THE WHITE-PINE BLISTER RUST. 11 



affected bark. In such cases the bark becomes scaly, while on the 

 green parts of the tree it is still smooth (figs. 1 and 2) . Oftentimes the 

 tree remains alive several years after being completely girdled with 

 the dead bark. In such cases, growth continues above the canker 

 until considerable swelling is produced. The fruiting bodies form each 

 spring, both above and below the dead area in the bark, additional 

 areas of the bark being killed each year (fig. 1). In this way the dis- 

 ease progresses sometimes 2 or 3 feet from its original point of entry. 



On currants and gooseberries the parasite is known to attack only 

 the leaves. Here it has two distinct forms, which may occur upon 

 either the currant or gooseberry, or both. The first or summer 

 form is seen on the lower surface of the leaves in the shape of small, 

 mealy, bright-yellow masses hardly larger than a pinhead, which may 

 be very sparse or may be so abundant as to form a continuous, 

 mealy layer over considerable portions of the leaf surface (PL I, B 

 and (7) . Upon shaking such a leaf, or blowing it, one will perceive a 

 cloud of yellowish powder set free. This form of the disease may be 

 found from the middle of June until the leaves fall, although it is not 

 so common in the autumn as in July or August. 



The second or autumn form of the disease upon currant or goose- 

 berry leaves usually is found from the latter part of July until the 

 leaves fall. It occurs in the form of small groups of 3 to 10 or 12 

 short, hairy outgrowths, scarcely a quarter of an inch long, ordinarily 

 arranged in small circles (PL I, D). These, like the summer form, 

 may be very scattering on the under surface of the leaf or may be so 

 abundant as to form a hairy coating on the entire lower surface. 

 These hairs are brownish in color, but in moist weather they assume 

 a grayish brown tint, Both of these forms occur upon the lower 

 surface of the leaf, and usually one must turn a leaf over in order to 

 find out whether it is affected or not. While the disease may kill 

 small areas of the upper leaf surface, so that they look brown or 

 yellowish, this symptom can not be depended upon when one is 

 searching for the trouble. 



LIFE HISTORY OF THE PARASITE. 



The fungous parasite causing the white-pine blister rust is known 

 by two different technical names. The form upon the pine is known 

 as Peridermium strobi, while upon the leaves of currants and goose- 

 berries it is known as Cronartium ribicola. The latter name is now 

 ordinarily used for the organism, both upon pines and upon currants 

 and gooseberries. The life history of this parasite is very complex, 

 but it is paralleled by a great number of related fungi. 



A period of incubation follows the infection of white pines. This 

 period may vary from less than one year up to six or more years, 

 during which time there is absolutely no external indication of the 

 disease. Then the bark begins to swell at the point of infection 



