116 FOREST PROTECTION 



The tree weeds, e. g., Halesia (Mohrodendron) , gum and 

 calmia, might be exterminated in days to come with the help 

 of their fimgus enemies. 

 E. Synopsis of the orders of damaging fmigi. 



I. Order Phycomycetes. Family Peronosporece. 



The mycelium is unicellular. The propa- 

 gation is effected by numerous branching hyphae 

 forming at their tips little sacs or sporangia in 

 clusters or chains (conidia). These are carried 

 by wind to other plants where they germinate 

 at once, forming a tube that penetrates the leaf. 

 If the leaf is wet, the contents of the sporangia 

 break up into a number of zoospores which de- 

 velop the characteristic hyphae of the fungi. 

 Sexual reproduction occurs in most species and 

 consists of a gametangia cut off from the ends 

 of the hyphae and fertilized by male gametes 

 developed on branches (antheridia) of the game- 

 tangia bearing hyphae. The resulting thick walled 

 gametospore tides the fimgus over winter. 



American representatives are not fully known. 

 Some bad nursery fimgi belong to this family 

 (notably Phytophtora omnivora). 



II. Order Ascomycetes. 



1st. Family — Pyrenomycetes. 



Flask-shaped frutifications (peri- 

 thecia) are characteristic of this fam- 

 ily. Within the perithecia, which are 

 open at the top (angiocarpous), occur 

 numerous asci, each containing eight 

 spores. Preceeding the formation of 

 perithecia, conidiospores are usually 

 formed which are especially efficaci- 

 ous in disseminating the fimgi. Ex- 

 amples: Nectria on maple and beech. 



2nd. Family— Discomycetes. 



Distinguished by open gymnocar- 

 pous apothecia (cup-shaped recepta- 

 cles, bearing freely exposed asci). 



The Discomycetes are unimportant 

 for the American forester, none being 

 observed as damaging our trees. Rhy- 

 tisma acerinum frequently forms large 

 black incrustations of pseudo-paren- 

 chyma on the leaves of maple, conidia 

 developing in the summer and mature 



