322 Wisconsin State Horticultural Society. 



Uncinula would be of advantage, would here be of little avail ; 

 because the fungus is not confined to the surface, but pervades 

 the whole plant, and in fact does not grow through the breathing 

 pores into the air until it has already traversed a considerable part 

 of the interior of the leaves and stems. The first warning of the 

 presence of the fungus, viz., the white spots on the under surface 

 of the leaves, is not to be interpreted as showing that the disease 

 is beginning, but that it is already far advanced. To sprinkle 

 sulphur on the leaves is quite useless in the case of the Peronos- 

 pora, for it will not affect the miss of the fungus which is in the 

 interior. The only thing which will check the disease is to dimin- 

 ish the moisture in the air, but that, unfortunately is beyond 

 human control. If the season happens to be dry, all very well; 

 if very wet, then the Peronospora, once started, will grow in spite 

 of everything. 



The disease spreads from plant to plant durtng the summer by 

 means of the conidial spores. They may be carried about by the 

 wind and rain, just as the conidial spores of the Uncinula; but 

 when they fall on a place sufficiently moist they germinate, but in 

 a different way from the conidial spores of the Uncinula. The 

 contents of the spores separate into a number of distinct bodies, 

 which break through one end of the spore and escape, leaving 

 the empty spore-wall behind. The bodies which escape, to which 

 the name of " Zoospores " is applied, swim about by means of 

 two hair like threads called "cilia," which are in constant mo- 

 tion. Being verv small, they are able to move about in the mois- 

 ture which is found on the ground and on plants when it is not 

 dry weather. They swim about for only a short time, and then 

 the cilia drop off, and the Zoospores come to rest. They then 

 give off threads like the conidial spores of the Uncinula, and the 

 threads penetrate into the interior of the grape plants on which 

 they may be. Once inside, the threads constitute a mycelium, 

 which extends through the plant at a rate corresponding with the 

 pxternal moisture, and finally the threads make their way through 

 the breathing pores into the air, and produce new spores. It will 

 be seen that the conidial spores of the Peronospora have an ad" 

 vantage over those of the Uncinula, because they produce a num 



