MARKET DISEASES OF FRUITS AND VEGETABLES 37 



until they begin to soften. It appears first as small, umber-colored, 

 nondepressed, vaguely bounded firm spots usually centering at a speckle 

 or other blemish. It may reach a diameter of one-half inch in 3 or 4 

 days at which stage it becomes slightly sunken, and a watery rot with 

 no'definite boundaries spreads rather slowly into the flesh. The decay- 

 ing flesh has a rancid unpleasant odor that penetrates the sound tissues 

 of the fruit. 



The fungus also affects leaves and twigs on which it is carried over 

 from one season to the next. Pruning dead twigs from the tree and 

 spraying with a bordeaux and sulfur mixture have been recommended 

 for control of the disease. 



(See 67, GS. 116.) 



FLESH DARKENING 



Avocado fruits of different varieties vary in the normal color of 

 their flesh. In certain varieties the flesh is sometimes rather gray. 

 and during certain seasons, possibly because of abnormal conditions of 

 growth, the flesh of the commercial varieties may be unusually dark. 

 The flesh may also darken in overripe or very soft fruit or when the 

 seed begins to grow. On the market hard, dark areas are sometimes 

 found in fruit from Florida and the West Indies which is believed 

 to have grown on seedling trees. 



Flesh discoloration is also commonly caused by chilling or freezing 

 the fruit. In general it occurs when the fruit is held below 40 z F., 

 although certain varieties can withstand storage temperatures as low 

 as 37 z without injury. Avocados of the West Indian race are more 

 susceptible to chilling injury than those of the Guatemalan and Mexi- 

 can races. 



In slight chilling injury only the tissues of the fibrovascular strands 

 may be darkened. After more severe chilling the skin takes on a brown 

 or scalded appearance and the flesh, particularly around the base of 

 the seed, becomes a gray color which changes to brown as injury in- 

 creases. Badly chilled fruit fails to soften normally when subsequently 

 held at room temperatures. Fruits just beginning to soften are more 

 susceptible to chilling injury than green or riper fruit. Fully softened 

 fruit may be held at 32 z without injury. 



The average freezing point of a number of varieties of avocado 

 fruits has been reported as 27. '2 z F. Immature and partially mature 

 fruits may freeze at a slightly higher temperature. If the fruit freezes 

 before it softens, the flesh becomes penetrated by many small cracks 

 and remains tough and rubbery. 



(See 31, 67, 77, 83, 8\, 8-5. 120.) 



RHIZOPUS ROT 



(Rhizopus nigricans Ehr.) 



Rhizopus rot is the most rapidly developing decay of ripe avocados 

 at room temperature. It is not found on the fruit until it begins to 

 soften but is fairly common on roughly handled fruit ripened at high 

 temperatures. The decayed area is extensive, dark brown and. if the 

 atmosphere is moist, is covered with coarse white to gray mycelium 

 bearing the typical spherical black sporangia. If the atmosphere is 

 dry, little mold growth will occur and the sporangia will be borne on 



