270 Minnesota Plant Diseases. 



Tar spot of willow [Rhytisma salicinwn (P.) Fr.]. This dis- 

 ease is very similar to that of the tar spot of maples. (See the 

 latter.) The tar spots become black in August and September 

 and may be formed in great numbers. Sometimes leaves are 

 completely covered by the spots and are consequently seriously 

 injured. When the disease threatens to become serious the 

 spotted leaves should be collected and burned in fall. 



Ring fungus or ring disease of cone-bearing trees (Rhizina 

 inflata QueL). This fungus is a member of the morel group of 

 sac fungi, but might almost as well be included in the cup fun- 

 gus group. The fruiting-body is a flattened, crust-like object 

 of very dark brown or blackish color, fleshy in consistency and 

 sticky in moist weather. It may attain a width of several inches 

 and is usually irregular in shape. It is found at the base of 

 trees or on old stumps and is commonly a saprophyte. It is 

 attached to the soil or tree stump by numerous strands of the 

 mycelium which run into the substratum for some distance. 

 The sacs of the fruiting body are long cylinders and contain 

 eight spores each. The mycelium may under favorable conditions 

 become parasitic on the roots of trees. It grows around the 

 root, killing the tissues, and may thus ultimately effect the death 

 of the entire tree. It has not yet been reported from Minnesota 

 but is known in Wisconsin and very probably exists in our 

 state. The fruiting bodies should be destroyed. 



The green mold rot of timber (Species of Penicillium). 

 There seems to be some evidence that the common green 

 molds, which are so conspicuous as saprophytes on starchy food 

 materials and on cheese, are capable in themselves of causing 

 rot of timbers. Such a mold thrives on the starchy material 

 which is stored up in the medullary rays of woody tissues and 

 from this point invades the woody fibres. The green mold very 

 frequently accompanies other rots and may assist in the disin- 

 tegration of the wood. It is doubtful, however, that it is ever 

 in itself alone a very serious cause of the decay of timbers. The 

 ordinary mass of green mold is composed of thousands of 

 minute, brush-like clusters of strings of green spores which are 

 exceedingly resistant and can retain their power of germina- 

 tion for a long time. The winter spores are formed in sacs, 

 produced in closed capsules, which open only by irregular split- 



