Causes of Diseases in Crops 87 



system. 3. Destruction of leaf, flower, and fruit 

 without disturbing the root system. 



FACTORS WHICH FAVOR THE SPREAD OF FUNGOUS 

 DISEASES 



The amount of soil moisture may either protect 

 or predispose a certain crop to fungous disease. 

 For instance, in dry seasons and with a limited rain- 

 fall, truckers lose heavily from asparagus rust 

 (Puccinia asparagi). In this case, the lack of soil 

 moisture weakens the plants, making them therefore 

 more susceptible to rust. An excess of water, such as 

 is found in poorly drained soils, undoubtedly favors 

 the spread of damping off, and the numerous 

 root rots. Weather conditions exert a powerful 

 influence on the prevalence or absence of plant 

 diseases. Wet weather favors the spread of 

 downy mildews (Peronosporaceae). Late blight 

 of potatoes (Phytophthora infestans), downy mil- 

 dew of lima beans (Phytophthora phaseoli), and 

 many other similar diseases, are really wet weather 

 troubles. 



HOW PARASITIC ORGANISMS ARE DISSEMINATED 



Fungi may be carried from place to place as bits of 

 mycelium, as spores, or as sclerotia. Fungi produce 

 enormous numbers of spores, not all of which find 

 their way to receptive healthy plants. Large num- 

 bers are destroyed by exposure to sunlight and air, 

 others fall on crops upon which they are unable to 



