‘lmported’ seedlings a threat to Canty trees

Press, 16 July 1983, Page 6

‘lmported’ seedlings a threat to Canty trees

By

DERRICK ROONEY

The Forest Service fears that the forest-planting boom in Canterbury may backfire if growers who are unable to obtain planting stocks from local nurseries are tempted to break the law by going outside the province to obtain seedlings. Two serious pine diseases — one capable of weakening or killing plantation trees, and the other of stunting the growth of nursery seedlings — which the Forest Service is anxious to keep out of Canterbury may be introduced if stocks of certain conifers are brought in from infected areas. The diseases are “terminal crook,” which affects seedlings in their first year of growing, and the dothistroma needle-cast blight, which affects older trees. Radiata pine, the major plantation tree in New Zealand, is highly susceptible to both diseases, and Corsican and ponderosa pines are also vulnerable. The importation of pine, larch, spruce, and Douglas fir seedlings to Canterbury from infected areas is prohibited under a regulation promulgated in 1967. ’ The Forest Service fears that because of the shortage of planting stocks in Canterbury, some people will court disaster by flouting the regulation. Most tree nurseries in Canterbury are either sold out or running low on stocks of popular timber species, as a result of a planting boom, caused partly by the introduction of new forestry encouragement grants which meet 45 per cent or more of the cost of establishing and maintaining plantations, and partly by optimistic forecasts of demand for forest products. “Growers can now qualify for assistance to establish species such as Corsican and ponderosa pine in the high country, and the needle blight could be a problem with these two species in particular if it becomes established in Canterbury,” said the senior forestry extension officer, Mr Theo Russell.

The needle-blight disease was first reported in New Zealand in 1964, in a plantation near Rotorua, and has spread to most parts of the country except Canterbury and North Otago. The first reported case of “terminal crook” was in a nursery at Woodhill, near Auckland, in 1963.

“Both these diseases would be expensive to control by both the State and private tree growers,” said Mr Russell. If the diseases occurred in Canterbury, intensive spraying of nurseries and aerial spraying of woodlots would be required. “The main point is that Canterbury is free of these diseases, and it is in all our interests to keep the status quo,” said Mr Russell.

Dothistroma is not generally regarded by foresters as a serious threat to plantations on the dry plains sites, where climatic conditions do not favour its establishment, but it could become established in the wetter foothills districts, such as the Hanmer State Forest area, and on Banks Peninsula. It is well established in pine plantations on many sites on the western side of the Alps. Since 1967, the Forest Service has had power, under the Forest Disease Control Regulations, to prohibit the carriage of tree seedlings from infected districts to dothistroma-free areas. As the whole of the North Island, part of Marlborough,

Nelson, the West Coast, and part of Southland are infected areas, Canterbury growers are prohibited from importing seedlings of pines, larch, or Douglas fir from any district except North Otago. Dothistroma pini originated in western North America, and has been introduced to Europe, Africa, and Chile as well as New Zealand. It kills the needles of pine trees, thus reducing the vigour of the tree and, in extreme cases, killing it. The disease spreads by minute spores, which can probably travel long distances in mist or cloud. The intensity of outbreaks is apparently related to the rainfall in early and late summer, and at those times heavy and continuous rainfall can cause a dramatic increase in infection. The effects of the disease are most severe on pines. Douglas fir and larch are less susceptible. Radiata pine trees older than 15 years may develop some immunity to the disease, but Corsican and ponderosa pines are vulnerable at any age.

The initial symptom of the disease is the development of brick-red bands on the needles. These zones are distinctly marked off from the rest of the leaf. Small black spots, containing the

spores, later appear on the red zones, and eventually the leaf dies. In a severe

infection, almost all the leaves on a tree can be killed. Since 1966, a committee of representatives of the Forest Service and major private forest owners has administered a national campaign to control the disease. The committee arranges and supervises contracts for the bulk purchase of fungicide and the aerial spraying of forests, and advises forest owners on control methods. “Terminal crook” is a disease which stunts pine seedlings in nursery beds. In affects the terminal bud, causing the upper part of the stem to bend over like a shepherd’s crook, and stopping upward growth. Some plants are reinfected in their second year, but usually the disease affects only plants in their first 12 months of life. At one year old, infected plants may be less than half the height of normal seedlings. As the seedlings grow older they usually develop immunity to the disease, and most will reach an acceptable size for planting if grown on for a second year in the nursery. However, this increases production costs, and requires double the nursery area to produce the same number of plantable seedlings annually. Chemical sprays to control the disease are available, but cost about $1 per 1000 seedlings to apply-