BACTERIAL DISEASES OF PLANTS 



PERIOD OF GREATEST SUSCEPTIBILITY 



In certain diseases the brief seedling stage of the plant is 

 the one most subject to attack, e.g., Stewart's disease of maize 

 due to Aplanobacter stewarti, and brown rot of tomato and to- 

 bacco due to Bacterium solanacearum, but many bacterial 

 diseases of older plants are also rather strictly time-limited. 

 In both groups it is a question of abundant immature tissue. 

 To the latter class belong the numerous leaf-spots, fruit-spots, 

 and blights, e.g., black spot on the plum and peach, due to 

 Bacterium pruni, the fire-blight of the pear, apple, quince, etc., 

 due to Bacillus amylovorus and the blight of the mango due to 

 Bacillus mangiferoB (Figs. 2, 3,). In such cases, so far at least 

 as they occur in temperate climates, the disease appears in the 

 spring and the greater part of it occurs during a brief period 

 in the early summer, in which growth of roots, leaves and shoots 

 is proceeding rapidly and there are many young and succulent 

 parts. The cause of the disease may and often does remain on 

 the plant over winter in a latent or semi-latent condition (walnut 

 blight, pear blight, plum canker, etc.), but the active period is 

 limited to three months, more or less, of actively growing weather 

 in which developing tissues, subject to infection, are abun- 

 dant. With the end of rapid growth and the hardening^of the 



