48 U. S. DEPARTMENT OF AGRICULTURE. 



without a microscopic examination or cultural test. An 

 examination of the entire lot, however, will reveal the pres- 

 ence or absence of typical signs of freezing injury. The 

 frozen tissue of a tuber usually is set off from the healthy 

 tissue by a more or less straight purple or brown line which 

 extends across all the tissues of the tuber, or by a gray, 

 chalky, brittle layer of starch. On the other hand, most 

 Fusarium species generally tend to rot the center -of the 

 tuber faster than the cortical tissue, leaving a shell of sound 

 tissue enclosing a rotted center. 



An examination of the surface of a tuber may not be suf- 

 ficient to determine whether the tuber is affected* with late 

 blight or Fusarium tuber rot. Both produce sunken dis- 

 colored areas. The late blight tuber rot, however, causes 

 more of a metallic luster than the Fusarium tuber rot, and 

 usually the sunken area is not shrunken and shriveled. In 

 tubers affected with late blight, the diseased tissues under- 

 lying the discolored areas are solid and dry and have a 

 feathery edge, while in the Fusarium tuber rot the diseased 

 tissue, if dry, contains cavities or is watery and soft and in 

 cross-section is set off from healthy tissue by a sharp, smooth 

 edge. 



Tubers affected with the soft, wet, brown type of Fusa- 

 rium tuber rot can be differentiated from tubers affected 

 with the slimy soft rot by the absence of the foul odor so 

 characteristic of all bacterial soft rots. Fusarium-infected 

 tissue is not slimy even though it is soft and disintegrated. 



Deterioration of tubers due to Fusarium tuber rot is some- 

 times rapid and often complete. Infected tubers are a men- 

 ace to healthy ones. Immature tubers, cut and bruised 

 tubers, tubers with second growth knobs which are easily 

 broken off, tubers affected with other diseases such as late 

 blight tuber rot or blackleg, and tubers with frozen areas 

 are an easy prey for the Fusarium species causing tuber rot. 



One or more forms of Fusarium tuber rot occur in practi- 

 cally all potato districts. Fusarium tuber rot is much more 

 common in northern than in southern potatoes. It does 

 develop, however, in southern potatoes shipped north and 

 causes a very soft, watery rot or a stem-end rot. The orig- 

 inal infection may occur in the field, in transit or in storage, 

 but in most cases the rot develops and spreads in transit and 

 storage. In a few cases, such as jelly-end rot and black field 

 rot, the disease develops in the field. 



The Fusarium tuber rot in potatoes from the Central 

 Western States, especially in the Early Ohio stock from 

 Nebraska and Minnesota, and the Burbank and Netted Gem 

 stock from Idaho, is called powdery dry rot. This term 

 refers more to the appearance of the spore masses of the 

 fungus than to the diseased tuber tissues. The Fusarium 

 tuber rot in potatoes from the Eastern States is designated 

 as tuber rot or dry rot and generally is of the wet type. The 



