386 Minnesota Plant Diseases. 



Rust of Pyrola \Chrysomyxa pirolae (DC.} Rostr.]. On all 

 species of Pyrola found in the northern part of the state occurs 

 a Pyrola rust. It is found chiefly in the spring and often occurs 

 in great abundance. The cluster-cup stage is not yet known, but 

 is probably to be found upon some needle-leaved evergreen tree. 

 The summer spores are by far the most abundant and appear in 

 early summer. Their spore groups often completely cover the 

 lower surface of the leaves with a golden-orange powder-mass. 

 The winter spores form darker, crust-like masses, which are much 

 less abundant. See Leaf Rust of Pines, Chapter XVI. 



The rust of milkweeds [Cronartium asclepiadeum (Willd.) 

 Fr.~\. This rust is very well known in Europe as a parasite upon 

 many species of the milkweed family. It seems to be very rare, 

 or entirely wanting, on milkweeds in this state, but what is prob- 

 ably the same rust has been found upon the leaves of the oak, not, 

 however, in great abundance. The winter spores, preceded by 

 the summer spores, are formed on the above-named hosts, and 

 the cluster-cups probably on the pines. It is the disease which is 

 produced upon pines that is of chief importance. See Leaf 

 Rust of Pines, Chapter XVI. 



Cluster-cup rust of various wildflowers (Species of Aecid- 

 iuni). Many of our spring wild flowers, such as buttercups, may- 

 flowers, columbines, squirrel-corn, Dutchman's breeches, Old 

 Man's beard, white cohosh, Solomon's seal, false Solomon's seal, 

 lilies, evening primrose, elder, violet, etc., are attacked by the 

 cluster-cup stages of rusts. In comparatively few cases is the 

 connection between these and the winter-spore stages known. 

 These cluster-cups are found, often in great abundance, from 

 early spring until midsummer and are usually found on yellowish 

 spots, on the under surface of the leaf. On the upper surface of 

 the same spots are formed the little accessory spore-capsules 

 known as pycnidia. These are flask- or pear-shaped bodies, 

 opening in small yellow or black dots, onto the upper surface of 

 the leaf. They often exude a sugary solution. Their behavior 

 is problematical but they are probably vestiges of former func- 

 tional male, reproductive cells. 



A few of these cluster-cup diseases, such as the cluster-cup 

 rust of gooseberries and currants, have been considered in detail 

 Most of them are of minor importance economically, so that pre- 

 ventive means need not be considered. 



