DISEASES OF WHEAT 149 



mention. Very little specific information can be found as to 

 the extent of damage caused. Such scattering data as have 

 been collected can be most advantageously presented under the 

 subject of insurance. 



FROSTS. The most usual injury by frost is the winterkilling 

 of fall wheat. This may occur whenever the ground freezes to 

 any appreciable depth, and in two ways. The plants either 

 freeze to death, or are lifted out of the soil by alternate 

 freezing and thawing. A good covering of snow is very pro- 

 tective. Seeding with a press drill lessens the danger. Frost 

 may also injure wheat when it is filling, or it may cause the 

 stems to burst after they have jointed. 



HOT WAVES OR WINDS are most liable to occur during a period 

 of drought. It is thought that these waves can be forecasted 

 for a period of about four days. At such times the eastward 

 circulation of the atmosphere is practically suspended, and radi- 

 ation is at a minimum. A hot wave is defined as a period of 

 three or more consecutive days with a maximum temperature 

 reaching or passing 90 F. In years when hot waves are un- 

 usually severe, the harvest returns are decreased by one-fourth 

 in quantity, and the quality is quite inferior. The heat seems 

 to mellow the ground, however, and to put it in such ideal con- 

 dition as to increase the crop of the following season. Hot 

 winds have a velocity of 20 to 30 miles per hour, a tempera- 

 ture often ranging from 100 to 106 F., and 20 to 30 per cent 

 of relative humidity. The roots cannot supply moisture, even 

 if it is present in abundance, as fast as it is evaporated from the 

 leaves by this great blast of hot, dry air. The cells are com- 

 pletely desiccated, and the whole structure of the plant col- 

 lapses. A hot wind is most destructive immediately after a 

 rain, which temporarily checks and lessens the transpiration of 

 which the plant is capable. In the United States these winds 

 are most apt to occur in the central prairie regions. In Argen- 

 tina, a similar dry, hot wind known as the pampero comes up 

 suddenly, destroys all vegetation "and even cracks furniture 

 and timber in buildings.' 7 A wind-break of trees, or anything 

 else that tends to lessen the movement of the air, has a remedial 

 effect. 



EXCESSIVE MOISTURE. This may be injurious in a number of 

 ways. If too much water is present, the wheat may be 



