236 DISEASE IN PLANTS. 



In the leaf-curl the yellow and flaccid appear- 

 ance of all the leaves of a stalk, or even of the 

 plant, is the striking symptom, and the stem soon 

 droops and blackens just above the soil, a white 

 mould appearing also at the black spots. Subse- 

 quently black spots appear higher up, and bacteria 

 gain an entrance. The stolons rot, and eventually 

 the roots and the leaves wither. The tubers 

 appear sound, but are small ; they are apt to rot 

 in the store, the vascular zones turning brown. 



This leaf-curl has been ascribed to Pleospora, 

 Polydcsmus, Verticillium, and other parasites, as 

 well as to excessive manuring and other agencies, 

 but it still needs explanation. 



Rot of Potato tubers in the soil, or in store, 

 may be brought about by very different agents. 



If PhytopJitJioi^a has obtained access, the fungus 

 hyphae spread between the cells, starting from 

 the haulm, and cause the flesh to turn yellowish 

 and then brown in patches. On the exterior are 

 discoloured patches, depressed, with the flesh 

 beneath ^brown and soft. The m3'celium spreads 

 mostly in the outer layers, which though they 

 turn deep brown remain firm. 



Wet rot of potatoes may be due to various 

 fungi, and, in excess of water, to putrefactive 

 bacteria {e.g. Clostridium), which destroy the cell- 

 walls. The flesh becomes soft, then soup-like, and 

 finally putrefies to a liquid mass with a vile smell 

 of butyric acid, etc., in which the starch grains 

 may be seen floating. 



Tubers are often found with the cork burst and 



