542 SPECIAL PLANT PATHOLOGY 



phism of the asci where these figures vary. Each ascus contains eight 

 ascospores which measure 4 to S/x (Fig. 42). 



Potato {Solanum tuberosum, L.) 



Late-blight {Phytophthora infesians, deBy). — Historically, this is 

 one of the most interesting of fungi, for in 1845 the potato crops 

 of the British Isles, especially Ireland, were decimated by the late 

 blight to such an extent as to cause a severe famine in Ireland. This 

 famine caused the emigration of hundreds of thousands of people from 

 the Emerald Isle to America and the British parliament in order to 

 alleviate the distress of the poor repealed the corn laws, and thus 

 began the free trade policy of that country. 



Formerly, it was thought that the potato disease was distributed 

 widely in America, but it is now known to be most prevalent in New 

 England, in New York and the Canadian provinces, where the potato- 

 growing industry is an important one. It has a wide range in Europe 

 and is known throughout Great Britain and from France to Russia, 

 being especially favored, as it was in 1845, by warm damp weather in 

 the summer months. 



The disease is characterized by leaf spots which first appear at the 

 margin, or apex of the leaf, and spread over its surface until the 

 leaf presents a dark somewhat water-soaked appearance. These spots 

 are brown in drier weather and in all cases a withering of the leaf fol- 

 lows the attack of the mycelium. The disease is known as dry-rot, 

 when it develops in the tubers, for the hyphas enter the cells, as haus- 

 toria kill the cells, and the condition of the tuber known as dry rot 

 is produced, which may be found especially in the stored tubers. 



The hyphas of the late-blight fungus are unicellular and they spread 

 through the intercellular spaces of the host sending filamentous haus- 

 toria into the cells of the leaves, or tubers. From this internal myce- 

 lium, long branched (dendritic) conidiophores grow out through the 

 stomata and the branches bear either laterally, or apically, egg-shaped 

 conidiospores, which measure 2 7 to 30/x by 1 5 to 20/i. The conidiospores 

 on germination form eight biciliate zoospores, which are motile for a 

 brief time perhaps not longer than an hour. If one of these swarm 

 spores finds its way to a leaf, germination speedily follows and the 

 hyphal germ tube enters the interior of the leaf either through a 

 stoma, or by boring a hole through the epidermis. 



