126 INTERNATIONAL CONGRESS OP VITICULTURE 



STUDIES ON PLASMOPARA VITICOLA (DOWNEY MILDEW 



OF GRAPES). 



By C. T. GREGORY, 

 Cornell University, Ithaca, New York. 



Historical. 



Early writers frequently mention the "mildews" and "blights" which 

 "sickeneth" the vines but these troubles were attributed to some visitation 

 of Providence or to atmospheric disturbances. As late as 1870 the disease 

 was not supposed to be caused by any organism, although a conjunct fungus 

 was always found. Saunders (18G2) v as of the opinion that the vine was 

 previously injured and partially decayed before the fungus was able to gain 

 foothold. 



Lippincott (1866) in a summary of the various theories of the origin of 

 the disease held at that time, states that they were essentially as follows: 

 that it is spontaneously developed, that the spores are inhaled through the 

 stomates and distributed throughout the plant and that during moist weather 

 the plants become gorged with water resulting in an incipient stage of 

 decomposition so weakening the plants that they are unable to prevent the 

 formation of the mildew. Lippincott, himself, offers the explanation that 

 ozone in the air prevents the formation of the mildew and that during moist 

 weather this prophylactic gas is not present, hence the abundance of the 

 disease during such periods. 



Farlow (1876) described the disease and correctly attributed it to the 

 fungus, Peronospora viticola De Bary. 



Name and Classification. 



The fungus was first collected in America by Schweinitz, in 1834, but 

 was erroneously referred by him to Botrytis caca Link. It was studied by 

 Berkeley and Curtis (1848) who described it as a new species, Botrytis viti- 

 cola. De Bary (1863) later studied the fungus carefully, described and 

 figured the sexual as well as the asexual stage and referred it to the genus 

 Peronospora, naming it Peronospora viticola. 



Schroeter (1886) subdivided the genus Peronospora into Peronospora 

 and Plasmopara. The differences may be briefly stated. The conidiophores 

 of Plasmopara are monopodially branched, never clearly dichotomously as 

 in Peronospora. The ultimate branches or sterigmata of Plasmopara are 

 dichotomous or trichotomous and after the dissemination of the conidia 

 their ends are flat or concave. In Peronospora the sterigmata are always 

 dichotomous and the ends are acutely or obtusely pointed but never flat. 

 Finally the germination of the conidia of Plasmopara is typically by means 

 of zoospores or by the entire contents of the spore slipping out and then 

 producing a germ-tube. Rarely a germ-tube may be produced directly. The 

 conidia of Peronospora always germinate by means of a germ-tube. 



Berlese and de Toni (1888) redescribed the fungus as Plasmopara viti- 

 cola. 



