Department of Plant Pathology, University of the Orange Free State, P.O. Box 339, Bloemfontein 9300, South Africa
Department of Botany, University of Stellenbosch, Stellenbosch 7600, South Africa
Pringlea antiscorbutica R. Br. (the Kerguelen cabbage) is a monotypic species endemic to five sub-Antarctic islands. It does not occur elsewhere and is the only brassica found in the sub-Antarctic region as a whole (1). On Marion Island (46° 54′S, 37° 45′E) the distribution and abundance of the cabbage has declined alarmingly over the past 20 years and the plants increasingly are showing symptoms of microbial pathogen attack. Leaves display brown, water-soaked lesions and sometimes whole plants collapse into a black, slimy residue. Small sections of lesions were sampled, surface sterilized, and placed on water agar and potato dextrose agar (PDA) to which streptomycin (0.1 g/liter) was added. Nonsterilized sections were placed in petri dishes on moist, sterilized, filter paper. Plates were incubated at 15°C in the dark. Botryotinia fuckeliana (de Bary) Whetzel (conidial state; Botrytis cinerea Pers.:Fr) was consistently isolated. Gray to white mycelium grew rapidly on PDA and after 10 days abundant black sclerotia (2 to 16 mm long) were observed on the medium. To confirm pathogenicity, four detached leaves of P. antiscorbutica were each inoculated with a single, 5-mm-diameter PDA plug of B. fuckeliana. Four leaves with noncolonized PDA plugs were used as controls. The leaves were placed with their petioles in sterile water in a transparent incubation chamber. Chamber temperature (minimum 3.9°C, maximum 7.7°C and mean 6.4°C) and light (100 µmol s-1 m-2 photosynthetic photon flux density) approximated field conditions quite closely but relative humidity (annual mean for the island, 81%) was on average 10% higher in the chamber. First symptoms were observed after about 48 h. After 5, 7, and 8 days, brown, water-soaked lesions averaged 14 × 7, 28 × 11, and 36 × 18 mm, respectively. Control leaves showed no symptoms. Reisolations from lesions produced B. fuckeliana. Identity of the pathogen was confirmed by the South African National Collection of Fungi Biosystematics Division, Pretoria. The sub-Antarctic climate is probably conducive to the rapid spread of B. fuckeliana and to its ability to infect P. antiscorbutica. Other factors, such as the recent introduction of the diamondback cabbage moth (Plutella xylostella) to the island and grazing damage by an introduced slug species (Deroceros caruanae) probably exacerbate the threat offered by the fungus to this plant species, one of the last and perhaps the only, remaining relic of an extensive circum-Antarctic flora.
Reference: (1) A. J. Dorne and R. Bligny. Polar Biol. 13:55, 1993.