TOBACCO DISEASES AND THEIR CONTROL. 



41 



ment brown leaf spots may develop (PL XIII, fig. 2). Marked 

 stunting, with yellowing, may eventually follow. 



Before attributing the above-described symptoms to nitrogen or 

 phosphoric-acid starvation the roots of the plants should be care- 

 fully examined for root injury by the root parasites previously de- 

 scribed. At the same time the weather and soil conditions in gen- 

 eral must be taken into account. Chemical analysis of the soil may 

 aid in determining which element is lacking, but trial application of 

 fertilizing elements is the most reliable method to determine the 

 fertilizer needs of any particular soil. 



INJURIES DUE TO PHYSICAL AGENCIES. 



The tobacco plant is subject to a number of injuries resulting from 

 the action of physical agencies which may be grouped together. 

 These deserve brief consideration under the subject of diseases, be- 

 cause they may sometimes be mistaken for plant diseases. 



Injuries of this nature which frequently appear on tobacco are 

 rain-spot, hail cuts or spots, frosted or frozen tobacco, sun-scalded 



Fig. 22. — Lightning injury to tobacco. This apparently diseased area was produced 

 by lightning striking the field. 



and sunburned tobacco, wind whipping, bruises from handling, 

 spray injury, sand beating, and lightning damage (fig. 22; PI. Ill, 

 fig. 2). To these might also be added the large group of insect in- 

 juries, some of which are sometimes mistaken for disease injury. 

 Most injuries of the above-named types soon after taking place are 

 ordinarily too evident to warrant description (fig. 23). Some time 

 after the damage has occurred, however, and especially when affected 

 leaves have been cured or fermented, considerable difference of opin- 

 ion may arise as to the source of the damage found. 



UNIMPORTANT OR HARE DISEASES NOT OTHERWISE CLASSIFIED. 



A considerable number of abnormalities occur in rare or isolated 

 cases in tobacco fields, regarding which little is known as to causal 

 relations, since sufficient importance is not attached to such cases to 

 warrant invest igatiom Some of these diseases have not been de- 

 scribed at all or not more than once in the literature, so that com- 

 parisons are difficult to make. 



One of the more common abnormalities of this nature is sometimes 

 known as marbling or variegation, but in foreign countries goes un- 



