TOBACCO DISEASES AND THEIR CONTROL. 3 



THE NATURE OF PLANT DISEASES. 



Before giving separate consideration to each disease it is well to 

 recognize the general facts in regard to- plant diseases and their 

 control. 



The most fundamental consideration to* be borne in mind is that 

 most of our plant diseases are caused by fungi or bacteria (germs). 

 These organisms (PL I, figs. 1 and 2), usually so extremely small in 

 size that they can be seen only with high-power microscopes, are 

 capable of attacking and feeding upon plant tissues, thereby injur- 

 ing the plant. The particular organism causing the injury is called 

 the parasite, and the plant attacked is conveniently referred to as 

 the host. Each parasite usually produces a distinct type of injury, 

 so that it is ordinarily possible to> recognize the disease readily by 

 the general symptoms. In the case of most diseases caused by para- 

 sites it is possible to isolate the fungus or bacterium concerned and 

 to grow it in pure culture, that is, free from all other living organ- 

 isms,, and thereby study many aspects of its character and behavior. 

 By inoculating plants with such cultures it is possible to prove posi- 

 tively that the organism is the cause of the disease in question and to 

 study many problems bearing on the control of the disease. 



Diseases of the above-described type are infectious, that is, under 

 favorable conditions they can spread from one plant to another. 

 They may be harbored in the soil, or by certain agencies may be 

 carried over unfavorable periods for infection and later spread to 

 susceptible plants which become diseased, environmental conditions 

 permitting. The part played by weather conditions in plant diseases 

 is an important one, but from the standpoint of disease control is 

 of little importance, since weather can not be controlled. About 

 the only' practical method for the control of plant diseases is there- 

 fore that of preventing infection either by eliminating the parasite 

 or else by safeguarding the host plants from its attack by spraying 

 or by the use of resistant varieties. 



A second class of plant diseases are those generally referred to 

 as the nonparasitic group. Such diseases are either known to be 

 or believed to be not due to parasites but result rather from unfavor- 

 able soil or climatic conditions which disturb the normal progress of 

 plant development. Several other types of injury may be included 

 here, however. The principles of control are therefore radically dif- 

 ferent from those used against parasitic diseases. 



Another class of troubles which must be considered in the case of 

 tobacco and which strictly does not fall into either of the above 

 classes is damage occurring during the curing, fermenting, or storage 

 processes, when the plant tissue is dead. While these troubles usually 

 are of the same nature as the parasitic diseases, in that they are due 

 to fungi and bacteria, the organisms concerned are termed sap- 

 rophytes because they live and grow normally on dead tissue. 



Insect damage is not considered in this bulletin except in so far 

 as it plays a part in spreading plant diseases. 



LOSSES FROM DISEASE. 



The annual losses caused by diseases of tobacco in the United 

 States are difficult to estimate with any degree of accuracy. Fairly 



