4 BULLETIN 212, U. S. DEPARTMENT OF AGRICULTURE. 



flowers, it is believed that Peridermium cerebrum can enter young 

 seedlings or the tender portions of more mature growth without first 

 having the bark broken. Entrance in this manner must, out of 

 necessity, be aided by sufficient moisture for germination and to per- 

 mit a rapid penetration by the young mycelium. On the sandy 

 plains of the Great Lakes region rain water disappears almost immedi- 

 ately and the sand becomes heated about the isolated tree groups, 

 causing a rapid evaporation from the surface of the trunk and 

 branches and leaving the moisture content of the outer bark at a 

 minimum. In whatever maimer the fungus may enter its host, 

 directly or through wounds, the number of galls and imperfect 

 branches is usually much less on trees of the sandy barrens than in 

 more moist regions. 



In swampy areas the jack pine grows in close stands. Here the 

 percentage of infected trees is much greater. The trunks of the 6 to 

 12 year old jack pines are often covered with swellings stunting the 

 growth of the trees very rapidly (PL I, fig. I). Trees so infected 

 never reach maturity and may continue living for ail indefinite 

 period in a stunted condition, to be finally blown over by the wind 

 or broken down by the snow. The 1 to 4 year old seedlings are quite 

 often attacked. With these, as is often the case with larger trees 

 which through mechanical injury may become infected at the ground, 

 the gall is formed directly at the base of the main stem. When a 

 seedling is infected there or higher up on the stem, it develops into a 

 deformed growth after the manner of a witches'-broom (PL I, fig. 2) 

 and never attains a height of more than 2 or 3 feet. The perennial 

 mycelium of the fungus thrives in the cambial layer and in the living 

 parts of the sapwood. Trees with a single infection on the trunk 

 occurring at the age of 4 to 6 years are known to support the living 

 mycelium of the fungus to the advanced age of 70 to 80 years. Usu- 

 ally, however, the excessive production of resin in the infected 

 tissues infiltrates the woody portion of the trunk, and the sap supply 

 is cut off so that death results in a comparatively short time (fig. 2) . 

 This is especially true in young seedlings. Peridermium galls are 

 frequently observed a foot or more in diameter. Trees supporting 

 galls of this size had succumbed in every instance to the disease. 



Some knowledge of the damage done by Peridermium cerebrum to 

 the jack pine may be obtained from notes of a pathological survey 

 by the writer in the national forests of Michigan. Out of 100 trees 

 of an average plat on dry sandy soil, not selected because of any 

 pronounced diseased condition, 50 per cent were heavily infected, 

 while only an occasional tree out of a second hundred on similar but 

 moister soil was absolutely free from the disease. Out of 100 trees 

 taken from the swamp type, practically all were infected. Not all 

 the trees were infected seriously. A tree bearing a single branch gall 



