Defense responses to Fusarium oxysporum f. sp. asparagi and F. proliferatum
were compared after root inoculation of the asparagus fern, Asparagus dens
iflorus vars. Myersii and Sprengeri, and cultivated asparagus, A. officinal
is cv. Guelph Millennium. Both varieties of A. densiflorus exhibited a hype
rsensitive response with rapid death of epidermal cells within 8-24 h and r
estricted the fungal growth. In A. officinalis roots, rapid cell death was
not found, and necrotic lesions were observed 8-14 d after fungal inoculati
on. Peroxidase and phenylalanine ammonia-lyase activities increased signifi
cantly in inoculated A. densiflorus but not A. officinalis plants. Local an
d systemic induction of peroxidase activity was detected after pathogen ino
culation in root and spear tissues, respectively, of A. densiflorus. POX ac
tivity decreased in roots of inoculated A. officinalis by 8 d post-inoculat
ion. Germination and germ tube growth were inhibited when spores of F. oxys
porum f. sp. asparagi were incubated in root exudates and on root segment s
urfaces of inoculated A. densiflorus plants exhibiting hypersensitive cell
death. Spore germination of F. proliferatum and three fungi non-pathogenic
to cultivated asparagus was inhibited as well. Rapid induction of hypersens
itive cell death in A. densiflorus was associated with restriction of funga
l growth, and activation of peroxidase and phenylalanine ammonia-lyase, two
defense enzymes thought to be important for plant disease resistance.