694 Journal of Agriculture, Victoria. [10 Nov., 1917. 



pores, by wliich. the germ tubes can gam eutry into the interior of the 

 tissues, where the mycelium grows raiiiilly, fonning the characteristic 

 " oil spot." Genainatiou of conidia is impossible unless in a drop of 

 water, and this must remain on the surface of the leaf for a sufficient 

 time. Even though germination has occurred infection cannot take 

 place unless one or more of the zoospores can reach a stoma or breathing 

 pore; they are incapable of penetrating the outer skin of the vine at 

 any other point. The stomata of the vine are to be found almost 

 exclusively on the under surface of tlie leaf. There are scarcely any 

 on the upper side. Hence infection occurs almost exclusively on the 

 under side of the leaf. 



According to Kavaz (see p. 696), the period of incubation in France 

 is usually seven days. After the lapse of this time, the oil spot is fully 

 formed and ready to produce spores in enormous numbers should weather 

 conditions prove suitahle; otherwise no harm can result, though it 

 continues to be a source of danger, since in mild, but dry weather, it 

 can remain latent for a varying time, only awaiting moist conditions 

 to permit spoi-ulation. The oil spot stage may thus be compared to a 

 loaded gun, only needing the pull of the trigger, in the shape of a spell 

 of wet weather, to discharge an enonuous number of spores. Should 

 wet weather continue, and copper spray protection be absent or insuffi- 

 cient, wholesale new infection will occur. 



If, on the other hand, hot weather should supervene, even though 

 the oil spot has duly incubated, further infection need not be feared. 

 The spots become darker and dry off to dead leaf tissue, soon losing 

 their power for further harm. Professor Ravaz describes (Progres 

 Agricole, 22nd July, 1917) how an invasion, that of 3rd July, "had 

 been checked in its development by the very hot weather of 4th July 

 (91° F. in the shade) and following days, which .... destroyed 

 almost the totality of the old spots and the germs they bore. . . . 

 Everything was dry, both leaf tissue ' (damaged) and the innumerable 

 white efflorescences they bore. . . . The conidia under the mirco- 

 seope were shrivelled or showed a granular content. At any rate, they 

 were no longer able to germinate. 



Conidia do not retain their vitality long* — according to Gregory, they 

 last about a week under normal conditions. In hot weather they soon 

 shrivel up and die; they are unable to last from one season to another. 

 Nature has devised another means by which the fungus can perpetuate 

 itself and survive the winter period. Towards the close of the season the 

 mycelium produces, in the interior of the leaf, a varying number of very 

 resistant bodies called oospores or eggspores (see Fig. 2) ; these have a 

 tough envelope, which lasts through the winter, only germinating the 

 following spring, when each one gives rise to one single large conidium, 

 which germinates by zoospores, as described above. These are splashed 

 by rain on to the lower leaves of the vine, whence infection spreads with 

 extraordinary rapidity. Unless very wet weather prevails, the oospores 

 will not germinate, nor vsdll the zoospores be able to infect the tissues 

 of the vine ; in other words, no outbreak is possible. These oospores 

 are so resistant that if sheep are fed on infested leaves they will be 

 found uninjured in the sheeps' droppings. 



♦ studies on Plasmopara Viiicola by C. T. Grpgorj', Cornell University, in official report of the 

 session of the International Congress of Viticulture, San Francisco, Cal. July. 1915. 



