BLISTER RUST ON WHITE PINE

J. F. MARTIN, PERLEY SPAULDING.

White pine blister rust is a fungus that attacks and destroys the highly valued white, or five-needled, pines. It spreads to pines from its alternate hosts, currants and gooseberries, without which the fungus cannot infect white pines. Thus the disease is controlled by removing the alternate host plants, commonly called ribes, in the vicinity of white pines.

A point to note is that white pine blister rust is an introduced disease, not a native. Most native tree diseases are curbed by natural conditions, so that in a given outbreak they are fatal only to individuals or groups of individuals. But introduced diseases are free from the natural controls of their native habitat; in their new environment, if the conditions are unusually favorable, they sometimes become epidemic and destructive, although the existence of an entire tree species is rarely endangered.

White pine blister rust was first found in North America at Geneva, N. Y., in 1906. It occurred on cultivated ribes, and the bushes were promptly destroyed. The disease was found again in 1909 in new plantations of eastern white pines, large numbers of which had just been imported from Europe to fill a heavy demand for forest planting stock. The shipments went to most of the Northeastern and Lake States, and to eastern Canada. Many of them contained infected trees, and so the disease was widely distributed within the native range of eastern white pine. The State officials concerned immediately agreed on concerted action to find and destroy all infected pines and remove all ribes within 500 feet of the diseased plantations. They hoped thus to eradicate the fungus. The action delayed the spread of the disease, but in 1913 it became evident that infection had spread to the native white pines. By 1915 all hope of eradicating the fungus from North America was abandoned. Efforts then were concentrated on local control to prevent serious damage in stands of eastern white pine.

In 1921 the disease was found near Vancouver and in northwestern Washington on western white pine. The origin of that outbreak was a shipment of white pine nursery stock made directly from France to Vancouver in 1910.

Out of experiences with white pine blister rust, chestnut blight, and some forest insects that were known to have been imported from abroad came the enactment in 1912 of a Federal Plant Quarantine Act. The first quarantine under it prohibited further importation of white pines. Later the interstate movement of white pines and ribes was regulated to prevent spread of the disease by shipment of infected host plants. The affected States also enacted laws to control the blister rust or promulgated quarantines and regulations under established pest control laws pertaining to control of blister rust. Such action has been taken by 32 States. In 1917 a Federal embargo was placed on the movement of white pines and ribes from the Eastern States to points west of the Great Plains to prevent westward extension of the disease through the shipment of infected host plants. This embargo was lifted in 1926 after it became evident the disease had become widely scattered in western white pine forests. Adjustments were made in the Federal white pine blister rust quarantine from time to time to take care of problems created by the natural spread of the rust into uninfected territory and the removal of ribes in control areas.

White pines are among our most valuable and desirable forest trees.

Of the eight native species, three are among our leading timber species. They are the eastern white pine, which grows from Georgia to Maine and west to Minnesota; the western white pine, which is found chiefly in the Panhandle of Idaho and nearby parts of Montana and Washington, and the sugar pine of Oregon and California. They are a forest resource of great commercial importance. They are adaptable to a wide range of site conditions, they make rapid growth, and they lend themselves to forest management. Their timber, as it stands in the forest, is worth several hundred million dollars; its manufactured value is much greater. The younger growth is the timber crop of tomorrow. Both eastern and western white pine are used for reforestation. The eastern white pine also is used extensively in landscaping homes, parks, buildings, memorials, and like places.

Numerous logging, milling, and manufacturing industries that employ thousands of people and form the economic basis of many communities depend on the three species for raw material. The wood is soft, durable, fine-grained, easy to work, and excellent for patterns, matches, doors, window sashes, toys, and many other products. In northern Idaho and nearby parts of Washington and Montana, forest industries are a main support of the economic and social life. These industries, in turn, depend on western white pine, which represents about three-fourths the value of the forest products of the region. Without the white pine, the harvesting and utilization of associated trees would not be profitable.

The other five species grow at high elevations along the mountain ranges west of the Great Plains. They produce little timber, but they have considerable value in other ways in protecting water supplies, preventing soil erosion, and making scenic and recreational areas.

Now all eight native species of white pines are endangered by blister rust. To save them will take united and sustained action by public and private agencies to bring the disease under control and keep it suppressed in pine-production areas.

