56 U. S. DEPARTMENT OF AGRICULTURE. 



this stage the dried-out starchy remnants of the tissue are 

 separated from the uninjured tissue by a dark brown corky 

 layer. 



Very frequently the starchy or corky layer is not formed 

 before infection with Fusarium takes place, or it is subse- 

 quently broken and admits the fungus. Severe freezing 

 injury generally affects all tissues straight across the tuber, 

 while Fusarium rot develops most rapidly in the pith of the 

 tuber, leaving a shell of cortical tissue. This characteristic 

 sometimes is of service in differentiating freezing injury and 

 Fusarium tuber rot. 



Thawed tissue, especially when in a warm damp atmos- 

 phere, is predisposed to slimy soft rot. In fact, most se- 

 verely frozen potato tissue is dead and sooner or later 

 becomes infected with the bacteria which cause this rot. 

 The bacteria get into the watery tissue, which is an excellent 

 growing medium for them, through breaks in the skin, and 

 cause a slimy, mushy, or soupy rot which has a very offen- 

 sive odor. Frequently the much wrinkled skin of such tubers 

 bursts, and the neighboring potatoes are smeared with the 

 slimy disintegrated tissue which dries out grayish and 

 chalky when exposed to the air. 



It is very difficult at times to differentiate freezing injury 

 from injury due to scalding. This is especially true when 

 the first or last freezes of the season occur. 



The severe general type of freezing injury is marked by 

 a killing of all the cells of the potato. The less severe type, 

 popularly known as chilling, is definitely localized and 

 restricted to the most susceptible tissues. It is known as 

 freezing necrosis. It appears first in the stem end, and at 

 times only the conducting tissues are killed and appear yel- 

 lowish-brown to black. Very often the conducting tissues 

 in the pith also are killed and appear as a net resembling 

 net necrosis. The storage tissues also may be affected, either 

 in extensive regions, or in small pockets. Starchy tissues so 

 affected are grayish to steel blue, even black in color, gran- 

 ular, and very tough and leathery. 



Sometimes the thawing tissues are pinkish to red in color 

 when freshly cut. These colors change quite rapidly in the 

 air, becoming gray, brown, or dirty red. These colors should 

 not be confused with the pinkish color of the vascular ring 

 or pith tissue of some of the red varieties of potatoes. At 

 times white potatoes which have not been exposed to low 

 temperatures also have a pink color. Potato tissues when 

 freshly bruised and exposed to the air often are reddish in 

 color. 



Potatoes subjected to freezing temperatures may show 4 

 none of the symptoms enumerated above but only a tend- 

 ency to excessive wilting and flabbiness. The cut tissue may 

 appear glassy and abnormally moist. When cooked they 

 may turn black in spots or may have a sweet taste. This 



