Corynespora cassiicola causes leaf spots on the ornamental plant Allamanda cathartica in Brazil
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The necrotrophic leaf spot fungus Corynespora cassiicola is recorded for the first time attacking Allamanda cathartica in the host’s center of origin in Brazil. The pathogen was identified using morphological features as well as molecular analysis and its pathogenicity was confirmed by fulfilling Koch’s postulates.
KeywordsApocynaceae Dematiaceous asexual morph Garden plant Occurrence Pathogenicity
A dematiaceus fungus was regularly found growing on the center of the yellow allamanda leaf spots. Conidia were transferred to potato dextrose-agar (PDA) plates with a sterile fine pointed needle and pure culture colonies were obtained. One isolate was selected and deposited in the culture collection of the Universidade Federal de Viçosa (Acc. No COAD 1770). Fourteen-day-old colonies grown at 25 °C under continuous light (light provided by two white and one near-UV lamps placed 35 cm above the plates) in PDA and potato carrot-agar (PCA) were described following the terminology of Crous et al. (2009). For color terminology Rayner (1970) was followed.
Fungal structures were scraped from lesions with a scalpel, mounted in lactoglycerol and observed with an Olympus BX 51 microscope. The fungus had the following morphology - conidiophores amphigenous, cylindrical, straight to slighlty curved, 322 ̶ 644 × 5 ̶ 8 μm, 4–6 septate, pale brown, smooth; conidia solitary or catenate, cylindrical to subcylindrical, 60 ̶ 160 × 3 ̶ 5 μm, 9 ̶ 24 pseudoseptate, pale brown, smooth (Fig. 1b and c). In culture: Fast-growing (8 cm after 7 days in PDA), umbonate, cottonose, pale olivaceous gray with mouse gray center surrounded by hazel rim, hazel reverse, not sporulating (Fig. 1d).The morphology of the fungus on yellow allamanda was almost identical to Corynespora cassiicola (Cc) as described by Ellis and Holliday (1971).
In order to confirm the morphological identification a study of the ITS region (as performed by Dixon et al. 2009) was conducted. DNA was extracted from a hyphal colony of COAD 1770 growing on PDA with Wizard Genomic DNA Purification Kit (Promega) by following the manufacturer’s instructions. Primers-pair ITS4 e ITS5 were used in PCR amplification and sequencing was performed by Macrogen (Seoul, Korea). The resulting sequence was deposited in GenBank (Accession No MF962868) and a BLASTn search resulted in the sequence of COAD 1770 having 100% identity with various Cc isolates from many hosts e.g. Plectranthus barbatus (FJ852592) further confirming the identification of COAD 1170 as Cc.
To confirm its pathogenicity, an aqueous suspension of 1 × 104 conidia mL−1 of COAD 1770 (obtained from fourteen-days-old cultures on PDA) was sprayed on five healthy one-year-old plants until runoff, whereas two separate plants were sprayed with distilled water, serving as controls. These plants were transferred to a humid chamber where they were incubated for three days, and then transferred to a greenhouse bench. Numerous leaf spots such as those observed in the field were observed after four days of inoculation while controls remained healthy. Corynespora cassiicola was found sporulating on lesions and pure cultures of equivalent morphology to those of COAD 1770 were obtained after transfer of conidia present on the lesions onto PDA plates.
Although Cc is a fungus known to have a broad host range and has been recorded as a pathogen of A. cathartica in Brunei Darussalan, Guam, Samoa and continental USA (Farr & Rossman, 2017), these records are only associations in published lists without any information on taxonomy or pathology and without reference to voucher specimens or cultures. Additionally Cc appears in the Brazilian list of records of fungi on plants (Mendes & Urben, 2017), but this was found to be erroneous. It was based on information from a conference abstract (Lucon et al. 1996) where the authors only dealt with in vitro growth of Cc. Lucon et al. (1996) vaguely mentioned that Cc is able to cause, depending on the environmental conditions, significant damage on eleven genera of crop and ornamental plants, including Allamanda. These authors did not provide any definitive evidence that these Cc × host associations were based on occurrences detected in Brazil. Therefore, we believe that our paper provides the first unequivocal record of Cc on A. cathartica in Brazil. Although yellow allamanda is Brazilian plant species and is a widely grown ornamental plant, little is known about its pathogenic mycobiota inBrazil.
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