Already the disease is established and is spreading in all commercial white pine belts. It is present throughout the range of eastern white pine except in the extreme southern fringe. In the West it has invaded the entire range of western white pine and of sugar pine as far south as Eldorado County in California about 210 miles south of the Oregon border.

Further, the disease acts relentlessly and insidiously. The fungus destroys pines by girdling the limbs and trunk. Young seedlings are girdled in a short time. They die and disappear and leave no evidence that they ever existed. Diseased saplings may live several years before they succumb. Infected mature trees survive 20 years or more; if the disease is detected in time, most of them can be salvaged. No hope exists, however, for adequate future supplies of white pine if the young reproduction is killed or excessively thinned by the disease.

RIBES appear during the early formation of the forest stands. They reach maximum development in about 20 years. Then they decline. They grow from seeds, sprouts, and layered stems. Ribes usually are absent or sparse on light, sandy soils. They vary from few to many on the heavier soils. They persist indefinitely in places in the forest that are permanently open. They are suppressed by shade and root competition in fully stocked stands. Ribes are carried over from one forest generation to the next by viable seed stored beneath the litter on the forest floor. When this debris is disturbed by logging, fire, or other causes that expose the stored seed and increase the intensity of light, conditions become generally favorable for seed germination and growth of the ribes. Under those conditions, young ribes begin to produce seed in 3 to 5 years and seed storage begins anew. Ribes usually develop in abundance from seed following single light burns in forests.

In young stands, the crowns of the old ribes that are still alive may produce sprouts that grow rapidly. Double burns and severe single burns destroy the seed and crowns, and create ribes-free conditions except in wet and rocky places where they may survive the fire. The production and longevity of ribes seed, disturbances of the forest floor, shade, plant competition, and fire are factors in ribes suppression that receive careful consideration in planning control operations and forest-management practices.

SPORES, the reproductive bodies of fungi, serve the same purpose for fungi that seeds do for ordinary crop plants. The blister rust spores are minute, dustlike particles that are easily carried by the wind. One kind of spore, produced in the diseased bark of white pines in the spring, cannot infect pines; they infect only ribes.

On ribes leaves, two kinds of spores are produced, an early- and a late-summer form. The early form can infect ribes leaves but not white pines. It is a repeating form, producing several generations in a season and causing local disease intensification on ribes. The late form infects white pine needles but not ribes. The fungus grows in the needles until it reaches the bark. There it causes spindle-shaped diseased areas called cankers. From 2 to 5 years after infection of the needles, spores begin to develop in the diseased bark. Thereafter each spring a new crop of spores is produced; they infect ribes and again start the life cycle of the fungus.

Blister rust may reach new localities by shipment and planting of infected white pine or ribes and by wind-borne spores from infected pines. Distance is not a limiting factor in the spread of the disease by shipment of infected host plants. Investigations in western North America showed that the disease was spread by wind-borne spores from infected pines to ribes over distances of several hundred miles. From infected ribes to the pines, however, the spread usually does not exceed 900 feet. Under especially favorable weather and topographic conditions, the spread sometimes extends for a mile or more. The amount of pine infection rapidly lessens as the distance from diseased ribes increases.

The different native white pines are highly susceptible, although there is some evidence of resistant trees among species. Native ribes species vary widely in their reaction to the rust. Some are highly susceptible, while others seldom take the disease. That fact, however, has had no significant effect on the spread or control of the disease, because susceptible ribes species are well distributed throughout the range of the white pines.

In new localities the disease follows a definite course. It starts as a single infection or several scattered infections on ribes and is transmitted to nearby white pine. Two to five years later the diseased pine produces spores that infect nearby ribes. In turn, the local pine infection increases. Alter this situation has developed in several spots, a favorable rust year causes abundant and widespread infection on the ribes and pines, and many pines die.

The white pines would be doomed within a few years were it not that the rate of spread of the fungus depends on the simultaneous occurrence of several factors: Widespread infection on ribes, abundant production of pine-infecting spores, and a favorable combination of temperature and moisture conditions. The integration of such conditions over extensive areas fortunately are infrequent, and new infection of pine is light in most years. Only in an occasional year is it so general as to cause widespread damage.