First Report of Ceratocystis fimbriata Causing Black Rot on Arracacia xanthorrhiza in Brazil

Abstract

Peruvian carrot (Arracacia xanthorrhiza Bancroft) is a starchy root crop used for human consumption in Brazil (Henz 2002). In June 2014, in the city of Poço Fundo in Minas Gerais (southwest Brazil), we observed plants of A. xanthorrhiza showing yellowing symptoms in the older leaves, followed by wilting and death of the plant. Black rot was observed in the roots as well as in propagation material (corms), from which we detected the presence of perithecia, typical of Ceratocystis spp. Thirty-one samples were collected. The symptoms were detected in a single, 5-ha plantation (random distribution) of a local grower. The fungus was grown on malt extract agar (MEA). Fifteen isolates were obtained. The perithecia taken from colonies were dark brown to black, globose, with base measuring 105 to 180 × 103 to 179 µm. The perithecial neck was long and exhibited the same color as the base, 269 to 643 × 18 to 28 µm at the base and 12 to 17 µm width at the apex, with ostiolar hyphae light brown to hyaline and 23 to 47 µm long. The ascospores were hyaline, hat shaped, 1.9 to 3.5 × 3.1 to 4.6 µm. The flask-shaped endoconidiophores were hyaline or pale brown, septate, 2.2 to 4.1 µm wide at the base, 2 to 5 µm wide at the widest point, and 33.6 to 159 µm long, producing chains of cylindrical endoconidia. Cylindrical endoconidia were hyaline and 6.4 to 18.6 × 2 to 4 µm. The endoconidiophores producing doliform endoconidia (barrel-shaped) were not observed on MEA. Barrel-shaped endoconidia were hyaline, 13.9 to 19.4 × 6.5 to 8.4 µm. The aleurioconidia were brown, ovoid to pear-shaped, produced singly or in chains, 8.3 to 13.9 × 6.8 to 11.2 µm. One of 15 isolates was used for molecular characterization, deposited as CML 3304 in the Mycological Collection of the Federal University of Lavras. After DNA extraction, amplification and sequencing of the ITS (internal transcribed spacer) region was conducted using the primers ITS1 and ITS4 (White et al. 1990). The generated sequence had 559 bp and was deposited in GenBank (Accession No. KP892530). A nucleotide BLAST search was conducted to assess the similarity with other sequences deposited in GenBank, and the results showed homology (99%) with C. fimbriata (Accession No. KJ439342). Thus, based on morphological and molecular analysis, the fungus was identified as belonging to C. fimbriata species complex. To test pathogenicity, plants of A. xanthorrhiza were inoculated with a spore suspension of 2.5 × 106 conidia/ml. Ten plants were inoculated by wounding the stem base and subsequently depositing 500 µl of spore suspension. As a control, 500 µl of sterile distilled water was deposited on stem base of 10 plants. After 60 days in a greenhouse, the fungus was able to cause dark discoloration at the stem base and in the roots. The pathogen was recovered from the infected tissues, completing the Koch’s postulates. There are records of C. fimbriata causing black rot on roots of Ipomoea batatas and Colocasia esculenta (Clark and Moyer 1988; Harrington et al. 2005), however, this is the first report of C. fimbriata causing black rot on A. xanthorrhiza.