324 



STATE HORTICULTURAL SOCIETY. 



Fig. 



-Downy Mildew of the Grape. 



Peronospora viticola. 



1. Section of leaf (greatly magnified) showing conidial stage. 



2. Showing the same, less magnified. 



3. The same, showing the effect of the fungus, the leaf brown and shrivelled. 



4. Germinating conidia. 



5-8. Development of conidia through zoospore and swarm-spore stages. 



9. Spore. 



10. Germinating spore. 



11-13. Fertilization of oogonium and development of the oospore. 



14. Section of leaf greatly magnified, showing mypelial thread passing between the- 

 cells, and sending its haustoria into them. 



15. Branching mycelium, the spots representing haustoria. — After Viala. 



place, germinate with great rapidity. The contents swell, become seg- 

 mented, and finally burst forth as ciliated swarm-spores. They move about 

 for a time, and then, having come to rest, drop off their cilia and 

 soon throw out a mycelial thread which develops into a new fungus, if 

 the conditions are favorable. In this way, the downy mildews rapidly 

 reproduce themselves during the summer, generation following generation 

 in quick succession. As winter approaches, the fungus sets about the 

 development of spores with thick coverings, that will enable them to with- 

 stand the winter's extremes. These are developed at points where two 

 threads cross or come in contact. Each thread gives rise to a | rounded 

 protuberance, one corresponding to the pistil, and the other to the anther 

 of flowering plants. The contents of one pass into the other, and the 

 development of the oogonium commences. It finally becomes covered 



