98 REPORT OF THE COMMISSIONER OF AGRICULTURE. 



irregular oiitline. Attaclied to one side of each, mass are two very- 

 fine, hair-like cilia, by the vibration of which it swims about in the 

 water with an animal-like motion; hence these bodies have been 

 named zoospores. In from fifteen to twenty minutes the cilia fall 

 off, the zoospore comes to rest, assumes a definite outline, takes on a 

 cell-wall, and immediately pushes out a prolongation or germinal 

 tube, which penetrates the epidermis, and, continuing its growth 

 within the tissues of the leaf, develops into what we have called the 

 vegetative or mycelial portion of the fungus. 



The germination by zoospores is the most common, and perhaps we 

 might say the normal habit of the Peronospora of the vine; and ex- 

 perience in culture leads to the belief that no form of germination 

 will take place except in the presence of water. A damp atmosphere 

 is insufiicient; there must be the actual presence of water, in the form 

 of drops of rain or dew, to effect the formation of the zoospores. 



Temperature exercises a considerable influence over the germi- 

 nation of the conidia, that which is most favorable being between 

 75° and 95° F. At lower temperatures germination takes place more 

 slowly; but the temperature may be reduced to 82° without destroy- 

 ing the vitality of the conidia. Exactly how long these bodies will 

 retain their vitality in a moist atmosphere has never been determined, 

 but it is known that dry air, particularly a dry wind, is destructive 

 to them. Experiments have shown that ina dry atmosphere the co- 

 nidia contract in a very short time and shrivel up, or burst and lose 

 their contents. 



During the summer or season of growth the Peronospora expends 

 its energies in the production of the conidia, whose office is the imme- 

 diate dissemination and propagation of the fungus. To tide over the 

 season of winter another spore-form is produced, which is furnished 

 with thickened walls, and is still further protected by being embedded 

 within the tissues of the host plant. These are the result of a special 

 sexual process, and are termed oospores, or, more popularly, "winter 

 spores." Their formation begins as a slight swelling at the ends of 

 branches of the mycelium. This swelling finally attains a diameter 

 of about jifoo of an inch, assumes a spherical shape, and the cell- wall 

 covering it becomes thickened and pale yellow in color. _At one side, 

 arising from the branch that bears the oogonium or sack in which the 

 oospore is developed, another and smaller body is formed, which is 

 termed the antheridium. The antheridium, without detaching itself 

 from its support, comes into close contact with the oogonium at an 

 early period, and later on, by a special mechanism, the granular pro- 

 toplasmic contents of the former is doubtless conveyed to the latter, 

 by which means it is fertilized, and the contents of the oogonium then 

 develops into an oospore. 



The germination of the winter spore has never been satisfactorily 

 determined; but, however it may take place, it probably does not occur 

 until the spring or early summer following its formation. 



ACTION OF THE FUNGUS. 



On the leaves. — Pale green or yellowish spots of irregular size and 

 outline appear upon the upper side of the leaves, and corresponding- 

 points on the lower surface soon exhibit the outside development — 

 the spore-bearing filiaments of the fungus— in the form of white 

 patches, that are very conspicuous on the smooth-leaved varieties of 

 grapes. As the disease progresses the yellowish spots of the upper sur- 



