GENERAL PART I 
CONCERNING PLANT DISEASES IN GENERAL 
As defined in the introduction, a plant is called diseased when 
it fails to show normal vigor and normal condition of its parts. 
‘The manner of disease attack is extremely varied and the conditions 
set up as a result of disease are accordingly of many different kinds. 
We learn to recognize disease by the symptoms shown in the plant; 
these symptoms will at times be readily interpreted and on other 
occasions they will prove misleading. Nothing is plainer than the 
necessity for continuous observation of growing plants if one is to 
be in a position to interpret the symptoms of disease. 
Fig. 1. Roots of white burley tobacco plant attacked by broom-rape. Each of these masses 
attached to the root shows beginning of the plant which will grow upin larger dense form, and pro- 
duce an abundance of blossoms and seeds but no leaves. Each one of these must have started from a 
buried seed of the broom-rape, Orobanche Ludoviciana Nutt. 
Diseased conditions may be due to the very obvious attacks of 
certain parasitic seed plants which lack leaf-green or chlorophyll'in 
their tissues and must subsist on other plants somewhat after the 
manner of parasitic fungi. The dodders which attack the clovers, 
alfalfa, onions, etc., belong in the class of parasitic seed plants of 
the genus Cuscuta. Their seeds are liable to be harvested with the 
, (311) 
