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First Report of Peroneutypa scoparia Causing Cane Dieback in Kiwifruit in Chile

Castilla-Cayuman, A., Lolas, M., Díaz, G. A.
Plant disease 2019 v.103 no.2 pp. 373
Actinidia deliciosa, Cadophora, Diaporthe, Diatrypaceae, ascospores, bark, canes, chloramphenicol, conidia, cultivars, culture media, dieback, discoloration, fungi, genes, growing season, hyphae, internal transcribed spacers, internodes, kiwifruit, leaves, mortality, mycelium, orchards, pathogenicity, perithecia, pycnidia, sodium hypochlorite, surveys, temperature, tubulin, vascular tissues, vines, winter, wood, Chile
During the 2013 to 2014 growing season, a survey of three commercial kiwifruit (Actinidia deliciosa ‘Hayward’) orchards located in the Maule Region (35°25′S, 71°40′W), consistently reported the presence of kiwifruit showing symptoms of dieback of lignified canes in 11 to 25% of the plants. Kiwifruit vines showed short internodes, chlorotic leaves, cane dieback, and cane mortality. A vascular cross-section of the affected canes showed a hard, light-brown discoloration of the vascular tissues. To isolate the causal agent, 30 diseased canes were collected from cordons obtained in six kiwifruit vines and were cut into 5-cm² pieces and surface disinfected with 0.5% NaClO for 2 min. After removing the bark, small wood pieces (0.5 × 0.5 cm) were plated onto potato dextrose agar (PDA) amended with 200 ppm of chloramphenicol and incubated at 22°C. After 7 days, hyphal tips were transferred to plates with PDA and incubated at the same temperature for an additional 30 days. Fungal colonies were white to gray, with fluffy aerial mycelium and forming gray to black conical pycnidia with whitish conidial masses. Conidia on PDA (n = 50) were unicellular, hyaline, filiform, slightly curved, and 16.2 to 20.1 × 1.5 to 2.0 µm. Stromatic multiple black perithecia were isolated from the symptomatic canes in the field. Ascospores (n = 50) obtained from black perithecia were unicellular, hyaline, and allantoid, measuring 3 to 5 × 1 to 2 μm. Based on the observed morphological characteristics, these isolates were preliminarily identified to belong to species in the Diatrypaceae family (Carmarán et al. 2006; Trouillas et al. 2010). Molecular identification was determined using three representative isolates (UT21MAD, UT01CH, and UT46CH) by sequencing the internal transcribed spacer (ITS1-5.8S-ITS2) gene and a partial sequence of the β-tubulin (BT) gene (Glass and Donaldson 1995; White et al. 1990). BLAST analysis showed homology of 99 to 100% with Peroneutypa scoparia (Schwein.) Carmarán & A.I. Romero strain ex-type (GenBank accession nos. GQ293962 and GQ294029). Sequences of the three isolates were deposited in GenBank (KY552741 to KY552743 for ITS and KY698413, KY671246, and KY698414 for BT). Pathogenicity tests were conducted on lignified dormant canes (DC) (n = 4 DC per plant) in six kiwifruit vines of 8-year-old cultivar Hayward using the same three isolates. A single pruning wound at the top of the DC (winter, July) was inoculated using a 5-mm mycelial plug obtained from the margin of 7-day-old colonies growing on PDA. The wounds were sealed with Parafilm. Four control DC in two plants were inoculated in the same way with sterile PDA plugs. The experiment was conducted twice. Nine months after inoculation with the fungus, canes inoculated were cut from the plants, and light brown lesions on the inoculated DC (from 50 to 75 mm in length) and dieback symptoms were observed. Lesions in control DC were short (10 mm in length) and sterile. Koch’s postulates were fulfilled by reisolating and identifying the three isolates of P. scoparia by morphological and molecular characteristics. Recently, Cadophora malorum and Diaporthe species were described causing cordon dieback in kiwifruit vines in Chile (Díaz and Latorre 2018; Díaz et al. 2016). To our knowledge, this is the first published report of P. scoparia causing cane dieback in A. deliciosa worldwide.