Something evil this way comes?

Written by      

Ian Brown

In 1993, when New Zealand Geographic was celebrating five years of publication, the magazine produced a lengthy feature on Pinus radiata, “the prince of pines.”

Among the many issues covered was a consideration of the pine “monoculture” that makes up most of this country’s forestry stands. Critics such as Albany nurseryman Graeme Platt denounced our reliance on a single species as a dangerous practice but, by and large, any perils were played down by the forestry industry.

Hadn’t pines survived attack by the wood-boring wasp Sirex in 1946, and the pine needle blight Dothistroma pini, which arrived in 1962? Couldn’t our new clones of genetically superior trees beat anything else that might be out there?

We may yet have the opportunity to find out, for a fresh threat to the pine hegemony has emerged. While it is still distant from these shores—as far as we know—it may prove hard to keep out.

What we term sap or resin, the Americans call pitch, and a disease named pitch canker has been known from the south-eastern US since 1946. There the fungus Fusarium subglutinans pini infects several species of pine, including slash and loblolly pines. Infected trees develop lesions on shoots, cones, bark and roots.

On shoots and branches, needles beyond the lesion die, and when bark is removed from the canker, the underlying wood is found to be soaked with resin. If a number of branches are infected, much of the crown of the tree may die. Infected cones often abort without maturing.

On the trunk of the tree, lesions may reach 30 cm in diameter, and resin may dribble down the bark for a metre, but bole lesions are usually a late stage in the disease. Young trees, up to three or four years old, may get a single infection at soil level which kills the whole tree.

Many trees die com­pletely, others linger deformed and debilitated. Seedling mortality is exten­sive, with root rot and other rather nonspecific symptoms. In the warm, wet south-east (a climate not dissimilar to New Zealand’s) the disease is largely spread by rain splattering infectious propagules into the air and the wind ferrying them from tree to tree, but the fungus also seems to require some sort of wound to gain entry into the tree.

In 1986, Pinus radiata infected with pitch canker were found in Santa Cruz County, California, about 120 km south of San Francisco. The disease has since spread in coastal California from San Diego to north of San Francisco, and is expected to go further north. San Francisco is at the same latitude as Hamilton.

Pinus radiata has turned out to be particularly suscep­tible to the disease, but 40 other species of pine occur­ring in California have either been found naturally infected or have been experimentally infected. Some of these species are of European origin, and it is thought that probably no species of pine is immune to the disease.

A few Douglas fir have been attacked, although that species is neither generally nor seriously affected. No other species of conifer is affected.

In California, the fungus seems to be spread in a different manner from in the south-east. There it has been isolated from a number of insect species which live around and in susceptible trees, including moths, bark beetles, cone and twig beetles, weevils and possibly aphids. Experimental contamination of some of these insects has led to their infecting plants, and insects are now implicated as major vectors in the spread of the disease. When insects burrow into or feed from infected plants they pick up spores, which they then carry to healthy trees and infect as they feed upon them.

Seeds from infected trees are also frequently infected, probably reducing rates of regeneration, and there is a suspicion that even pollen could carry the disease.

In California, Pinus radiata is not much regarded as a timber species (they say it has too many pests), but it is grown in plantations as a Christmas tree—often from seed purchased from New Zealand—and is widely planted as an amenity tree along highways, in parks and public gardens, etc.

Because lesions develop within a month or so of infection, the Christmas-tree business is literally wilting, and the transport­ation of infected Christmas trees around the state is thought to have contributed to the spread of the disease.

Many disfigured and dying amenity trees have been felled, and the State Senate has approved the expenditure of several million dollars to fund research into the disease. There is serious concern that pitch canker will travel inland into the large commercially important pine forests of the Sierra Nevada, although lower temperatures at higher altitudes may impede its spread.

In the laboratory, infections have not established at temperatures lower than 10°C, but in summer even the moun­tains are a lot warmer than this.

New Zealand-sourced Pinus radiata have proved just as susceptible to pitch canker as the Californian material, and Fletcher Challenge Forests is now preparing to test the susceptibility of 80 of its New Zealand clones in California.

Once a tree is infected, nothing can be done to cure it. Reports from California indicate that no fungicides or insecticides are effective against the disease, and lopping off infected branches is of little value. However—a ray of hope—even in areas where the disease is well established, a few Pinus radiata—less than 5 per cent—seem to be unaffected. Perhaps a small minority of trees are naturally resistant; this is certainly the hope behind the Fletcher Challenge trials.

Needless to say, pitch canker could devastate New Zealand’s forestry industry, since more than 90 per cent of our forests consist of Pinus radiata, with Douglas fir the second most impor­tant species.

While we lack the exact insect species that are implicated as vectors in California, the wide range of insects involved there suggests that we would have a good chance of having local equivalents.

It is not even certain that insects would be required to spread the disease here, as our wet climate is more akin to the south-eastern US, where insects seem of little importance. Our practice of extensively pruning young trees produces an abundance of major wounds which would provide ideal entry points for this disease.

How good are our chances of keeping the fungus at bay? Pine pitch canker has spread in the past decade to South Africa, Japan and Spain, which does not auger well for us, and it also occurs in the Caribbean.

For several years we have had restrictions on imports of conifer seeds from the US, and our port and airport interception services are vigilant. But fungal propagules are invisible and long-lived, and our success at intercepting even insect pests has been limited.

A succession of serious insect pests on eucalypts have established here in recent years, and tussock moth and Mediterranean fruit fly were eliminated only with great effort, and only after getting through the net.

Recently, examples of Monterey pine aphid (Monterey pine is what Ameri­cans call radiata) were found at Sydney airport on a shipment of avocados from New Zealand. This pest was not known from New Zealand, but a search of pines around the orchard the avocado came from yielded many aphids. This pest has now been found in pine forests throughout the North Island, and we didn’t even know it had arrived! It could possibly serve as a vector for the canker fungus.

Another possible avenue for the disease to reach here is via soil—an unnoticed passenger on many a campervan, tent, item of used logging equipment or simply on a car. Most used camping gear brought into the country has been found to have plant debris and probably soil adhering to it.

Used logging equipment has been imported from the US in recent years. And remember that Japan, source of more than a few used cars, has this pest now.

Wood can also carry the disease, and although we import no pine from the US, there is at least a possibility that lumber from some of the dying trees in California might be used in packing crates and the like. Infected beetles could also arrive here in the wood of packing cases.

Early symptoms of the disease are not especially distinctive, and in California it is thought that the canker may have been present in some areas of spread for a couple of years before it was spotted. By then it was well established and impossible to stop.

A group from the New Zealand Farm Forestry Association recently visited North America and described stands of Pinus radiata infected with pitch canker as looking like Chernobyl.

Some of the party were so afraid of bringing the disease back to New Zealand that they left their boots in California. How many tourists would be prepared to follow their example?

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