Science > Research > Biological control strategy for management of dwarf mistletoes

Development of biological control strategy for management of dwarf mistletoes

Simon F. Shamoun 

[ Background | Results | Discussion | Future research | References ]
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Background

Dwarf mistletoe (Arceuthobium spp.) are serious pathogens of conifer species and trees of any age may be retarded, deformed, or killed by these parasitic plants (Fig. 1). Losses result from reduction in growth (e.g. in BC, estimated volume loss in mature stands is >3.7 million m3/year), degradation of merchantable wood, reduction in cone production, infection entry points for decay fungi, and reduction in reproductive fitness. Our research is investigating a strategy of interference and interruption in the mistletoe life cycle and seed dispersal. The biological control strategy is to suppress mistletoe through inoculation with pathogenic fungi. Research has focused on fungal collection and identification, elucidation of fungal biology, establishment of biotechnology methods for pathogenicity screening tests, and development of formulation and delivery technologies under field conditions. The goal is to reduce spread from affected trees to seedlings following stand openings by forest harvesting, in particular under partial cutting system, or silvicultural activities (Fig. 2 & 3).

Fig. 1. Mature trees infected with dwarf mistletoe parasite

Fig. 2 & 3. Dwarf mistletoe seed dispersal from infected retention trees

Results

During 1996-1998, a collection of mycoflora associated with dwarf mistletoe was generated and three fungal hyperparasite were identified which attack either mistletoe shoots, seeds, or the endophytic system: Colletotrichum gloeosporioides (Fig. 4), Nectria neomacrospora (Fig. 5) and Cylindrocarpon gillii (Table 1). Pathogenicity screening tests evaluated fungal virulence on mistletoe shoots and seeds in laboratory and greenhouse tests, and a field trial. For field applications, an invert emulsion (oil-in-water) formulation was selected due to enhancement of fungal growth, protection from desiccation, and adhesion properties.

Fig. 4.  Colletotrichum gloeosporioides,  causing mistletoe stem and seed disease.

Fig. 5.Nectria neomacrospora, on mistletoe infection sites on hemlock.

A novel protocol for in vitro tissue culture of western hemlock dwarf mistletoe successfully produced callus for the first time with this species. Mistletoe callus is being assayed to elucidate the cellular interactions between mistletoe and hyperparasitic fungi and light microscopy has revealed both inter-and intra-cellular colonization (Figs. 6 & 7).

Fig. 6. A novel protocol for tissue culture of w. hemlock dwarf mistletoe.

Fig. 7. Screening of potential biocontrol agents through challenging callus with inoculum.

Discussion

The biological control strategy presents a management tool that could integrate well with newer ecological and environmental systems being practiced in forests. Smaller cut-blocks and partial harvesting in dwarf mistletoe infected stands will increase the ratio of perimeter to area and thus increase the rate of dwarf mistletoe re-invasion. A biological control strategy which enhances natural disease events and interrupts the mistletoe life cycle and seed dispersal may offer an effective management tool to reduce the impact of dwarf mistletoe in conifer sites.  

Future research

• Refine formulation and delivery technologies of candidate biocontrol agents for hemlock and lodgepole pine dwarf mistletoe pathosystems (Fig. 8).
• Elucidate the population structure and genetic variation of A. tsugense and A. americanum and lead isolates of biocontrol agents within and between populations over the entire geographic range in Canada and the USA.
• Identify genetic resistance and its mechanisms in conifer species to dwarf mistletoes.

Fig. 8. Potential inoculum dispersion systems.

Table 1. Identification of fungi collected from dwarf mistletoe (Arceuthobium tsugense subsp. tsugense) on western hemlock (Tsuga heterophylla) from field sites 1996-1998.

References

Deeks S.J., Shamoun S.F. and Punja Z.K. 1998. Tissue culture of western hemlock dwarf mistletoe. In: Proceeding of the Canadian Society of Plant Physiologists. 41: 9-10

Hawksworth, F.G. and Weins, D. 1996. Dwarf mistletoes: biology, pathology, and systematics. Agric. Handb. 709, USDA, Forest Serv., Washington, D.C. USA

Kope, H.H., S.F. Shamoun, and C. Oleskevich. 1997. First Report of Colletotrichum gloeosporioides on Arceuthobium tsugense subsp. tsugense in Canada. Plant Dis. 81:1095.

Shamoun, S.F. 1997. Managing Dwarf mistletoe- A biological and genetic control approach. Information Forestry, Dec. 1997, CFS-Victoria publication.

Shamoun, S.F. and Dewald, L. 1998. Control of dwarf mistletoes by biological, chemical and genetic methods. In: Mistletoes of North American Conifers. B. Geils, J.C. Tovar and B. Moody (eds.) (In press).

Other publications by Simon F. Shamoun

Other publications about dwarf mistletoes

[ Background | Results | Discussion | Future research | References ]