57© SPECIAL PLANT PATHOLOGY 



tion to a type of non-inheritable "net necrosis" of potato tubers which 

 has developed under conditions which suggest frost injury and this 

 hypothesis has been confirmed by chilling experiments. Tubers 

 "frozen solid" are totally killed and collapse when thawed, and if the 

 chilling stops with incipient ice crystalUzation, such interior tissues as 

 are most sensitive may be killed. Such frozen tubers are normal in 

 external appearance but when cut open they show that the most 

 sensitive internal vascular tissues are discolored and are killed. There- 

 fore, moderate exposure to freezing temperature may produce either 

 "ring" or "net" necrosis, the blackened vascular tracts penetrating the 

 fundamental tissue cells filled with starch. Tubers vary individually 

 in their sensitiveness but in general the best types of "net necrosis" 

 have been secured by about two hours exposure to + 5°C. with similar 

 results on exposing them to — i°C. for eight and one-half hours to — 9° 

 C. for one hour. Slightly more severe treatments, or unequal exposures, 

 may give frozen spots with corresponding dark blotches involving the 

 general parenchyma. The stem end of the tuber is always more 

 sensitive than the other end. 



Apple Fruit Spots. — This disease of the fruit of the apple is also 

 known as Baldwin-spot, bitter-pit, fruit-pit, pointe bruns de la chair 

 and stippen. It is cosmopolitan in its distribution, being found wher- 

 ever apples are grown. It has recently received the attention of a 

 number of mycologists and a number of explanations as to its cause have 

 been given. The most recent study seems to indicate its non-parasitic 

 character. The observed spots are dark in color, circular or some- 

 what angular in outline, from one-eighth inch or less to one-fourth inch 

 in diameter. Although distributed over the surface of the pome they 

 appear most commonly on the blush, or sun-exposed side. A lenticel 

 forms the center of the sHghtly depressed areas or "pocks," which con- 

 sist of necrotic tissue. The injury is superficial extending only sHghtly 

 into the pulp. Pathologists appear to have agreed that the disease is 

 due to extreme variations in the water-supply of the apple tree during 

 the growing season. 



McAlpine,^ an Australian mycologist, has published four quarto 



1 Eastham, J. W.: Bitter Pit Investigation, Phytopath. 4: 121-123, 1914 

 Brooks, Charles: Bitter Pit Investigations, Piiytopath. 6: 295-298, 1916 

 Crabill, C. H. and Thomas, H. E.: Stippen and Spray Injury, Phytopath. 6 

 51-54, 1916. 



