THE PERONOSPOREtE OF IOWA. 



By T. H. McBRIDE and A. S. HITCHCOCK. 



The Pcronosporcce are delicate fungi mostly parasitic on other 

 and higher plants. So far as they are popularly recognized at 

 all, they are variously reckoned with moulds, mildews and 

 blights. The botanist distinguishes four well defined genera, 

 Pythiam.Phytophthora^ Pcronospora, and Cystopus, all of which 

 are known in this country, three appearing in the accompanying 

 list of Iowa species. As will be observed our species are all 

 parasitic and affect a very considerable number of plants, few 

 however of any economic value. The different species run 

 through the entire season; some appear on the leaves of their 

 respective hosts before the latter are fairl}- above the ground 

 in the spring, some attack the cotyledons of bursting seeds, 

 while others, as the species of Cystofiis and Pcronosfora Icp- 

 tosfcrma, come late in the fall, the last named whitening the 

 leaves of Artemisia in September and October, simulating the 

 hoar frost before which at length host and parasite sink 

 together. As already intimated the Peronosporas attack the 

 leaves of plants they infest, but sometimes the young and 

 growing stems are by no means exempt. The mycelium of 

 the fungus spreads through all the succulent tissues of the leaf 

 attacked, passing between but not through the cells in all direc- 

 tions diverting to the development of the parasite that which 

 should go to the development of the host. The fungus is 

 reproduced in two ways: by spores formed in special recepta- 

 cles developed within the tissues of the host, and by conidia, 

 spores more simply produced by abstriction from the ends of 

 certain branches of the mycelium which project, either by way 

 the stomata or of the ruptured epidermis, from the surface of 

 the leaf, or supporting structure w'hatever it may be. It is 

 by means of the conidia and the modes of their abstriction and 

 germination that the current classification is effected. Most 



