88 Diseases of Truck Crops 



thrive, while a relatively small proportion find ideal 

 conditions on the proper hosts. 



Wind. If we consider the microscopic minuteness 

 of fungous spores we shall appreciate how easy it is for 

 winds and air currents to become carriers of these 

 spores. 



Water. Water is another important agent which 

 helps in carrying and disseminating fungous spores. 

 The latter may be actually carried in streams from one 

 territory to another, or by rain washing and splashing 

 from plant to plant. The spores of Phytophthora in- 

 festans, for instance, the cause of late blight of Irish 

 potatoes, are spread about from plant to plant by rain. 



Seed-Borne Diseases. A large number of our truck 

 crop diseases are introduced with the seed. This 

 is often brought about unconsciously or through 

 carelessness. Seeds and tubers may carry fungous 

 pests as bits of mycelium in the interior tissue. An 

 example of this is the bean anthracnose {Colleto- 

 trichum lindemuthianum) (fig. i6), which is carried 

 as mycelium within the seed. The late blight of the 

 Irish potato is carried in a similar way within the tub- 

 ers. Seeds and tubers may also carry fungous pests 

 as spores or sclerotia which adhere to the exterior of 

 the seed coat. The smut of onions, for instance, is 

 carried as spores on the onion seed. The Rhizoctonia 

 disease of Irish potatoes is carried as sclerotia on the 

 surface of the tuber. The same is true for ntmaerous 

 other diseases. The methods of prevention of seed- 

 borne diseases is taken up on page 99. 



Insects. Little do we realize as yet the importance 



