14 MISC. PUBLICATION 168, U. S. DEPT. OF AGRICULTURE 



organism, the black rot fungus, and other agents, at the edges 

 of which the bitter rot fungus has established itself. Infection 

 may occur in warm, wet weather when the spores are washed 

 down on the apples. 



CONTROL MEASURES 



Control measures include removal of bitter rot mummies and 

 repeated spray applications according to the schedule recom- 

 mended by the local agricultural experiment station. When bitter 

 rot is present at harvest careful sorting and prompt storage and 

 cooling help to hold the rot to a minimum in the pack. Apparently 

 it does not spread from one fruit to another. 



(See 12, 59, 65, 111, 113, 1U, 122.) 



Black Rot 



{Physalospora obtusa (Schw.) Cooke 4 ) 



OCCURRENCE AND SYMPTOMS 



Black rot is a disease of apple, pear, and quince that is found 

 in most of the producing sections of the United States east of 

 the Rocky Mountains. Practically all varieties of pome fruits are 

 susceptible, but the greatest losses usually occur on early-maturing 

 varieties. 



Black rot is characterized in its early stages by brown rotten 

 spots on any part of the apple (pi. 5, G). These spots vary greatly 

 in size and are usually irregular in outline. Under orchard con- 

 ditions they may show zones of different shades of brown; for 

 that reason the disease is sometimes called ring rot. In late stages 

 the spots enlarge, become dark brown to black, and sometimes 

 show numerous small black pimples (pycnidia) scattered irregu- 

 larly over the surface. In these pimples are produced the spores, 

 the chief means by which the fungus is distributed and propa- 

 gated. Apple flesh affected with black rot is usually quite firm. 

 Where infection occurs in fruit held at ordinary temperatures 

 after removal from storage, the flesh may become soft and mushy. 



CAUSAL FACTORS 



Black rot is caused by the fungus Physalospora obtusa, which 

 has two spore stages, both capable of producing the disease. 

 The fungus attacks the leaves and the wood, as well as the fruit. 



Under favorable temperature conditions, 50° F. or higher, 

 rotting of the fruit takes place in the orchard, in transit, in 

 storage, and on the market. Black rot does not ordinarily spread 

 from one fruit to another. 



Infection may occur at injured places in the skin such as worm 

 holes, bruises, limb rubs, hail injuries, and rain cracks. The rot 

 develops freely if conditions are favorable and may finally involve 

 the whole fruit. Infection may also follow spray or frost injury 

 at the blossom end, producing what is known as blossom-end 

 rot. The rot may develop steadily from the time of infection and 

 finally involve the whole apple; or it may progress for a short 

 time, then become quiescent, and start up again when conditions 

 become more favorable — for example, when the fruit is shipped 



4 Syn., Physalospora cydoniae auct. Am. and P. malorum (Pk.) Shear. 



