Dutch elm disease and biological control
Scion is the leading provider of forest-related knowledge in New Zealand
Formerly known as the Forest Research Institute, Scion has been a leader in research relating to forest health for over 50 years. The Rotorua-based Crown Research Institute continues to provide science that will protect all forests from damage caused by insect pests, pathogens and weeds. The information presented below arises from these research activities.
From Forest Health News No. 22, July 1993.
Naturally occuring strains called d2 strains of the dutch elm disease fungus Ophiostoma novo-ulmi carry cytoplasmically located mycoviruses that are transmitted to normal strains by hyphal anastomosis. These viruses reduce the fitness of the fungus by affecting spore production, spore viability, growth rate and pathogenicity. d2 isolates have potential as biological control agents and may be a useful tool within an integrated DED control programme. The major goal of introducing the d2 strain is to reduce the percentage viability of spores, thereby reducing the chance of initiation infections during maturation feeding of the elm bark beetle Scolytus multistriatus . Overseas literature suggests that depending on conditions 1000-10 000 viable spores are required for infection. This figure would increase to 50 000 if the spores were d-infected. Since S. multistriatus is a much smaller bodied insect than S. scolytus, the principal vector overseas, the chance of inoculation with enough spores to facilitate infection is very low. This reasoning is the basis for consideration of the d2 strain as a potential biocontrol agent. Preliminary laboratory testing of New Zealand isolates of 0. novo-ulmi has shown that transmission of the virus can be effected from a d2 infected strain to the fungi attacking elms in Auckland. Growth rate of the d2 strains is significantly slower than Auckland strains, making it a poor competitor as a saprophyte. This and other factors, including loss of the d2 factor during the yeast phase in the xylem of the host tree may reduce the effectiveness of the d2 agent as a biocontrol agent.
A lot more work will be necessary before a sound decision on the suitability of the d2 strain for use in New Zealand.
(Mark Self )
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