I 



14 MISC. PUBLICATION 198, U. S. DEPT. OF AGRICULTURE 



CAUSAL FACTORS 



Experimental evidence indicates that one of the causes of internal 

 decline is the withdrawal of water from the fruits by the leaves dur- 

 ing periods of low humidity and high temperature. Freezing appears 

 to be another cause, since it was noted during the picking season of 

 1922 that fruit from trees that had been frozen the previous winter 

 showed a great deal of the decline, whereas fruit from trees damaged 

 only slightly by freezing or not at all showed very little. 



CONTROL MEASURES 



The following suggestions have been made for preventing internal 

 decline: (1) Get as much humus as possible into the soil to aid in 

 aeration and water penetration and to increase the water-holding 

 capacity of the soil; (2) irrigate to maintain optimum soil moisture 

 conditions; and (3) use windbreaks to overcome some of the effect 

 of drying winds. 



(See 1, 13, 15, 16, 17, 24, 1$, 43.) 



MEMBRANOUS STAIN 



Membranous stain, or membranosis of lemons, is characterized by a 

 browning or darkening of the membranes or carpellary walls between 

 the segments (pi. 7, and D). The central core tissues and the inner 

 tissues of the rind also may be affected. The disease can be detected 

 only when the lemons are cut and can be seen best when the segments 

 are pulled apart for longitudinal view. 



The disease is more prevalent in fruit picked in cool, damp weather. 

 It is extremely responsive to storage temperature ; lemons held at 40° 

 F. become seriously affected, whereas, curiously enough, those held 

 at either 32° or 60° seldom develop sufficient stain to be seriously 

 damaged. For the prevention of this and related diseases a storage 

 temperature of 55° to 58° has been found most satisfactory. 



(See 24, 48, 43, SO.) 



OIL SPOTTING (OLEOCELLOSIS) 



Oil spotting is found on citrus fruit from all producing regions. 

 It is worse and probably most commonly seen on lemons and limes, 

 but it is also found on other citrus fruits, especially oranges harvested 

 early in the fall before they have lost their green color (pi. 11, B). 

 The commonest form of the injury occurs as irregularly shaped yellow, 

 green, or brown spots in which the oil glands of the skin stand out 

 prominently because of slight sinking of the tissues between them 

 (pi. 11, A). The yellow spots develop on fully colored mature fruit, 

 whereas the green spots develop on fruit that was green in color when 

 picked. Brown spots are either a later or older stage of spots that 

 were originally green or of spots that were more severely injured. 

 Spots seen on fruit on the market are usually not more than one-half 

 inch across. Some of those observed on fruit in groves and packing 

 houses are so large as to involve the greater part of the fruit surface. 

 In either the green or the brown stage they become more evident after 

 the fruit has been subjected to the degreening process. 



