Impact of a disease and a defoliating insect on houndstongue (Cynoglossum officinale) growth: implications for weed biological control

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.