Causes of Diseases in Crops 75 



prevalent, usually diaring Jtily. It may be foimd in 

 sandy as well as clay loam fields, and the contour of 

 the land seems to have no influencing efifect. The 

 following is Jones and Gilbert's explanation of the 

 phenomenon: "When an electric stonn breaks sud- 

 denly following a period of dry weather and the first 

 rain wets the topsoil, there remains a layer of. dry 

 earth between this wet surface and the moist soil 

 underneath, which is a poor conductor of electricity. 

 When the lightning strikes the wet surface spot, it 

 disperses in all directions, horizontally and then 

 downwards into the earth, following lines of least 

 resistance. The plant stems and roots with their 

 abundant water content are better conductors than 

 the layer of dry soil just mentioned, and so the 

 electric current passes through them. The tissues 

 may thus be variously injured or killed, depending 

 upon the amount of current passing through them. " 



FROST INJURY 



The greatest profits in trucking are generally made 

 when crops are available for the early market. 

 This means that truckers must be prepared to meet 

 losses directly due to spells of frost. Not all 

 truck crops are equally sensitive to frost, but we 

 have as yet no crops which are absolutely frost 

 proof. When the temperature at which condensa- 

 tion of moisture in the air takes place is below freez- 

 ing, ice may form in the intercellular spaces, and 

 the plant is then destroyed, without any frost 



