454 



AMERICAN FORESTRY 



The blister rust is not an insect as many suppose, but 

 a fungus; that is, a plant which has no leaves and feeds 

 on the live tissues of other plants. When a white pine 

 tree is attacked, the invisible "roots" or mycelium of the 

 blister rust fungus start to grow into the base of the 

 pine needles soon after the spores are blown to them 

 from currant or gooseberry leaves. The disease con- 

 tinues to spread from the needles to the twig below, then 

 into the branch and finally the trunk of the tree. After 

 an indefinite number of years, but usually two to three, 

 the spore bearing sacks push through the pine bark. 

 These sacks are bright orange in color, about the size 

 of a grain of corn, and give the pine bark a blistered 

 appearance, hence its common name. Eventually, the 

 tree dies. 



The spores formed in the blisters germinate and grow 

 when they fall on a currant or gooseberry leaf. About 

 ten days later little yellow or rusty brown spots appear 

 on the under surface of the leaf. Late in the summer 

 the currant or gooseberry leaves frequently become so 

 completely covered with these spots that the entire lower 

 side of the leaf has a rusty aspect. During the summer 

 and fall, dark brown horn-like growths, not much 

 larger than a pin point, also develop on the under side 

 of the leaves. These "horns" bear the spores that cause 

 the disease on white pine trees. 



Very rarely, the blisters break out on pine twigs a 

 year after they are infected. At other times it takes 

 many years for the disease to become visible. In one 

 instance, trees infected with the blister rust at the time 

 they were brought into this country from Germany in 

 1902 apparently first produced "blisters" this spring 

 a period of sixteen years for development. Eighteen 

 years is the longest time on record for the period be- 

 tween the time the disease entered the tree and the first 

 appearance of the "blisters." 



Aside from the peculiarity of this disease in attacking 

 the pines only through the medium of currants and 

 gooseberries, the most important .point bearing on its 

 control is the distance to which the spores are carried 

 from currants or gooseberries back to the pines. Infor- 

 mation on this point is furnished by a careful study of 

 an area of infected native white pines in one of the 

 New England states. This study was made recently by 

 Mr. George A. Root and Mr. H. E. Grupe. The topog- 

 raphy of the area is rolling, flattening out into an 

 open river-valley on the north. The low hills are covered 

 with dense stands of almost pure native white pine. 

 Between these stands are open fields and pastures, with 

 here and there scattered pines. All of the pines were 

 carefully examined within a radius of one and one-half 

 miles of the infected area. 



The infection was traced to cultivated currants which 

 grew in a garden at the eastern edge of the infected pine 

 area, and which were heavily diseased with blister rust 

 when destroyed in 1917. The individual pine infections 

 seemed to date back to 1912-13. The original source 

 of the infection is not known, but it is possible that it 

 was brought in on the currant bushes from the nursery 



where they were purchased. Within a radius of 500 

 feet of these bushes there were 1360 pine trees, of which 

 43 out of each thousand were infected. In a zone 500 

 feet wide outside of the central circle, there were 7840 

 white pines, of which 33 trees out of each thousand were 

 attacked by the rust. In a 500-foot zone around the 

 second circle (that is in the zone lying between radii of 

 1000 feet and 1500 feet from the diseased currants), 

 there were 14,710 pine trees, and only 4 trees out of each 

 thousand were infected, or less than one tenth as many 

 as at the center of the area. 



The disease progressed westward and slightly south 

 from the currant bushes, following the prevailing winds. 

 Pines growing in the open and along the borders of 

 the woods were most severely affected. Beyond this, 

 to the west, three diseased trees were found, slightly 

 less than 1800 feet from the infected currants at the 

 center of the area. However, as these were close to 

 other cultivated currants, known to be diseased in 1917, 

 it was concluded that these infections probably were not 

 part of the larger infection area. 



Trees of all ages and sizes were attacked by the blister 

 rust, without discrimination. In each 100 infected pines, 

 36 trees were 1 to 12 years old, 45 trees were 13 to 24 

 years old, and 19 trees were 25 years old and upward. 

 More than half of the youngest age group (1-12 years) 

 were diseased on the trunk, while of trees 13 to 24 years 

 less than one-tenth had the trunk attacked. The trees 

 above 25 years of age had no disease on the trunks, but 

 only on the branches. This does not mean that the 

 j trunks of the larger trees will not become diseased, but 

 i only that sufficient time has not elapsed for the disease 

 to have run its course. Usually it requires a number of 

 years for the fungus to work its way down the branches 

 of a large pine, girdle the trunk and kill the tree. Small 

 trees succumb to the disease more quickly and even these 

 may live 3 to 6 or more years after the disease is appar- 

 ent on the branches. But however slowly the disease 

 progresses, death is none the less certain. It is especially 

 important, however, to protect young pines by the re- 

 moval of currant and gooseberry plants, because a tree 

 25 years old, though attacked, may live long enough to 

 produce a saw log, but a tree 5 or 10 years old will die 

 from the blister rust long before it becomes merchant- 

 able. 



Within a radius of 4 to 12 miles of the original infec- 

 tion area just described, there were about 20 secondary 

 pine infection centers. All of these were near cultivated 

 currants and gooseberries, and most of them were small 

 twigs on a few trees, indicating recent attack. The most 

 probable explanation is that the diseased pines in the 

 oldest infection area produced spores in great quantities, 

 infecting nearby currants and gooseberries. As the 

 disease can pass directly from one currant or gooseberry 

 bush to others, the disease thus advanced from the 

 nearby currants to the currants in the surrounding 

 country. 



White pine blister rust work in the New England 

 States and in the Champlain Valley in New York is 



