Causes of Diseases in Crops 83 



attributed to the work of parasitic organisms. It 

 is often noticed on tomatoes (fig. 14) and beans. 

 Various causes may lead to it. A period of warm 

 weather accompanied by .cool nights, or by sudden 

 drops of temperature, will induce many truck crops 

 to shed their blossoms. In this case truckers are 

 helpless, for weather conditions are not controllable. 

 Blossom drop may also be brought about when too 

 much nitrogen is applied to the soil in the form of 

 mantire, hen manure espedaliy. To overcome this, 

 the fertilizer in the soil must be balanced by the 

 addition of 600 pounds of acid phosphate and 150 

 pounds of muriate of potash per acre. 



C. Diseases of Unknown Origin 



MOSAIC 



This trouble extends practically to all parts of the 

 host except the roots. To the tomato grower the dis- 

 . ease is very important, for it may reduce the yield 

 of his crop by 50 per cent. 



Symptoms. Mosaic is readily distinguishable by 

 a ydlow dotting or mottling of the leaf, presenting 

 in some instances a beautiful mosaic structure (fig. 

 15), whence its name. Affected leaves linger for a 

 time, but they eventually lose aU of their chlorophyll. 

 Another symptom is a curling of the leaves resembling 

 the curling induced by green aphids, but in this 

 case the insects have no association with it. The 

 disease makes its appearance after the seedlings are 



