E. M. Doidge 117 



External Characteristics of Disease. 



The appearance of leaves attacked by the bacterial blight is quite 

 distinct from that of leaves attacked by the common leaf spot fungus 

 (Sejitogloeum mori Bri. and Cav.) ; the spots being smaller, darker in 

 colour and more numerous. 



The first indication of infection is the appearance on the under side 

 of the leaves of very minute water-soaked areas (Plate XIX, fig. B) ; these 

 increase somewhat in size though individual spots rarely exceed 2 mm. 

 in diameter. After some days the spots begin to discolour, and gradually 

 become dark brown or almost black. They are always angular and 

 often become white in the centre. The attacked tissues become quite 

 dead and later fall away leaving holes in the leaf-tissues, and the leaves 

 assume a torn and ragged appearance (Plate XIX, fig. A) ; they are 

 sometimes completely reduced to tatters. Frequently a large number of 

 infections on a young leaf occur in the neighbourhood of a vein, in 

 which case the vein is affected and becomes wrinkled up and growth 

 ceases in the affected region, with the result that the leaf is distorted 

 (Plate XX, fig. A). Badly affected leaves sometimes turn yellow and fall. 



On the shoots infections are also frequent, and usually take place 

 in rapidly growing tissues. They first appear as short, water- soaked, 

 somewhat raised streaks, which may increase to a length of 3-4 cms. 

 The infected portions later become sunken and discoloured (Plate XXI). 

 If the whole of the circumference is affected the young shoot dies, and 

 trees attacked by blight are readily detected by the numerous bare, 

 dead twigs which they bear (Plate XX, fig. B). 



This blighting of the young shoots when they appear in spring 

 is also largely responsible for the stunted appearance of the trees, as 

 it is only when the rains are exceptionally late and the disease conse- 

 quently slow in spreading that the trees are able to make any appreciable 

 amount of new growth. 



The tissues in the affected streaks being killed and the rest of the 

 stem continuing to grow, a tension is set up which results in the formation 

 of longitudinal cracks in the diseased parts (Plate XXI). 



