58 A Bacterial Spot of Citrus 



goes half way round the stem ; after a time the central part loses its 

 water-soakecl appearance and turns brown (chestnut brown to olive 

 brown) and becomes shghtly sunken (Plate IX, a and h). The dis- 

 coloration spreads out to the edges of the infected area which ceases 

 to increase in size. In transverse sections the discoloration does not 

 appear to extend into the wood, but if the bark be peeled off the surface 

 of the woody cylinder appears slightly yellow. 



Towards the end of the season, i.e., in the spring, a considerable 

 amount of gummosis may be observed from the affected stem tissues, 

 the gum oozing very largely from the severed leaf bases. In a number 

 of affected branches received at this time of the year the leaf subtending 

 the flowering shoot had been destroyed, and it was a question whether 

 the whole inflorescence would not have dropped before any fruit was set. 



If the disease were confined to the fruit it would hardly be expected 

 that it would cause any considerable damage in the districts with 

 a summer rainfall and a dry winter, but the fact of its attacking the 

 branches and causing the destruction of the leaves causes the trees to 

 present a defoliated appearance towards the end of the season. It 

 may be anticipated therefore, that even in a climate where the fruit 

 ripened during the dry season considerable damage would be done to 

 the trees during the summer, and their vitality considerably lowered. 

 An exceptional winter rain such as fell in the Transvaal during last 

 July would be sufficient to start the spread of infection amongst the 

 fruit. 



Leaf infections are comparatively rare, occasionally a small dis- 

 coloured area develops in the leaf tissues, a thorn prick or small wound 

 serving as the starting point of infection. The dark brown spot is 

 surrounded by a yellowish zone and the injured tissues eventually 

 fall out leaving a hole. (Plate IX, c). 



Inoculation Experiments. 



An organism was plated out from lemons received from Simondium 

 in November, 1914; with a pure culture of this a number of lemons 

 were inoculated by needle pricks, and in four to five days these showed 

 a number of small sunken dark-coloured spots. Owing to the pressure 

 of work and the fact that the lemon season was nearly over, the work 

 was discontinued, and was not resumed until July, 1915. 



At the beginning of this month the disease again became very 

 active, shortly after the first steady rains in the Western Province, 

 and specimens of infected fruit (both lemons and navel oranges) were 



