238 DISEASES OF TROPICAL PLANTS CH . 



the full-grown roots it causes decay spots of various 

 sizes, which upon removal of the skin appear green in 

 colour. Care should be taken to select clean seed roots 

 and clean plants, and rotation of crops should be 

 followed. 



Dry Rot. This disease is due to Phoma batatae, 

 Ell. & Hals. It occurs on the underground portion 

 of the plant, causing the upper end of the root to 

 become wrinkled and covered with small pimples or 

 fruit bodies. The disease works downward, converting 



Q 



the interior into a dry powdery mass, with but little 

 change in colour. The only remedy known at present 

 is to collect and burn the diseased roots. 



Soft Rot. This disease is due to that widely dis- 

 tributed organism known as Rhizopus nigricans, Ehr. 

 (Figs. 11 and 12). It attacks the roots and does its 

 greatest damage in storage. It causes the root to 

 become shrivelled and the interior to become black with 

 the enormous quantity of spores. It is accompanied by 

 a very disagreeable odour. All diseased roots should 

 be destroyed. 



Other Diseases. Other diseases which have been 

 reported as more or less severe on sweet potatoes, 

 especially in temperate zones, are Rhizoctonia root rot, 

 the root rot caused by Ozonium omnivorum, Shear. ; 

 stem rot caused by Nectria ipomoeae, Hals. ; the soil 

 rot caused by Acrocystis batatas, and the scurf which 

 is caused by Monilochaetes infuscans. Also Lasio- 

 diplodia tubericola, E. & E., which was found in 

 Louisiana on potatoes imported from Java and also from 

 Florida ; Coleosporium ipomoeae, which causes reddish 

 brown spots on the leaves ; an undetermined species of 

 Marasmius, which has been reported by Stockdale ; 

 an undetermined species of Penicillium, which causes 

 a dry, chalky coloured rot which is usually not detected 

 until the root is cut ; Pliyllosticta bataticola, E. & M. , 

 which causes a leaf spot ; Albugo ipomoeae-panduranae, 

 (Schwein) Swingle, or white rust, which also attacks 

 the leaves. 



