54 Diseases of Economic Plants 



Frost Injury. ^^ — Varieties of apples differ from each other 

 ill their susceptibihty to injury from cold. The injury may 

 be local, on single twigs, or spots on twigs, body, or branch; 

 or general, affecting the whole tree. The damage may be 

 apparent externally or show at first only in a discolored 

 cambium. '' Sun scald," in which the frosted bark occurs as a 

 cankered area and either i:)eels off, or clings tightly and is 

 sunken, is a frost injury. Twigs may l;e killed outright, 

 especially if the wood has not ripened properly. Late spring 

 frosts injure, as is well known, the buds, blossoms, etc. 

 Frost cankers may form the threshold for the entrance of 

 various fungi and thus pave the way to disease, e. g. black- 

 rot or bitter-rot cankers. A cover crop tends to lessen frost 

 injury, by diminishing excess moisture in the fall, while 

 avoidance of cultivation late in the season hastens maturing 

 of the wood. 



Bitter-Pit ^^ consists of a pitting of the fruits with slightly 

 sunken circular spots varying from mere dots to 5 mm. in 

 diameter, which finally become brown. The tissue at the 

 bottom of the pit is dead, brown, and spongy. Similar lesions 

 may occur deep within the tissue of the ajaple. No parasite 

 is present and the trouble seems to be due to improper water 

 relations. The disease causes considerable loss over a large 

 range of territory. Thinning the fruit to a proper set lessens 

 the damage. Pitted fruit should either be consumed at 

 once or stored or shipped under low-temperature conditions 

 (30°-32° F.,— 1°-0° C.) 



Jonathan-Spot '''•'' "^ occurs on several varieties of apples 

 including the Jonathan, Esopus, Wealthy, Ortley, and Wolf 

 River. The spots are circular, 2-15 mm. in diameter, 

 abruptly but slightly depressed, light brown in color, and 

 are more superficial than those of bitter-pit. The disease 

 may appear on fruit still on the tree or may develop during 

 storage or transit. The cause is unknown. Fruits of variety 

 tending to spot should be consumed early or placed in cold 

 storage. 



Scald '''^ is a storage trouble particularly serious on im- 



