Syagrus oleracea (Mart.) Becc. (bitter coconut), a palm tree species that is native to central Brazil, has been increasingly cultivated in this country for heart-of-palm production. Epidemics of a necrotic leaf spot of unknown etiology have been recorded on bitter coconut plants in transplant nurseries and plantation since 2008. The first symptoms appear as small, yellow, hydrotic flecks on young or mature leaves that evolve to necrotic brown streaks that run parallel to the leaf veins. Usually, yellow halos occur around the lesions and hydrosis is common during lesion expansion. Necrotic lesions can reach up to 40 mm in length and 10 mm in width, and the lesions often coalesce, causing extensive tissue damage. During a survey in a 3-year-old bitter coconut plantation in Maringá County (coordinates: 23°23′51.25″ S, 51°57′02.09″ W; elevation: 507 m) in the state of Parana, a dozen symptomatic leaves were collected with the aim of elucidating the etiology of this disease. Conidia and conidiophores typical of Cladosporium were frequently observed on the diseased leaf tissue under natural field conditions as well on the surfaces of disinfected leaf tissues kept in a humid chamber for 48 h at 25 ± 2°C with a 12-h photoperiod. Five monoconidial cultures growing on potato dextrose agar (PDA) medium were obtained from different leaves showing leaf spot symptoms. The cultures were grown on PDA to induce sporulation. At 7 days after incubation at 25 ± 2°C and a 12-h photoperiod, gray to gray-olive colonies were observed. The conidiophores were macronematous, erect, oblong, branched, 1 to 5 septate, and 75.0 to 120.0 × 1.90 to 3.20 μm. The ramoconidia were cylindrical or oblong, 0 to 2 septate, and 28.0 to 40.0 × 2.8 to 3.6 μm, with a truncate base of 1.9 to 2.2 μm; secondary ramoconidia were cylindrical or oblong, 0 to 2 septate, 8.0 to 31.0 × 2.2 to 3.1 μm, with 3 to 5 distal conidial hila; intercalary 1-septate conidia were 5.5 to 17.0 × 2.1 to 3.4 μm, with 1 to 3 distal conidial hila; terminal 1-septate conidia were catenulate and 2.2 to 4.2 × 1.8 to 3.1 μm. Species identification was performed based on morphology and DNA sequence data (1). Portions of the elongation factor 1α (551 bp; TEF) and actin (213 bp; ACT) genes were amplified by PCR. A BLAST search of the GenBank database revealed that the TEF (KC484658 to KC484662) and ACT (KC484663 to KC484667) sequence fragments from isolates Gua1, Gua2, Gua3, Gua4, and Gua5 had 100% identity with the accessions HM148616 and HM148371 of Cladosporium perangustum (1). Isolates were tested for pathogenicity against bitter coconut. Ten potted plants with 4 to 6 fully expanded leaves were inoculated with each isolate by spraying a suspension of conidia (105 spores per ml) onto leaves until runoff using a handheld spray bottle. Non-inoculated controls (10 plants) were sprayed with distilled water. The plants were kept in a humid plastic chamber at 20 to 26°C. All examined isolates were pathogenic to bitter coconut, causing symptoms identical to those described above 12 days after inoculation. All inoculated tissues were plated onto PDA to confirm the presence of the pathogen. Live cultures are being maintained at the Micoteca/URM/UFPE (www.ufpe.br/micoteca), Brazil. To our knowledge, this is the first report of a disease caused by C. perangustum on S. oleracea worldwide, and the study provides valuable plant disease diagnostic information for the palm hearth industry in Latin America.
Reference: (1) K. Bensch et al. Stud Mycol. 67:1, 2010.
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