DANGEROUS FOREST DISEASES IN WESTERN 



EUROPE 



Scots Pine Blister Rust 



F. Roll- Hansen 



Norwegian Forest Research Institute, Vollebekk, Norway 



Cronartium flaccidum (Alb. & Schw.) Wint. is very similar in 

 morphology, biology, and disease symptoms to C. ribicola, the cause 

 of white pine blister rust. It forms perennial cankers on the stem and 

 branches of pines. Resin flow and dark color of the bark are char- 

 acteristic. TVTiere the bark is killed, the growth stops, and the canker 

 becomes more or less flattened. But above and to the sides of the 

 canker the growth is usually greater than normal. Small branches and 

 twigs may get spindle-shaped swellings. The cankers grow from year 

 to year, and much faster in the longitudinal direction than around 

 the circumference. Small stems may become girdled in a few years, 

 big stems after 10 to 50 or more years. 



Spermogonia are flattened, inconspicuous, 2 to 3 mm. in diameter. 

 Aecia are formed from May to July, along the attacked bark of twigs, 

 and at the border of older cankers. They are blisterlike, 2 to 8 mm. 

 long, 2 to 3 mm. wide, and 2 to 4 mm. high. The wall is thin and 

 hyaline, the yellow-orange color of the spores showing through the 

 wall. Peridium cells are 15 to 40/t in diameter, the walls are verrucose 

 and 4 to 6/i thick. Aeciospores are roundish — ellipsoidal, or somewhat 

 polyhedral, 22-31 X 15-22/x. The greater part of the wall is 3-4.5/1 

 thick and coarsely verrucose; the rest of the wall is thinner, 2-3/x 

 thick, nearly smooth, because the warts are much lower. The wall 

 itself is colorless, but the content gives the spores a yellow-orange 

 color. Uredo- and teliospores are formed on the dicotyledonous 

 hosts on the lower side of the leaves. Uredia are small, up to 0.25 mm. 

 in diameter, pustular. The peridium has a small opening at the top. 

 The uredospores are ellipsoidal or obovoidal, 20-27 X 15-20/x; the wall 

 is spiny ; the pores are inconspicuous. Telia are columnar — filiform, 

 0.4 to 2 mm. long and 0.04 to 0.13 mm. in diameter, horny when dry; 

 they are formed in older uredia. Teleutospores are elongated ellipsoid- 

 al, the ends more or less truncated, 25-56 X 9-14/i. Basidiospores are 

 roundish, about 8/i in diameter. 



Life history. — Scots pine blister rust embraces one host-alternat- 

 ing and one nonalternating race. The host-alternating race (hetereu- 

 form) attacks as haplont some 2-needled pines. The dicaryophyt is 

 very pleophag, as shown in the host list. With Gentiana asc7epiadea 

 as differential host, the host-alternating race can be divided into two 

 formae speciales : F. sp. typica attacks the greater part of the dicotyle- 

 donous genera, but not Gentiana. F. sp. gentianea attacks Gentians. 

 but not Cynanchum. Paeonia is attacked by both the mentioned 

 formae speciales. 



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