Vegetable and Field Crops 259 



The cause of mosaic is unknown, but it has been repeatedly 

 shown experimentally that it can be communicated from 

 plant to plant by first touching a diseased plant and later 

 touching a healthy one. The disease then appears within 

 six to fourteen days. Thus the disease is spread largely by 

 topping. In nature it is probably spread by insects. The 

 virus appears to be present in all parts of an affected plant, 

 even in the trichomes, and, when inoculated locally, spreads 

 rapidly throughout the plant, though the disease is not 

 transmitted to the offspring through seed from diseased 

 capsules. Tomatoes, petunia, physalis, datura, solanum 

 (but not the potato), and pepper are susceptible to inocula- 

 tion from tobacco plants. 



The seed bed should be either new or thoroughly disin- 

 fected by steam or firing, and no tobacco refuse or tobacco 

 water used on it. Diseased seedlings in the bed, if any ap- 

 pear, should be destroyed. It is well to top, worm, and 

 sucker the healthy and sick plants separately, on different 

 days, thoroughly washing the hands with soap and water 

 before passing from the diseased to healthy plants. 



Orobanche. — This is a true flowering plant, whicli draws 

 its noinishment from the roots of the tobacco plant to which 

 it is attached. The orobanche occurs in small clusters, its 

 stalks are from 1-4 dm. tall, and entirely devoid of green 

 color. 



They should be pulled and burned before they produce 

 seed. Root-knot (nematodes), see p. 24. 



Curing-house " diseases." — When tobacco is cured by 

 natural heat, various rots may occur. Among these are: 

 Stem-rot, due to various fungi ; Pole-rot, consisting of small, 

 blackened areas upon the leaf, limited at first to the regions 

 near the veins. These spots may enlarge within two days to 

 cover whole leaves, and the contents of the entire barn may 

 become worthless, the leaf having become so wet and soft 

 as to readily fall apart. The cause of these rots is not cer- 

 tainly known; indeed there ma>' be several separate or- 

 ganisms, each capable of inducing such rot. 



