I'RKSIDK.NTIAL ADDKESS SKCTION C. 79 



involved and succunih. The leaves turn brown and dr\ up while 

 still hangniij upon the tree, and the twig will be found to be 

 (iead down to a certain point, which marks the advance of the 

 bacterium which is the cause of the disease. • 



Necrosis of the terminal twists is a very conspicuous symp- 

 tom of the mulberry blight (15), and also occurs in severe cases 

 of citrus canker. 



The mulberry blight, which does not seriously aftect the 

 common mulberry, is very severe on the black mulberry {Morns 

 nigra), or i'lnglish luulberry, as it is often called in this country; 

 so many of the young shoots are destroyed that the tree is 

 dwarfed, and I have seen trees fifteen years old that were not 

 more than 4-5 feet high. Olive shoots infected with Baclerium 

 Savastonoi are always dwarfed, and it has been observed (52) 

 that uninoculated sugar-cane stems soon surpass in height and 

 vigour those inoculated with Bacterium vascular urn. 



On citrus trees infected with canker, the fruit is undersized, 

 and on potato plants which have been attacked early in the 

 season by B. solanaccaruui the tubers remain small. 



Dwarfing is also a common symptom in the black rot of 

 cruciferous plants, but less frequently the disease manifests 

 itself in the over development of certain parts of the plant. 

 Cabbage ])lants affected with black rot in the stem often elongate 

 abnormally, and as such elongation is accompanied by a shedding 

 of the diseased leaves, the plant often comes to have a small 

 terminal tuft of leaves separated from the root by a long stem 

 bearing the scars of many cast-off leaves (51). 



A not uncommon symptom in leaf spot disease is distortion, 

 caused by imeven growth subsequent to infection ; this is con- 

 spicuous in blighted mulberry and walnut leaves. 



In other cases the position of the organs is abnormal ; the 

 leaves of tomato plants attacked by Bacterium, solanaccarum are 

 bent downwards, and so also are the fronds of the cocoanut j)alm 

 in which bacterial bud-rot has developed ; culms of Dactylis 

 attacked by Rathay's disease of orchard grass and buds of sugar 

 cane affected by Cobb's disease, develop knee-shaped curvatures 



C52). 



Adventitious roots develop on the stems of diseased tomato 

 and tobacco plants ; and the root system may show an abnormal 

 number of rootlets, giving rise to a tufted mass of fine fibrous 

 roots. This condition is designated " hairy root," and is common 

 in young apple trees attacked by Bacterium, tumefaciens. The 

 most common eft"ect of the parasitism of this bacterium is the for- 

 mation of the pathological overgrowths known as galls or 

 tumours ; these are frequently located near the surface of the 

 soil in the region of the collar, hence the name crown gall ; but 

 the irregular enlargements may appear on the roots or even on 

 the twigs, often developing in great numbers after the tree has 

 suffered severe hail injury. 



A diseased condition of the wood is often indicated l)v the 



