348 



THALLOPHYTES 



as the living content of the cells. Their presence in the leaf 

 causes a blackening of the veins and a yellowing of the mesophyll. 

 The disease may spread to the stem, where it clogs the vascular 

 bundles and destroys tissues. Plants attacked lose their leaves 

 and are dwarfed or killed. 



Potato Scab.^ — There are a number of organisms which at- 

 tack the Irish Potato and cause scabby areas and the decay of 



the tuber. Among this 

 group of organisms pro- 

 ducing scab there is one 

 of the higher forms of 

 Bacteria scientifically 

 called Actinomyces chromo- 

 genus (Fig. 301). Among 

 other bacterial diseases of 

 the Irish Potato, Black- 

 leg^ is of considerable im- 

 portance, especially in the 

 Southern States. 



Pear Blight.^ — This dis- 

 ease occurs on many fruit 

 trees, but is more serious 

 on Pears and Apples. It 

 is often called Fire Blight 

 or Blossom Blight. The 

 Bacteria enter the young 

 twigs, usually through the 

 flowers, and attack the 

 cambium and cortex. The 

 tips of the twigs with their 

 flowers and leaves soon 

 wilt, and in a few weeks 

 blacken and die. Sometimes when the attack is quite general, 

 scarcely a flower tip of an infected tree escapes. This not 

 only results in loss of fruit, but the tree is often so disabled 

 that death results. Figure 302 shows a Pear twig severely 



Fig. 302. — Fire blight on the Pear. 

 The tip of the branch is being killed by 

 the Bacteria. After Whetzel & Stewart. 



1 Potato Scab. Bulletin 184, Vermont Agr. Exp. Sta., 1914. 



2 Potato Tuber Diseases. Farmer's Bulletin 5U, U. S. Dept. Agr., 1913. 



^ Fire Blight Disease in Nursery Stock. Bulletin 329, Cornell University 

 Agr. Exp. Sta., 1913. 



