IOWA ACADEIMY OF SCIENCE 27 



death of the tree." It is certainly a very active agent in the destruc- 

 tion of the wood of the Box Elder in Iowa. The Box Elder {Negundo 

 aceroides) is one of the most easily injured of our trees, by wind storms 

 which often leave broken branches exposed to the attacks of the wound 

 parasites. 



The Oyster Fungus is an edible species. The spores germinate in 

 the pockets left in a wounded tree, the mycelium enters the wood where 

 it causes a slow disintegration; the wood frequently turns a pinkish 

 color. The white mycelium comes to the surface where the fleshy white 

 brackets appear ; sometimes one or two, but frequently more may occur. 

 The fruiting body 2-5 inches broad is generally convex or sometimes 

 nearly flat, firm and smooth, white or sometimes tinted with reddish 

 yellow or brownish yellow in color. The lamellas broad, extending 

 down on the stem, white. The stipe is usually curved, white or whitish, 

 smooth, 2-4 inches long; spores white, spherical. 



Root Rot Fungus. 



Root Rot Fungus {Polystictus versicolor Fr.) This is one of the 

 most common of our wood destroying fungi in Iowa, very common on 

 the soft maple, box elder, oak, apple, cherry, and other deciduous trees. 

 This fungus has been mentioned by a few writers like Earle, Galloway 

 and Woods, but has not been regarded as causing serious trouble. 

 However, F. S. Earle^^ many years ago attributed a root rot of pear 

 trees in Illinois to it. Galloway and "Woods^'' considered that it was the 

 cause of root rot of many trees. 



It is common in Europe as in the United States and was known to 

 Linnaeus, who described it. Fries, however, placed the fungus in the 

 genus' Polystictus'^*, the species becoming P. versicolor Fr}^ 



The spores germinate in the moisture which collects in the pockets 

 of wounds and in the wood forms a white mycelium, which occurs not 

 only on the roots, but in the interior of the wood as well. It causes 

 disintegration. Cherry and other fruit trees diseased have a tendency 

 to overbear; a gust of wind will cause such a tree to split more easily 

 in the crotches or to break off exposing the thoroughly disintegrated 

 heart and sapwood. According to Earle on pear trees the white my- 

 celium covers the root causing the decay, the growth of the tree be- 

 comes stunted and there is a tendency to overproduction of fruit. It 



"Pear Diseases Caused by Fungi. Trans. 111. Hort. Soc. 20:168. 

 "Yearbook U. S. Dept. Agrl. JS.96.-250, /. 55. 

 "Nov. Symb. 54. 

 '^Syst. Mycol. J. -368. 



