336 



Mosaic Disease of Potatoes. 



[Jl-LY, 



lated into a healthy one the disease is reproduced either 

 immediately or in the progeny of the inoculated plant. The 

 causal agent is not known, but it has been shown that fungi are 

 responsible and bacteria have not been found. It is clear, how- 

 ever, that the infective principle or virus is present in the cell- 

 sap, but the nature of this virus has so far eluded the search 

 of the highest powers of the microscope. Whatever the 

 infective principle may be it is in some cases so potent that 

 even the most minute quantities of infected sap conveyed by 

 small sucking insects such as aphides are sufficient to transmit 

 the disease. Among other important crops in which infectious 

 Mosaic disease occurs are those of tomatoes, beans, cucumbers, 

 sugar-beet, maize, sugar-cane and tobacco. 



In some cases (e.g., dwarf beans) Mosaic is transmitted 

 from season to season through the seed, but in others (e.g., 

 tobacco) this does not appear to be the case. In Potato ]Mosaic 

 it is carried by m,eans of the tubers, and thus, like Potato Leaf 

 Curl, it may, in a loose sense, be said to be inherited. 



Description of Potato Mosaic. — The syaiptoms of the 

 disease vary somewiiat, both in different varieties and in 

 different parts of the country, but the following account will 

 probably suffice to indicate the features by which Mosaic may 

 be recognised. 



The most obvious and distinctive character, and the one 

 from which the disease takes its name, is the mottling of the 

 foliage. The individual leaflets, instead of presenting a normal, 

 uniformly green appearance, are faintly mottled or mosaicked in 

 varying shades of green.* Usually^ coupled with the mottling is 

 a crinkling of the foliage — a waviness in the outHne of the 

 leaflets and other indications that the leaves are not normal . In 

 some varieties and in severe attacks this crinkling or puckering 

 becomes very marked. Typical Potato Mosaic may nearly 

 always be seen (in early summer especially"! in the very 

 susceptible varieties mentioned on page 839. Under certain 

 conditions, and especially if it only develops late in the season, 

 this mottling of the foliage may be very conspicuous, but the 

 plants otherwise appear healthy and may produce a good or 

 fairly good crop of tubers. 



In more severe attacks other symptoms are apparent. A 

 dwarfing tendency is very frequently manifested, and w^hen 



It shuulil be noted that this iiiotthng is a very faint one and quite different 

 from the bright yellow spots found not infrequently in certain early varieties. 

 The latter is a variegation and not ]\rosaic or any foini of disease. 



