TOBACCO DISEASES AND THEIR CONTROL. 33 



cally zonated was said to be due to Macrosporium longipes n. sp. : a 

 " white-speck " disease was attributed to Macrosporium tabacinum n. 

 sp. ; and an irregularly shaped spot, pale dirty brown, lighter in the 

 center, with a reddish zonate margin was attributed to Phyllosticta 

 nicotiana. No mention is made of inoculation experiments, and the 

 pathogenicity of these fungi is therefore questionable. So far as 

 known, they have not since been described on tobacco in America, 

 except for one recently reported occurrence of P. nicotiana in North 

 Carolina and a fungus, apparently Phyllosticta, on tobacco seedlings 

 in Virginia. 



A recently reported disease which is most commonly found produc- 

 ing a leaf-spot in seed beds, where it may also cause damping-off, 

 but which may also apparently appear in the field, has been found 

 to be due to a species of Fusarium, closely resembling Fusarium 

 affine. Exceptionally high moisture accompanied by some pre- 

 liminary weakening of the plant due to unfavorable conditions ap- 

 pears to favor infection. Only isolated cases are known, and the dis- 

 ease is not to be regarded as an important one. 



NONPARASITIC LEAF-SPOTS. 



Under this general term may be included a considerable number of 

 leaf -spots or injuries which differ considerably in appearance and 

 are evidently due to various causes arising from the effect of soil or 

 weather conditions on plant growth rather than following the at- 

 tack of any parasitic organism. No distinct line can be drawn be- 

 tween this class and other abnormalities of plants resulting from 

 malnutrition or other causes, but arbitrarily we may include those 

 diseases in which the early symptoms are mor or less characterized 

 by spotting of the leaves. This class of tobacco diseases has con- 

 siderable historical interest, since such diseases were apparently 

 among the earliest observed maladies of tobacco, or at least all early 

 observed spotting was attributed to nonparasitic influences. 



Such terms as " firing," " specking," " rusting," and " spotting," 

 with various modifications usually descriptive of the color of the 

 diseased areas, have been used from the earliest days of tobacco cul- 

 ture. Their origin has been laid to all sorts of causes, none of them, 

 however, with any satisfactory proof for the theories presented. 

 There can be no doubt that some of these diseases, such as frog- 

 eye, black-fire, and Wisconsin leaf-spot, were early included in the 

 list of diseases attributed to nonparasitic agents which are now 

 known to be due to specific organisms. The belief held from the 

 earliest time was that spotting of tobacco leaves was due to an excess 

 or deficiency of fertilizers or water in the soil or to a succession of 

 certain weather or atmospheric conditions. These theories are, of 

 course, sufficiently broad so that some of them are doubtless respon- 

 sible for some forms of spotting, but they are not of much help 

 in explaining the frequently distinctive types of nonparasitic spot- 

 ting which are now known to exist and that usually can not be 

 produced at will by subjecting the plants to variations of the above- 

 named conditions. In view of this fact, control measures for many 

 of these forms of spotting are not at hand. 



91061°— 24 3 



