SYMPTOMS OF DISEASE IN PLANTS. 233 



plants, is characterized by the mottled appearance of the leaves, or in 

 other words the leaf is somewhat variegated with yellow and green. 



When the leaf shows colored areas or spots that are more or less 

 definite and circumscribed, and different from any normal coloration, we 

 may be fairly certain that it is suffering from either an insect or fungus 

 disease, with the presumption in favor of the latter. 



White patches more or less definite or somewhat diffuse are generally 

 due to either some of the "Powdery Mildews" or to the "Downy Mil- 

 dews." We have a good illustration of this symptom in the powdery 

 mildews of the apple, peach, cherry, grape, gooseberry, wheat and other 

 grasses, the elm, catalpa, the lilac, honey-suckle, crimson rambler, and 

 other roses, golden glow, etc. In all of these cases the fungus that 

 causes the disease lives on the surface of the host plant, or is what 

 may be termed an external parasite. In the downy mildew of the grape, 

 which is an internal parasite, we often find the under side of the leaf 

 covered with irregular whitish patches, which are formed by the spore 

 producing parts of the fungus that emerge from the interior of the leaf. 

 It may be mentioned in this connection that the upper side of the leaf, 

 directly over these spots is at first a yellow color, but later changes to 

 a dark brown. Other downy mildews showing somewhat the same symp- 

 toms may be found on cucumbers, lettuce, spinach, and onions among 

 garden vegetables, and upon some of our forage crops like millet, clover, 

 and alfalfa. The "white rusts" generally show more definite whitish 

 pustules, which eventually rupture and allow the escape of the spores. 

 The white rusts may be observed upon cress, turnips, radish, cabbage, 

 horse radish and related plants and also upon the leaves of the oyster 

 plant. 



Many fungi that inhabit leaves have a localized effect and produce 

 more or less circular or slightly irregular spots of a marked yellow 

 color. In such cases the disease is generally characterized as a "leaf 

 spot." Symptoms of this kind may be noted in the leaf spots of the 

 oak and violet, and in the anthracnose of cucumbers and melons. Some- 

 times the change in color becomes more general and the whole leaf turns 

 yellow and falls from the tree as in the early summer stage of apple 

 scab. I 



Another marked symptom of disease is the occurrence of red or 

 orange spots or pustules. The "rusts" of our cereals and other crops 

 received their name from the fact that in the early stage of the attack 

 reddish or brownish spore producing pustules give the stems or leaves 

 a rusty appearance. Sometimes these rust spots may be few in number, 

 at other times so numerous that almost the entire surface of leaf or 

 stem is covered, and thus the vigor and productiveness of the crop is 

 seriously impaired. In all of our cereal rusts, black pustules appear 

 on leaves and stems as a later growth. The red pustules always pre- 

 dominate and give character to the disease in the "orange rust" c* 

 wheat and the "crowned rust" of oats, while the black stage or black 



