Minnesota Plant Diseases. 279 



and cottonwoods. Older trees are seldom if ever seriouslv 

 affected. Only the summer and winter spores occur on the 

 poplar and cottonwood while the cluster cup stage is found on 

 some other plant. 



The summer spore groups are small, bright-yellow, resin- 

 like cakes before maturity and when ripe produce a bright- 

 yellow powdery mass of spores. Abundant, sterile, club- 

 shaped cells are found intermixed with the summer spores. 

 The summer spores are very abundant on poplar leaves in 

 August. In early autumn the winter spores commence to 

 form and soon, small, dark-brown, crust-like spore groups 

 are produced, which later become black in color. Fallen pop- 

 lar and cottonwood leaves, particularly those from the lower 

 parts of the trees, are often entirely blackened and the under 

 surface is almost completely covered with the black crust of 

 winter spores. The winter spores pass the winter on the fallen 

 leaves and in the following spring germinate without separating 

 from the spore group. 



In case of a serious epidemic the leaves should be collected 

 and burned in the fall. 



The birch leaf rust [Mchimpsora bctulina (Pcrs.) II' int.]. 

 This rust is closely related to that of poplars and the rotation 

 of spore forms is similar. The appearance of summer- and win- 

 ter-spore forms also resembles the rust of poplars. 



The willow leaf rust [M clam psora salicis-caprac (Pcrs.) 

 ll'int.]. The willow leaf rust is exceedingly abundant on all the 

 species of willow in Minnesota. This rust is also similar in 

 general to that on the poplar leaf and is a closely related form. 

 The leaves of younger shoots are not uncommonly so badly af- 

 fected that they shrivel up and die. The winter spore groups 

 form similar black crusts to those of poplar rusts and rest over 

 winter; the spores germinate the following spring. This dis- 

 ease occasionally occurs in sufficient amount to threaten seri- 

 ously the cultivated willows. When occurring in such trouble- 

 some quantities the fallen leaves with their winter-spore groups 

 should be burned in late fall. 



The cedar apples of red cedar (Gymnosporangium macropus 

 Link and G. globosnm Fat 1.). (See Leaf Rust of Apples Dis- 

 eases of Orchards.) 



