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Disease Management

Quarantines and host removal

One of the earliest attempts to control blister rust was to destroy infected white pines as they were found, but the relatively long latent period of infection (the time from initial infection until symptoms or signs are produced) doomed this method. The next method attempted was to prevent further introduction and spread of the disease by laws (i.e., quarantines) that prohibited or restricted the movement of host plants from one region to another. The United States had no laws restricting the introduction of plants until the 1912 Plant Quarantine Act, which was a direct result of the introduction of blister rust. Quarantine No. 1 prohibited importing five-needled (i.e., white) pines into the country. A later quarantine, No. 26, prohibited taking currant and gooseberry bushes west of the Mississippi River. This was an attempt to restrict the disease to the eastern United States. But, as was later learned, the rust was already in the west, having been introduced on imported pines to Vancouver, British Columbia, in 1910. However, Quarantine No. 26 also authorized the destruction of cultivated currants, and this was accomplished in a relatively short time (Figure 10). The greatest effort at control was to destroy Ribes bushes from pine stands, but this method also failed. This attempt was based on the fact that spores from pine do not infect other pines, but that infection of pines came only from Ribes. Eradication of cultivated Ribes was relatively successful, but the abundance of wild Ribes spp. in remote and difficult terrain (Figure 11, Figure 12) made this impractical. Some eastern states still regulate the planting of susceptible, cultivated Ribes spp. (To find regulations of individual states, search for "ribes" at http://nationalplantboard.org/)


Figure 10

Figure 11	Figure 12

The eradication of wild Ribes was an extensive program and required much hand labor that often was not available. However, the blister rust control program got a tremendous boost when government relief programs during the Great Depression of 1933 provided work for the unemployed. The Civilian Conservation Corps was one of these programs and was designed to utilize labor on projects that would not compete with the private sector. These projects were primarily forestry and soil conservation. As many as 11,000 CCC men were employed in a single year for Ribes eradication in the national forests (Figure 13, Figure 14). In 1946, 175 people were employed for barberry eradication against wheat stem rust in the entire U.S. (see Stem rust of wheat lesson) vs. 2,500 people for Ribes eradication in the Northwest.


Figure 13	Figure 14

Rust hazard zones

High rust hazard zones occur where cool, moist air settles into depressions, such as valleys, stream bottoms and openings in forests, where air drainage is poor. In addition to creating favorable conditions for infection, air currents can carry spores for relatively long distances, i.e., several kilometers/miles, into these depressions. Generally, upper slopes and ridges are less conducive to blister rust infections than benches and valleys.

Pruning

Pruning to remove lower branches on young trees and removal of diseased branches (Figure 1) is recommended in some areas. Pruning of young trees should take place over several years, and no more than one-third of the branches should be removed at any one pruning. Removal of branches with cankers closer than 10-15 cm (4-6 in.) of the trunk is not feasible since the infection is probably already in the main stem. In addition to disease control benefits, pruning also promotes development of better quality wood for lumber. However, pruning is labor intensive and costly and requires a degree of judgment and skill.

Figure 1

Genetic resistance

The occurrence of apparently resistant trees in areas where the rust was severe (Figure 15) was recognized in eastern white pine as early as 1937. Surveys of heavily rusted sugar pine and western white pine stands led to selection of large numbers of apparently rust-free, and presumably resistant, trees as parent trees for disease resistance breeding programs. The programs for the western pines started about 1950. By crossing and backcrossing these parent trees and the subsequent progeny, a number of rust-resistant lines have been developed. These resistant trees are used to establish seed orchards and some are now producing seed for general distribution.

Figure 15

Several resistance mechanisms such as shedding of infected needles and slow canker development have been identified. Incorporating several different types of these mechanisms helps guard against the loss of resistance based on a single type.

Integrated management

At the present time, removal of highly susceptible Ribes spp., avoiding planting pines in high hazard zones, and planting resistant pines, if available, are recommended. In addition, pruning off lower branches and infected branches is suggested in some areas even though these measures are costly and require skilled workers. See Maloy, O. C. 2001. White pine blister rust. Online. Plant Health Progress doi:10.1094/PHP-2001-0924-01-HM for further information on management of white pine blister rust .

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