50 U. S. DEPARTMENT OF AGRICULTURE. 



The original infection takes place in the field. Both dis- 

 eases develop in the field. They probably do not develop or 

 spread in transit and storage, but do open the way for other 

 Fusarium tuber rots. 



Fusarium radicicola may cause in southern potatoes a very 

 watery leaky rot which involves the entire tuber and re- 

 sembles the watery soft rot of other vegetables caused by 

 Sclerotinia. This rot differs from jelly end and black rot 

 in that the whole tuber is affected and that the rot is watery 

 rather than jelly-like or firm. The diseased tissue is only 

 slightly discolored. Under certain conditions in transit, an 

 extensive mycelial growth and numerous bodies which re- 

 semble small sclerotia develop on the surfaces of affected 

 tubers. 



In this leaky rot of southern-grown potatoes caused by 

 Fusarium radicicola, the original infection takes place in the 

 field. This type of rot is favored by hot weather, and is very 

 frequently associated with scald. It is probable that this 

 disease, unlike jelly end or black field rot, develops and 

 spreads rapidly in transit. 



Ref. (7); (54). 



POTATO: POWDERY DRY ROT. 



Cause : A fungus (Fusarium trichothecioides) . 



Powdery dry rot is a type of the Fusarium tuber rot pre- 

 viously discussed. In the name powdery dry rot, the word 

 powdery does not refer to the appearance of the diseased 

 tissue but to the powdery masses of spores formed by this, 

 fungus. 



In this disease the affected region contains cavities sepa- 

 rated by dried brownish tissue and starch and lined with 

 whitish fluffy mycelium or powdery pink masses of spores. 

 In the early stages, the affected tissue is light brown to black 

 in color and is sharply delimited from the healthy tissue by 

 a layer of brown or black tissue. Affected tubers are much 

 lighter in weight than healthy tubers because of the exten- 

 sive hollow areas. This rot is most pronounced in the cen- 

 tral portion of the tuber, and as a result, affected tubers 

 often become more or less hollow shells. 



If tubers infected with powdery dry rot are stored in a 

 very moist atmosphere, or if they become infected with bac- 

 teria, the typical symptoms are partly masked by the pres- 

 ence of soft watery tissues of chocolate color, which gener- 

 ally are more extensive than the typical dry areas. 



Potatoes just taken from a car which has been in transit a 

 long time, or which was not cooled or ventilated properly, 

 show the symptoms of wet Fusarium rot rather than the 

 symptoms of powdery dry rot. However, upon exposure to 

 the air the typical dry rot symptoms soon develop. 



Powdery dry rot appears to progress more rapidly at 

 lower temperatures than the other Fusarium tuber rots. It 

 occurs mainly in Idaho and other Rocky Mountain districts, 



