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Leaf Anthracnose, a New Disease of Swallow-Worts Caused by Colletotrichum lineola from Russia

Black swallow-wort, Vincetoxicum nigrum (L.) Moench (= Cynanchum louiseae Kartesz & Gandhi), and pale swallow-wort, V. rossicum (Kleopow) Borhidi (= Cynanchum rossicum (Kleopow) Borhidi), are invasive plants belonging to the family Apocynaceae and are the targets of biological control efforts to control their spread in the United States. In 2010, a disease on a related species, V. scandens Sommier & Levier, was observed in the Krasnodar area of Russia. Disease symptoms were many small, dark red-to-purple leaf spots, approximately 2 to 5 mm in diameter, with white centers. Leaf spots were found on the upper leaf surface. Leaf tips and margins of leaves bearing many of these spots were necrotic. Symptomatic leaves were collected and sent to the BSL-3 containment facility at the Foreign Disease-Weed Science Research Unit (FDWSRU) of the USDA, ARS in Frederick MD. Surface-disinfested symptomatic leaves were incubated at 20 to 25°C in sterile moist chambers. After several days, acervuli and brown setae were observed inside the leaf spots. Pure cultures, designated FDWSRU 10-002, were obtained by transferring spore masses with sterile glass needles onto 20% V8 juice agar. Seeds of V. scandens, collected in Russia, were placed in a freezer at –20°C for 6 weeks and then germinated in sterile petri plates on moist filter paper. The seedlings were then transplanted and grown in a 20°C greenhouse under 12 h of light. Koch's postulates were fulfilled as follows: 2-month-old plants each of V. scandens, V. nigrum, and V. rossicum were inoculated with spores from 2-week-old cultures of isolate 10-002. Plants were inoculated by spraying an aqueous suspension of 10⁶ spores per ml onto each plant until all leaves were wet. Plants were placed in 20 to 24°C dew chambers for 18 h and then placed in a 20°C greenhouse. Two weeks later, diseased leaves with the same symptoms observed in the field were harvested from each species, and the fungus was reisolated from seven of seven inoculated V. scandens plants, one of two V. nigrum plants, and four of four V. rossicum plants. Measurements of fungus fruiting structures were taken from cultures grown on synthetic nutrient-poor agar (SNA) (1). Conidiophores were brown, septate, and branched. Conidia were one-celled, hyaline, smooth walled, ovoid to oblong, falcate, and 20.1 to 26.2 × 1.7 to 3.6 μm (mean ± s.d. = 23.5 ± 1.3 × 2.6 ± 0.4 μm). Lengths of the conidia conformed to the description of Colletotrichum lineola Corda (1), but the conidia were slightly narrower than described. To induce appressoria formation, approximately 10⁴ conidia were placed on sterile dialysis membranes on top of SNA in petri dishes that were wrapped in foil and incubated at 24°C for 24 h. After this time, appressoria were observed with a microscope at ×400 magnification. The appressoria were dark brown, smooth walled, ellipsoidal, and 5.5 to 25.5 × 3.6 to 12.1 μm (mean ± s.d. = 13.4 ± 4.0 × 7.3 ± 2.1 μm), which conformed to the description of appressoria of C. lineola Corda (1). DNA sequences of ITS1, 5.8S, and ITS2 were submitted to GenBank (No. HQ731491), and after BLAST analysis, aligned 100% to 15 previously identified isolates of C. lineola in GenBank. Voucher specimens of the fungus have been deposited in the U.S. National Fungus Collection and were designated as BPI 881105 and BPI 881106. Host range and efficacy tests are planned to determine the suitability of C. lineola for biological control of swallow-worts in the United States.

Reference: (1) U. Damm et al. Fungal Divers. 39:45, 2009.