TOBACCO DISEASES AXD THEIR CONTROL. 9 



consequence of atmospheric conditions or poor ventilation in covered 

 beds is also a predisposing factor. High temperatures are also 

 favorable for the development of Pythium, although Rhizoctonia is 

 apparently favored by relatively lower temperatures. Generally 

 speaking, the thicker the stand of plants the more favorable the con- 

 ditions for the spread of damping-off. 



Control. — Seed beds should not be located where danger of damp- 

 ing-off exists unless the soil is well sterilized. Ordinarily, damping- 

 off can be avoided by locating the seed beds on grassland or where 

 only grains or corn have been grown the preceding year, unless the 

 parasites are introduced with the animal manures used. Garden 

 soils, old plant beds, weedy patches, or areas where refuse has been 

 thrown are likely to be infested. TVell-drained sandy soils are best, 

 since they permit more rapid drying out after wet weather. Avoid 

 the common error of seeding too thickly. Water the beds thoroughly 

 rather than frequently, and if damping-off appears permit them to 

 become well dried off if possible. Ventilate the beds by removing 

 the covers during favorable weather. If the disease occurs only in 

 isolated small areas remove the infected plants and those immediately 

 surrounding, and if the plants are still young apply a formaldehyde 

 solution (1 part to 50 parts of water) to the infested area, leaving 

 off sash covers of such areas, however, to permit the escape of the 



The occasional occurrence of damping-off in steam-sterilized beds 

 may result either from insufficient heating of the soil or from rein- 

 festation due to unsterilized soil being accidentally transferred to 

 the seed bed. 



Plants showing lesions of damping-off on the stem should not be 

 transplanted to the field, since such plants may subsequently rot off. 



SORE-SHIN, STEM-ROT. 



Description. — The sore-shin type of disease has been reported from 

 various parts of the country under different names. In addition to 

 those given, sore-shank, canker, black-leg, collar-rot, bed-rot, or 

 damping-off may be considered synonymous as far as symptoms are 

 concerned. As a matter of fact, however, it has not been found that 

 these diseases are all one and the same, and it is not improbable that 

 at least three or more different causal organisms are concerned in 

 their production. 



As the names imply, this disease is characteristically a rot, nor- 

 mally located on the stem or stalk of the plant and usually starting 

 in the vicinity of the soil surface. This decay may appear only as a 

 blackening of the surface of the stalk, but more commonly it pene- 

 trates to the woody portion in the case of large plants or completely 

 through the stem in young plants. Under favorable conditions the 

 blackening may extend up the stalk of the plant for a considerable 

 distance and enter the midrib and veins of the leaves, causing them 

 to drop (fig. 5). Affected plants are usually dwarfed and yellowish 

 in color, owing to interference with the transfer of water and plant 

 food. The weakening of the plant at the soil surface also renders it 

 quite likely to be broken off at that point by the wind. If the dis- 

 ease does not encircle the entire stem the plant may partially or 



