v^' '^-^ - 



PERONOSPORALES 



31 



1, Conidia. These are asexiially produced on branching co- 

 nidiophores which project from the stomata on the under side of 

 the leaf {PLj,f. 4). They germinate soon after they are pro- 

 duced and their function is to rapidly spread the fungus during 

 favoring conditions. For this reason they are often called sum- 

 mer spores. In germinating, the contents of the conidium break 

 up into a number of zoospores which swim in the moisture cover- 

 ing the leaf like a film, sprout, penetrate the leaf through one of 

 the stomata and develop new centres of growth. The distribution 

 of the conidia is aided by the wind. 



2. Oospores. These are produced within the host by the fertil- 

 ization of an egg-cell of an oogone by a smaller organ known as 

 an antherid (/Y. j, f. j). This results in the formation of a new 

 cell, which becoming surrounded with a thick wall and is known as 

 the oospore. From the fact that this remains dormant some time 

 before germination it is often called a resting-spore, since its 

 function is to carry the life of the fungus over an unfavorable 

 period, it is also called a winter spore. It remains within the 

 tissues of its host until the spring or until their decay occurs when 

 it germinates after a manner similar to the conidia. 



The remedy for the disease is found in spraying the vines with 

 copper salts to prevent the germination of the summer spores. 



The rot of the Irish potato is due to a similar fungus which pro- 

 duces a wilt of the leaves, accompanied by the formation of conid- 

 ia similar to those of the grape. These conidia, however, are 

 produced in a slightly different manner, the conidiophore produc- 

 ing branches only after the terminal conidia are developed. The 

 disease extends to the tubers, forming brown, or black discolora- 

 tions, which end in the extensive decay of the entire tuber. The 

 mycelium is perennial, living from year to year in the tuber, and 

 will break out i»to an epidemic whenever favorable conditions 

 occur. 



The disease is to be controlled more by the judicious selection 

 of seed and land and the proper destruction of diseased potatoes, 

 though spraying may prevent the spread of the disease by means 

 of the conidia. 



The families and genera of the Peronosporales may be distin- 

 guished by the following table : 



