First Report of Coriander (Coriandrum sativum) Seedling Damping-Off Caused by Pythium irregulare in Brazil

Abstract

Coriander (Coriandrum sativum) is among the most important herbs cultivated in Northeastern Brazil. Root diseases are considered the main limiting factor for coriander production, drastically reducing the number of plants in affected areas and causing great economic losses (Noronha et al. 2015). In May 2015, soil samples from coriander-growing areas displaying seedling damping-off were collected in the county of Arapiraca, Alagoas State, Brazil. This disease affected between 90 and 100% of the plants in the field. Symptoms on coriander initially appeared as yellowing and wilt, and plants quickly declined owing to foot and root rot. To isolate the putative etiologic agent from soil samples, coriander seedlings were used as baits. A typical Pythium sp. isolate was obtained from symptomatic seedling tissues on potato dextrose agar (PDA), and the Pythium-derived culture was deposited at the Colecao de Culturas de Fungos Fitopatogenicos of the Universidade Federal de Alagoas (code COUFAL1986). The colony on PDA was cottony and white, with a growth rate of 22.8 mm/day at 25°C. Sporangia were globose and nonpapillate, 10.9 to 22.5 μm in diameter; spherical oogonia, 9.2 to 14.5 μm in diameter, being observed on one or two antheridia per oogonium (Van der Plaats-Niterink 1981). To confirm identification, partial nucleotide sequences of the internal transcribed spacer (ITS) and cytochrome c oxidase subunit 2 (Cox2) were obtained and deposited in GenBank (accession nos. KX869910 and KX869911). The ITS and Cox2 sequences showed 99% identity with sequences from Pythium irregulare culture collection CBS 250.28. Furthermore, the COUFAL1986 isolate grouped with other P. irregulare in a Bayesian phylogenetic tree inferred from nucleotide concatenated data (ITS and Cox2; S19898 deposited in TreeBASE). The pathogenicity of the COUFAL1986 isolate was confirmed on coriander plants (cultivated in a greenhouse) from the Verdao variety. Coriander seeds were sown (1-cm depth) in five 400 mL plastic pots (20 seeds/pot) containing soil (previously sterilized) infested with a mycelial suspension taken from a 7-day-old culture (grown on PDA at 25°C) and kept at 25 ± 2°C and a photoperiod of 12 h for 10 days. The control (three pots) consisted of sterilized distilled water. Seedlings showed wilt, yellowing, and foot and root rot 4 days after emergence. To fulfill Koch’s postulates, P. irregulare was successfully reisolated from symptomatic plants and molecularly identified. No disease symptoms were observed in the control treatment. Pythium spp. have been reported inducing coriander seedling damping-off in India (P. irregulare; Misra and Hall 1997), Italy (P. ultimum; Garibaldi et al. 2010), and Puerto Rico (P. dissotocum; Romero et al. 2012). To our knowledge, this is the first report of P. irregulare causing coriander seedling damping-off in Brazil.