8 Karl Hermann Fernow 



also, in positive cases, to record which mosaic was present. It will readily 

 be understood that the chances of error in this matter are greater than in 

 the simple statement as to whether or not a plant is affected with mosaic. 

 Two mosaics on the same species may be very distinct in their charac- 

 teristic expression and yet individual cases will occur in which it is difficult 

 or impossible to say with which mosaic one is dealing. In the brief dis- 

 cussions of the various mosaics which follow, it must be borne in mind 

 that the symptoms vary greatly with the conditions, not only in intensity 

 but even in kind. To describe all the possible symptoms that may be 

 produced by each mosaic would require very lengthy descriptions pre- 

 ceded by years of experience. Even the best of such would prove inade- 

 quate to the recognition of these mosaics by persons not already acquainted 

 with them. The descriptions are therefore largely comparative, and are 

 presented for the purpose of distinguishing between the various mosaics 

 when two or more are at hand on the same species. It will be found useful 

 to refer to the accompanying plates for more ready comparison. 



Datura meteloides Dunal and D. Stramonium appear to be subject to 

 both mosaic B and mosaic C. The symptoms of mosaic B (Plate I, 2) 

 are usually rather large irregular areas of yellowish green appearing on 

 the dark green background of the leaf, with little or no rugosity of the 

 leaf. Mosaic C (Plate I, 3), on the other hand, produces a very pro- 

 nounced rugosity, often causing the leaf to appear blistered, and the 

 leaves are not infrequently narrowed and twisted. The flowers are some- 

 times distorted, and are generally marked with blue or purple streaks on 

 the purple variety. It is not certain whether the streaking may not 

 occur also in connection with mosaic B. 



Lycopersicon esculentum Mill, is affected by both mosaic A and mosaic 

 B. The symptoms of mosaic A (Plate I, 6) are well known, this probably 

 being the mosaic of tomatoes generally referred to in literature. The 

 leaves show rather sharply contrasted areas of dark green and light yellow- 

 ish green. The mottling is usually most readily detected on the very 

 young leaves. It is often somewhat difficult to detect mosaic on tomato, 

 because of the narrowness of the leaves and the fact that the young leaves 

 are often normally lighter-colored and somewhat distorted. The dis- 

 tortion due to mosaic A is usually greatest in plants that have become 

 infected when very small. In such plants, the blade of the leaf may be 

 almost entirely suppressed, and the filiform leaves much twisted and 

 curled. In artificially inoculated plants, the distortion is generally not 

 so striking. 



The symptoms of mosaic B (Plate I, 5) on this plant are much less 

 striking. As the leaves become fully expanded they show a rather ill- 

 defined mottling. The symptoms may be easily mistaken for the effects 

 of soil or water conditions or of unfavorable temperature. As far as 

 observed, the very young leaves show no noticeable symptoms and no 





