8 



BULLETTX 1256, U. S. DEPARTMENT OF AGRICULTURE. 



only brownish or black lesions appear on the stem of the plant (fig. 

 4). In case of an attack of damping-oif when the plants are very 

 young the symptoms above described are not very evident, and the 

 first indication of disease may be a poor and uneven stand of yel- 

 lowish plants. 



Cause. — Damping-off may be caused by one or more of several 

 plant parasites. The most common organisms causing this disease 

 are, however, two fungi, Pythium debaryanum and Rhizoctonia 

 solani (Corticium vagum). The symptoms produced by these or- 

 ganisms are essentially identical, but the organisms themselves are 

 readily distinguishable under the microscope. Both of these fungi 



Fig. 4. — Tobacco plants affected with damping-off, or bed-rot. This disease is char- 

 acterized by a decay of the steins of young plants in the plant, beds. 



are very common soil-inhabitating organisms and attack a large 

 variety of other plants. They are most likely to be found in garden 

 soils or other soils where their common host plants have previously 

 been grown. They are apparently least likely to be present in soils 

 where only grasses or grains have been recently grown. Both or- 

 ganisms overwinter in the soil. They do not spread as readily as 

 many other parasites, since spores are not ordinarily produced as 

 abundantly, nor are they so readily spread through the air. 



Conditions favoring the disease. — Soils or plants remaining wet 

 for relatively Long periods furnish conditions especially favorable for 

 the spread of the disease. High humidity around the plants as a 



