76 PROTECTION OF PLANTS, 1921-22 



Infection 



The tubers transmit the disease as in the case of mosaic, but an additional 

 fact must be mentioned, namely, that in the progeny of plants with leaf-roll 

 there is a much more rapid depreciation in yield and accentuation of the disease 

 than is the case with mosaic. 



Tests conducted in Maine show that aphids again are the only insects of 

 importance in transmitting leaf-roll. The seed-cutting knife is not a factor in 

 the spread of the disease. 



Varietal Susceptibility 



Red varieties are usually more susceptible than white varieties. Garnet 

 Chili; Da vies Warrior, Irish Cobbler, Early Puritan, Early Six Weeks, Dakota 

 Red and Mclntyre develop the disease more severely than Carman, Green 

 Mountain, and Empire State. The last named at times appears to be free and. 

 at others is severely affected, due in all probability to lack of inoculation by 

 aphids. 



Control 



The same points mentioned for the control of mosaic apply to the control of 

 leaf-roll. 



GROUP 2. 



Diseases caused by Myxomycetes or Slime Moulds 



There are certain organisms which have not yet been definitely assigned to 

 the animal kingdom nor to the plant kingdom, because during part of the life 

 history they move, feed and grow in such a manner that they are more animal 

 like than plant like, while the reverse is the case at reproduction time. Such 

 organisms are known as "Myxomycetes" and usually they are saprophytic. 

 Two, however, are parasites causing diseases of considerable economic impor- 

 tance, viz., "Club root" of cabbage, cauliflower, brocoli etc., and "Powdery 

 scab" or "Corky scab" of potato. This latter disease is caused by a slime mould 

 known as Spongospora subterranea (Wallr.) Johnson. 



Powdery Scab or Corky Scab {Spongospora subterranea) 

 Occurrence 



The disease has been known in Europe for nearly eighty years being des- 

 cribed first by Wallroth in 1842 and it was noted by Barkeley in 1846 in England. 

 The former investigator, however, considered that the disease was a smut. 



