MOSAIC DISEASE OR CALICO 479 



FUNGOUS DISEASES 



Bed-rot or Damping Off. The rotting of young plants in the 

 plant bed is usually spoken of as damping off. It starts at or near 

 the surface of the ground. Planting in new soil may avoid the 

 trouble, and the various methods of treating the plant bed to kill weed 

 seed also destroy the fungus. Steaming is especially valuable for 

 killing fungous diseases, but roasting or surface firing should be 

 effective, too. Spraying with formalin, 1 to 50, will kill the fungous 

 spores if done before planting the seed. After the disease is started 

 the only way to stop it is to take off cheesecloth and thus lower the 

 temperature, and to limit the water so conditions will be unfavorable 

 to its development. Thinning the plants to admit air will also help. 



Root Rot or Black Root. This disease attacks roots of the 

 plants both in the bed and in the field, causing them to turn brown 

 or black. It can be controlled in the plant bed by the means sug- 

 gested for damping off, but the only practical remedy in the field is 

 to rotate and have clean soil to set the clean plants in. 



Brown and White Rusts. This disease is characterized by the 

 death of small spots in the leaves. These may run together and 

 cause a considerable part of the leaf to shrivel up. It seems to be 

 due to a number of conditions. Excess of water, of certain fertiliz- 

 ing material or of manure, deep cultivation, insufficient moisture at 

 certain stages, moist weather succeeded by hot weather, drops of 

 dew acting as a lens focusing the sun's rays may cause the brown 

 or white rusts. No practical method of control can be suggested. 

 However, tobacco should not be planted in fields known to produce 

 the disease. 



Mosaic Disease or Calico. This disease is characterized by the 

 mottled appearance of the leaf due to the presence of the light green 

 and dark green areas. In pronounced cases the leaf may be 

 wrinkled or corrugated. 



No parasite has thus far been found associated with the disease, 

 but it seems to be able to perpetuate itself. Plant beds should not 

 be placed where the disease has been bad before. When only a few 

 diseased plants occur they should be taken up and destroyed. When 

 the disease is likely to occur planting on poorly drained soil should 

 be avoided. The disease may be transmitted from one plant to 

 another in suckering and topping. For this reason the healthy 

 plants should not be topped by the same person at the same time 

 as the diseased plants. 



