50 MISC. PUBLICATION 16 8, U.S. DEPT. OP AGRICULTUKE 



containing ammonia the color soon changes to black. In mild cases 

 the injury does not extend beyond the tissue immediately beneath 

 the epidermis. With more severe exposure the lenticel spots may 

 become much enlarged and finally coalesce and the discoloration may 

 extend deep into the flesh. 



The presence of moisture is favorable to ammonia injury; pears 

 with a moist surface are more susceptible than those that are badly 

 wilted or those that have a dry surface. Immature pears have been 

 found more susceptible than mature ones, (ii/7, 170^ 186.) 



BLACK END 



OCCURRENCE AND SYMPTOMS 



Black end of pears occurs in many of the pear-growing regions of 

 the West and Northwest, but is more common and has caused greater 

 losses in California than elsewhere. Although the disease is found 

 most often on the Bartlett variety, probably because of the greater 

 number of Bartlett trees, it occurs also on other varieties such as 

 Anjou, Winter Nelis, Comice, Easter Beurre, and Clairgeau. Black- 

 end fruits are culled out rather carefully during the grading and 

 packing process and consequently are not often seen on the market. 



In the development of the black end, the first symptoms become 

 evident when the fruit is a third to half grown, as a protrusion of the 

 tissues around the calyx and an enlargement of the calyx opening. At 

 this time the epidermis over the affected portion appears tight and 

 shiny. As the disease progresses the calyx lobes turn black, the 

 tissues surrounding the calyx opening become woody, and a brownish 

 discoloration begins to form. This discoloration may appear at first 

 in separate spots, which later coalesce ; in other instances a large area 

 may become completely and uniformly discolored from the begin- 

 ning. The final color of the affected tissues is black. On many 

 specimens the discoloration is confined to an area extending back 

 from the calyx for a quarter to a half of an inch ; on some it covers 

 half the surface of the fruit (pi. 14, A, B) . The discoloration usually 

 does not extend deep into the flesh and sometimes affects only the 

 skin. Specimens are found occasionallv that are not discolored at 

 all but have hard gritty flesh around the calyx and the pointed or 

 peaked appearance that characterizes true black-end fruits. Black 

 end does not develop or spread in storage or in transit. 



A black-end condition of apples has been reported from Alabama 

 but is not known to occur on the market. 



CAUSE 



Black end of pears is a physiological disease ; that is, no bacteria 

 or fungi are found associated with it. In California, where most 

 of the investigations of it have been made, the disease is found 

 almost exclusively on fruit from trees grown on Japanese (Pyn^us 

 serotina Rehd.) rootstocks, hence the conclusion has been drawn 

 that this kind of rootstock is in some way the principal cause of the 

 disease. Black-end fruits are occasionally found, however, on trees 

 grown on other rootstocks. There is also evidence that abnormal 

 moisture conditions in the soil or a shallow-lying hardpan favor the 

 development of black end. 



