Conidial morphology, virulence, molecular characterization, and host–parasite interactions of selected Alternaria panax isolates on American ginseng

Abstract

The fungus Alternaria panax Whetzel causes leaf and stem blight of American ginseng ( Panax quinquefolius L.). Six isolates of A. panax, collected from widely separated ginseng-growing areas in North America, were characterized on the basis of their colony and conidial morphology, molecular characteristics, and virulence. Colony morphology on standard media and conidial morphology based on light microscopy showed some variation with geographic locality: Wisconsin isolates were slightly different from Ontario and British Columbia isolates. Isolates from each locality were equally virulent on American ginseng. Ginseng phytotoxin was present in the conidial germination fluid of all isolates. Sequences of the β-tubulin gene from all six isolates were identical as were the sequences of the internal transcribed spacer (ITS) region of the rDNA gene. The ITS1 and ITS2 gene sequences of the six isolates were also identical to those of American and Korean isolates of A. panax deposited in GenBank. Cellulase and pectinase were detected by agar biodiffusion assays of a representative isolate. Germination of conidia of this isolate on ginseng leaflets began within 2 h of application to leaf surfaces, and over 80% of conidia germinated within 12 h of application. Conidia produced one to several germ tubes that grew in all directions over the leaflet surface. The germ tubes formed terminal appressoria over stomatal openings, epidermal cells, and epidermal cell junctions. The frequency of direct penetration into ginseng leaflets was greater than penetration through stomata. Both retraction of plasma membrane and disruption of chloroplast membranes were observed 12 h after leaflet inoculation. At 24 h after inoculation, hyphae were present in both intercellular spaces and inside mesophyll cells. Severe disruption of plasma membrane, chloroplasts, and mitochondria was observed between 24 and 48 h after inoculation.