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Dutch elm disease in Napier, 1997

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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. 68, October 1997.

In Napier the Dutch elm disease infected area is confined to Sturms Gully - a mix of both reserve and private properties. There has not been any sign of the disease vector, the elm bark beetle (Scolytus multistriatus), and disease spread has been shown to be the result of root grafting (see FHNews 39: 1-2, Feb. 1995). As the beetle's range is slowly expanding, from its point of introduction in Auckland, it is only a matter of time before it reaches the infection site in Sturms Gully. For this reason it was essential the infected trees be removed.

In early March 1997 a communications and an operational strategy were designed by the Ministry of Forestry. Residents were contacted personally with the intention of keeping the operation as informal as possible. Following a press release a 'hotline' was set up. This approach was well received by the public, other government departments (e.g. DOC) and the local forestry industry.

All the elms in Sturms Gully were poisoned in the autumn. Poisoning was carried out by axing slots at the base of each tree and applying a systemic herbicide (ESCORT ®). The trees on council land were poisoned first (19-20 March) and then trees and elm hedges on private properties (26-28 March).

The poisoned trees were removed by Treescape Limited of Auckland and the Napier City Council in late August to early September. The cleanup operation was greatly facilitated by labour supplied by the council. Branches and stems to a diameter of 47.5 cm were chipped on site. The chip pile has been left to compost with the high temperatures generated killing any fungal mycelium in the wood and any fungal spores. The cooler surface layer of the pile will be sprayed regularly with a copper-based fungicide to kill any surviving spores.

Stems over 47.5 cm diameter (3 articulated truckloads) were buried on council land near the sea. The burial site consisted of mixed aggregates with a high salt content and logs were buried to a depth of no less than 2.5 metres. Where possible the remaining stumps were removed. But where the ground was too steep the stumps were debarked manually and roots exposed. The lack of debris left in the gully at the end of the operation was impressive and no small feat.

Three hundred trees (approx. value $1200) were supplied to affected residents not only as a sign of goodwill but also for erosion control and site restoration. Species were selected to suit the local environment of dry summers and high soil pH. Residents selected mostly natives including titoki, rimu, totara, kauri, pittosporum, corokia and coprosma. The council will not start replanting the reserve until next winter.

The final stage, over several years, is to prevent coppicing and remove seedlings. If this is not done then infected root systems of felled trees may survive through grafting with this regrowth. It has been arranged that Taskforce Green will supply labour to grub, pull or poison all elm re-growth.

(Brent Rogan, MoF)


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