144 INTERNATIONAL CONGRESS OF VITICULTURE 



the grape attains the size of a pea there are no stomates on its surface but 

 in their place occur a number of lenticle-like structures. Thus we find again 

 the reason that it is not possible to infect the berries except when they are 

 very small. It can be shown without difficulty that the older berries become 

 infected by mycelium which grows down through the pedicles and peduncles 

 into the fruit. This view is further justified by the fact that there are a few 

 stomates on the pedicle and peduncle thus allowing the possibility of infec- 

 tion at these points. 



According to Miiller-Thurgau (1911) slight tears or injuries may allow 

 infection through the upper surface. He bases this statement on certain ex- 

 periments in which this surface was punctured with a needle at the points 

 of inoculation. To verify this statement similar experiments were conducted. 

 Spore-laden drops of water were placed on the upper surface of the leaf at 

 numerous points. In some cases the leaf was punctured with a needle 

 through the drop and in other cases injured before the drop was placed 

 upon the leaf. Checks of drops on the uninjured upper surface and on the 

 lower surface were also run. No infections were obtained except through the 

 lower surface. It was then thought that possibly the openings were not large 

 enough. With this in view another series of experiments was conducted in 

 which the leaf was torn as much as possible without allowing the water to 

 penetrate to the lower surface. In some cases jagged tears fully 5 mm. long 

 were made. As before the results were all negative. Hence in our opinion 

 injuries to the upper surface do not permit infection unless the water is 

 permitted to gain access to the lower surface through the opening so that 

 the swarmspores may swim through the opening. 



To determine whether the swarmspores produced from conidia which 

 had fallen on the upper surface of the leaf, might be able to produce infec- 

 tion if conditions were such that they could gain access to the lower surface 

 the following experiments were performed. The leaves were held with the 

 lower surface firmly but not tightly against plates of perfectly clean glass. 

 Drops of water were than so placed that there was a continuous layer pass- 

 ing from the upper surface over the margin and between the leaf and the 

 glass. Fresh conidia were then placed in the water on the upper surface 

 as far as possible from the margin. In three cases out of five (60 per cent) 

 infections occured. In the other cases the explanation of the failure lies in 

 the fact that the film of water was broken on the upper surface of the leaf 

 probably before conidial germination. This indicates that this method of 

 infection is possible but that it may occur only in the most favorable con- 

 ditions because of the ease with which the film is broken. This is an exact 

 verification of the work of Ravaz and Verge (1911) relative to this same 

 question. 



Pathological Histology. 



The chlorophyll, the plastids and at times the nucleus of the cell are 

 eventually disintegrated. This process is gradual, being not only confined 

 to certain cells or groups of cells, but even to the individual plastids within 

 a cell, certain of them remaining green while others are entirely yellow. 

 The first microscopical evidence of this change occurs in about three days 

 after inoculation. 



