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



occur outside the Pacific coast area. It has been reported from 

 British Columbia, Australia, and southern Africa, and it probably 

 occurs in England. 



Stony pit is caused by a virus and can be transferred from one 

 susceptible variety or tree to another by grafting. If diseased 

 Bosc buds are placed in healthy Anjou trees, most fruits on those 

 trees become pitted by the end of the second season. Such control 

 of the disease as is possible depends on the use of budwood from 

 healthy trees only and the removal of severely affected trees. The 

 Bartlett variety is a symptomless carrier of the virus, and its 

 use is possible where the disease prevails. Less severely affected 

 trees of susceptible varieties may be top-worked to Bartlett if 

 compatible. A record should be kept of such top-worked trees, 

 since they form a virus reservoir that is always potentially dan- 

 gerous. 



In stony pit, masses, or lumps, of stone cells occur in the flesh 

 of pears just under the skin at the bottoms of dimplelike depres- 

 sions (pi. 12, G, H). When the pitting is severe and the pits are 

 numerous, the fruit may be very much distorted. The lumps of 

 stone cells are so hard that it is almost impossible to cut them. 

 The pits are often bordered by darker green rings, and corky tis- 

 sue may develop around the masses of stone cells. 



Corky spot, so far as known, occurs only on the Anjou variety 

 and has been reported only in California and the Pacific North- 

 west. It is characterized chiefly by a bumpy, uneven appearance 

 of the pear surface as the fruit approaches maturity (pi. 12, E). 

 Affected areas usually have a more yellow mature color than the 

 rest of the surface. When diseased fruits are peeled or cut, large 

 masses of brown or grayish necrotic tissue are seen to underlie 

 the spots (pi. 12, F) ; unlike the masses of stone cells of stony 

 pit, these masses offer little or no resistance to the knife, although 

 the fruit containing them sometimes becomes gritty, because of 

 the production of stone cells in the affected tissue. 



The cause of corky spot is unknown. Heavy dormant pruning 

 increases the percentage of fruits showing corky spots in the 

 Wenatchee, Wash., district, whereas in the vicinity of Hood River, 

 Oreg., the disease seems to be associated with black end, which 

 occurs only when the trees are on Japanese pear rootstock. Corky 

 spot is thought to be initiated during periods of high transpira- 

 tion in trees whose root systems are inadequate to supply mois- 

 ture during the critical period. Such inadequacy may be due to 

 root injury on Japanese rootstock resulting from rapid fluctuations 

 of soil moisture or other factors. 



Pits resulting from boron deficiency are found in Bartlett, 

 Bosc, and Anjou pears, but are not known to occur in other 

 varieties. These are shallow, with rather steep sides, and usually 

 have flat bottoms (pi. 12, C). The last-mentioned characteristic 

 makes them distinguishable from stony pit and corky spot. The 

 pits are easily cut and the spot of spongy tissue underneath them 

 is usually less extensive than in corky spot (pi. 12, D). 



Boric acid or borax applied at the rate of 30 to 50 pounds per 

 acre has been found to correct the trouble. One application in 

 early spring is usually sufficient for 3 years. 



(See 73, 7U, 96.) 



