REED: PHYTOPATHOLOGY 159 



inated in the flowers, finally forming a resting stage, so-called "gemmae," be- 

 neath the glumes, which later produced the infection in the young seedling. 



Leveille (1851) brought out his standard work on the powdery mildews, 

 describing the genera which, for the most part, are accepted today. De Bary 

 (1863) worked out the main points in the life history, describing the sexual 

 organs and Harper (1895, 1896, 1905) investigated the cytology of sexual re- 

 production and ascospore formation. From the taxonomic standpoint, Salmon's 

 monograph, published as a Memoir of the Torrey Botanical Club in 1900, was 

 a landmark in our knowledge of the powdery mildews. 



Among the downy mildews, potato blight has been the subject of intensive 

 investigation wherever potatoes are grown. Studies have been concerned not 

 only with the pathology and the control of the organism, but also with its life 

 history. Berkeley, in the late 1840's, made the first detailed studies. It remained 

 for Clinton (1911) to discover the oospores, Jones, Giddings, and Lutman 

 (1912), and Pethybridge and Murphy (1913) adding further data on the con- 

 ditions necessary for sexual reproduction. Gaiimann (1923) brought together 

 the results of his detailed studies on the genus Peronospora. 



Great strides have been made in the large group of the Ascomycetes and the 

 connection between the conidial and ascospore stages of many have been estab- 

 lished. L. R. and C. Tulasne (1853) described in detail the life history of 

 Claviceps purpurea, which causes the ergot of rye. Aderhold (1894) and 

 CHnton (1901) established the connection between the common apple scab 

 organism and the ascocarp known as Venturia inaequalis. Norton (1902) dis- 

 covered the apothecia of the brown rot of stone fruits, although Schroeter 

 (1893) concluded that the species of fungi causing brown rot belonged in the 

 genus Sclerotinia and Woronin (1898) showed that there were two distinct 

 species of this genus, 5'. fructigena and 5". cinerea. 



2. Physiologic specialization. Proper identification of hosts is basic 

 to an advance in the knowledge of pathogens which cause disease. Taxonomists 

 have been concerned largely with genera and species, while the agronomists and 

 horticulturists have been interested in the cultivated varieties. Students of the 

 parasitic fungi must necessarily be familiar with the host plants upon which 

 they grow since, in works dealing with their classification, the keys are largely 

 based upon the proper host identification, and Arthur's recent Manual of Rusts 

 (1934) is a fine illustration. 



One of the great advances in pathology since 1867 is the demonstration of 

 physiologic specialization. Schroeter (1879) called attention to this phenom- 

 enon in connection with certain rusts on Carex. The first important work, 

 however, was that of Eriksson (1894) who made an intensive study of Puccinia 

 graminis from the cultural standpoint. On the basis of his experiments, he rec- 

 ognized 6 jormae speciales — Avenae, Secalis, Tritici, Airae, Agrostidis, and 



