382 MANUAL OF FRUIT DISEASES 



Japanese plums are frequently attacked by Bacterium Pruni, 

 which fact may, in part, explain their susceptibility to wilt. 

 The wilt fungus may also enter the tree by way of wounds made 

 by cultivating and pruning. 



On infecting the plum-tree the fungus spreads out in the bark. 

 Later the medullary rays and sap-tubes are invaded. In the 

 latter the mycelium passes from one duct to another through 

 the pits in the walls. The wood is not destroyed to any notice- 

 able degree. But through the ducts the fungus spreads rapidly ; 

 it may pass from a branch into the trunk and thus kill the whole 

 tree. The attacked bark, medullary rays and cambium are 

 killed and finally turn black. During the invasion of the ducts, 

 gum is produced. This partially plugs the vessels and the sap- 

 flow is checked ; thus the leaves show a tendency to wilt. 

 Around the margin of an infected area may be found a layer of 

 gum-filled tissue. This sharply limits the fungus for a time. 

 Older trees are not able to limit the fungus in this way, hence 

 they suffer more. Trees eight to ten years old develop gum more 

 readily, yet not in quantities sufficient to prevent rapid spread of 

 the mycelium. While gum stops the fungus temporarily, the 

 deposit in the ducts injures the host. When the attack results 

 in gum deposition throughout a cross-section of a trunk or limb, 

 the affected member dies, apparently from a lack of water. 



Control. 



From the foregoing the following points should be borne in 

 mind where control measures are necessary : (1) the disease 

 is most serious in Georgia. It occurs also in North Carolina 

 and Alabama. Warmer climates seem to favor it. (2) The 

 causal fungus gets into the tree only through wounds. Of these 

 wounds, those made by the peach-tree borer, the black-spot 

 pathogene, cultivating and pruning are most prominent. 

 Borers and the black-spot organism prefer Japanese plums, 

 because they are budded on peach stock. It is therefore sug- 

 gested that if some stock other than peach could be satisfactorily 



