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 
Fic. 302. — Fire blight on the Pear, Cambium and cortex. The 
The tip of the branch is being killed by tips of the twigs with their 
the Bacteria. After Whetzel & Stewart.- 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 
' Potato Scab. Bulletin 184, Vermont Agr. Exp. Sta., 1914. 
® Potato Tuber Diseases. Farmer’s Bulletin 544, U.S. Dept. Agr., 1913. 
5 Fire Blight Disease in Nursery Stock. Bulletin 829, Cornell University 
Agr. Exp. Sta., 1913. 
