposited from isolate INPA 2743 were KX702275 (ACT), KX702273 (GADPH), and KX702271 (CHS). Bayesian inference analyses were performed with concatenated genes sequences (ACT, GAPDH, and CHS) and showed that the isolate INPA 1809 clustered with the ex-type specimen of Colletotrichum theobromicola (CBS 124945) in a clade with high support (posterior probability = 1) and the isolate INPA 2743 clustered with the ex-epitype specimen of C. truncatum (CBS 151.35) in a clade with high support (posterior probability = 1). Fifteen Welsh onion seedlings were used to verify pathogenicity and to complete Koch’s postulates. Five of the seedlings were sprayed with 106 conidia/ml spore suspension from INPA 1809, another five seedlings were inoculated with 106 conidia/ml of INPA 2743, and five seedlings were sprayed with sterile water. All seedlings were covered with plastic bags, 24 h after inoculation, and kept at 27°C in a greenhouse and a 12-h photoperiod. Typical symptoms of anthracnose (brown necrotic lesions) were observed 5 days after inoculation with both inoculated strains. The fungal species were recovered from symptomatic plants, while no symptoms were observed on the controls. Anthracnose on Welsh onion caused C. circinans was reported from Korea (Kim et al. 2008). In Brazil, C. spaethianum was reported causing the same disease on Welsh onion in a vegetable garden in Manaus, Amazonas (Santana et al. 2016). To our knowledge, this is the first report of C. theobromicola and C. truncatum causing anthracnose on Welsh onion in Brazil. The identification of these species will contribute to the adoption of correct measures of control and reduction of anthracnose losses in Welsh onion. © 2017, American Phytopathological Society. All rights reserved.