MARKET DISEASES OF FRUITS AND VEGETABLES 53 



by the black rot fungus, but they are firmer, drier, and more 

 uniformly black throughout. 



The causal fungus grows very slowly at low temperatures and 

 apparently cannot penetrate the uninjured skin of apples. 



(See 12, 59, 65, 127.) 



Spray Injury 



There are several types of spray injury to the skin of apples. 

 Some of them can be readily attributed to a particular spray 

 material, whereas others have various possible causes. 



Russeting is the most common form of spray injury. When it 

 is produced by bordeaux mixture the skin is often decidedly 

 rough, showing evidence of extreme injury. When caused by 

 arsenicals, lime-sulfur, or oils, the injury is likely to be less 

 serious and usually shows finer lines and markings. 



Russeting may be due to causes other than spraying, but the 

 effects are usually somewhat different. Spring frosts often pro- 

 duce russeting, but the injury is usually found in definite bands 

 and lines around the fruit. Mild forms of russeting may be due 

 to cold spring rains and other unfavorable weather conditions 

 and are sometimes difficult to distinguish from the milder cases 

 of spray injury. External cork caused by boron deficiency occurs 

 as rough, corky areas of irregular shape on any part of the apple. 



Bordeaux russeting is most likely to result from applications of 

 bordeaux mixture early in the season (pi. 3, A). Late applications 

 sometimes cause injury in the form of small red spots, especially 

 on green or yellow varieties such as Yellow Newtown. Varieties 

 most susceptible to roughening and russeting by bordeaux mixture 

 are Ben Davis, Grimes Golden, and Golden Delicious. 



Sulfur injury from lime-sulfur or other sulfur fungicides is 

 likely to be worse under high-temperature conditions. Mild cases 

 often take the form of bronzing or browning of the area most 

 exposed to the sun, whereas severe injury results in the forma- 

 tion of hard, leathery grayish areas, which may or may not 

 slough off (pi. 3, B) ; in the most severe cases there is also 

 cracking of the affected area of the fruit. The Grimes Golden 

 and Golden Delicious varieties are especially susceptible to injury 

 by lime-sulfur sprays. 



In addition to russeting, arsenicals are responsible for an- 

 other form of injury to the fruit known as calyx injury, or blos- 

 som-end injury. This appears as a dark-brown or nearly black 

 area around the calyx of the apple and often furnishes a place 

 of entry for the black rot fungus or other decay organisms. 



Oil sprays cause various types of injury on apples. One of the 

 most common is very similar to the calyx injury that results 

 from arsenical sprays. In other cases oil injury is exhibited by 

 russeting and deformation of the apple, which becomes gnarled 

 and roughened. Injury from oil sprays is much more likely to 

 occur on trees suffering from drought or on those low in vigor 

 from other causes. Spraying should be avoided during periods of 

 high temperature (90° F. or above), and care must be exercised 

 at all times to use an acceptable type of oil as well as to have it 

 properly emulsified. 



(See 36, U2, 81.) 



