White Mold of Houndstongue (Cynoglossum officinale) Caused by Sclerotinia sclerotiorum in Canada.

Abstract

Houndstongue (Cynoglossum officinale L.) is a rangeland weed introduced into Canada from Eurasia, and it can be highly toxic to livestock feeding in pastures (3). During 2004, houndstongue plants near Creston, BC, Canada developed water-soaked lesions with white mycelia and black sclerotia on leaves and crowns. Isolations from diseased leaf tissues and sclerotia on potato dextrose agar (PDA) at 20°C for 5 to 7 days produced fungal colonies with formation of black sclerotia 5 to 10 mm in diameter. A single hyphal tip isolate from houndstongue. Ss-HT-C. was compared with a sunflower isolate of S. sclerotiorum, sun-87 (1), for morphology and pathogenicity. For apothecial production, Ss-HT-C and sun-87 were grown on PDA in petri dishes at 10°C for 10 weeks, and sclerotia produced were harvested, placed on moist vermiculite in petri dishes, and incubated at 20°C under light for 3 weeks. Mature apothecia were excised, stained with acid fuchsin, mounted on slides, and examined for asci and ascospores with a microscope. There were no morphological differences between Ss-HT-C and sun-87, each producing an ascus with eight binucleate, elliptical ascospores, measuring 4 × 10 μm (width × length), supporting the identity of Ss-HT-C as S. sclerotiorum (2,4). For pathogenicity tests of Ss-HT-C and sun-87, mycelial plugs (8 mm in diameter) were removed from the margin of colonies grown on PDA for 5 days at 20°C, and placed on leaves of C. officinale plants that were grown in a greenhouse (20 ± 4°C) to the rosette stage. Inoculated plants were covered with clear plastic bags, kept in the same greenhouse for 3 days, and the diameters of the leaf lesions developed at inoculation sites were measured. The experiment was run twice with 30 plants per isolate and five leaves per plant. Uninoculated plants covered with plastic bags were used as controls. Experiments used a completely randomized design. Results of leaf inoculations showed that Ss-HT-C and sun-87 were pathogenic to hound-stongue. There was no statistical difference between isolates or trials. The frequency of leaves with lesions was 90% for Ss-HT-C and 93% for sun-87. The mean leaf lesion diameters were 32 and 35 mm for Ss-HT-C and sun-87, respectively. Leaves of control plants remained healthy. S. sclerotiorum was reisolated from leaves with lesions, but not from controls. After 14 to 21 days, new sclerotia, 5 to 10 mm in diameter, were formed on leaves of inoculated plants. The plants eventually died. This study confirms that S. sclerotiorum is the causal agent for the disease of hound-stongue in Canada, and to our knowledge, this is the first world record of infection of this weed by S. sclerotiorum.