OLIVE 145 



The disease appears as conspicuous swellings on the 

 trunk, limbs, twigs, or leaves. These swellings are 

 irregularly hemispherical or flattened on top, or nearly 

 spherical where they occur on twigs. About the end of 

 the first season's growth they crack from the top and 

 often become deeply fissured with the segments strongly 

 diverging. They are from a few millimetres to four 

 centimetres or more in diameter. Sometimes several 

 knots become confluent and cover a considerable area. 

 This is particularly liable to occur on a wound or on 

 the large natural swellings at the base of the tree. The 

 majority of the knots on twigs occur on leaf scars. 

 Many appear about old injuries. On the trunk or 

 limbs, knots are particularly liable to occur on the 

 callus or thickened bark about an injury. 



The knots are at first firm and fleshy, but the 

 interior soon becomes woody ; after maturity the outside 

 dries and becomes hard also. There may be abnormal 

 growth formed within the fissures of an old knot or at 

 the side of it. 



The injury to the tree consists in killing or spoiling 

 areas of conducting tissue of cortex and sapwood. 

 When many knots develop in a limb and die, the trans- 

 portation of water and food materials may be impeded, 

 and the limb itself die. Trees may collapse and die 

 in the same way. However, the disease is not usually 

 fatal to the tree for many years. 



The development of the knots is directly caused by 

 the presence within the cortex of the tree of a specific 

 bacterium, Bacterium savastanoi, E. F. Smith. 1 It has 

 been repeatedly shown that when a living culture of 

 this bacterium is introduced into the bark of an actively 

 growing olive tree the characteristic knot appears. 



The manner in which the specific bacterium which 

 causes the disease is carried from the knots and intro- 



1 See Bulletin No. 131, iv. Bureau of Plant Industry, United States 

 Department of Agriculture, for very full technical discussion. The organi>m 

 has generally been referred to Bacillus oleae (Arc.) Trev., but the name has 

 been based on so much confusion and error that Dr. Smith has rejected this 

 name and given a new one. 



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