Rl. Conner et al., Impact of a disease and a defoliating insect on houndstongue (Cynoglossum officinale) growth: implications for weed biological control, ANN AP BIOL, 136(3), 2000, pp. 297-305
The noxious weed houndstongue (Cynoglossum officinale) has become a major p
roblem on the forested rangelands in the interior of British Columbia. Howe
ver, recently the fungus Phoma pomorum and the ranchman's tigermoth (Platyp
repia virginalis) were identified as potential biocontrol agents of this bi
ennial weed. Infection by the fungus Ph. pomorum resulted in the formation
of large brown lesions on leaves of houndstongue. In culture, the fungus re
adily produced pycnidia with pycnidiospores measuring 5.7 mum x 1.7 mum. Th
e effect of Ph. pomorum and P virginalis on the growth of houndstongue was
examined over a six week period. Leaf age strongly influenced the intra pla
nt distribution of insect feeding and lesion formation with Ph. pomorum pri
marily attacking the older leaves, while P virginalis larvae prefer-red fee
ding on young leaves. Infection of leaves by Ph. pomorum resulted in their
premature death. The effect of infection by Ph. pomorum on plant weight var
ied among tests, but the disease usually increased the number or percentage
of dead leaves and reduced root biomass. A study of the effect of plant ag
e and disease on houndstongue showed that younger plants infected with Ph.
pomorum had a slightly higher percentage of dead leaves than older plants,
but that reductions in live leaf weight and root weight were similar for di
fferent age groups. Six weeks after exposure to feeding damage by P virgina
lis, there was no significant: effect of P. virginalis on plant weight eith
er acting alone or in combination with Ph. pomorum.