Damping Off. SOT 



In a crowded seed bed after a few plants have fallen, unless the 

 disease is checked, it will spread from these affected ones as centers 

 to others near them and thus from the one or several starting points, 

 the plants will fall until nearly or quite all of them have been killed. 

 Where the soil and atmosphere is quite damp and the temperature 

 conditions so high as to favor rapid growth of the fungus it will 

 grow out from the diseased part of the stem into or on the surface 

 of the soil for a few millimeters in extent as a very delicate cottony 

 mass or velvety pile. Where the adjacent plants are not too far 

 distant the superficial threads may thus reach them and communi- 

 cate the disease to them. In other cases minute motile reproductive 

 bodies called zoospores, or swarrri spores (perhaps more properly 

 zoogonidia), are developed in a manner to be described later. These 

 swim in the soil water to the more distant seedlings and thus spread 

 the disease. 



Sometimes there will be seen quite a profuse growth of a mycelium,, 

 which on the surface of the soil may spread several centimeters in 

 extent. Usually this profuse growth is that of another fungus, a 

 lihizopus, or Mucor, or in other cases a different ''damping off" 

 fungus to be described in a later paragraph. 



If the tissues examined as described above from a seedling which 

 has not remained long after falling over perhaps the condition of 

 the mycelium described will be the only phase of the plant (for the 

 fungus is a plant) at that time present. If it has been dead for 

 sometime, however, there will probably be seen here and there on 

 the hyphae a number of rounded or spherical bodies, three to live 

 times the diameter of threads of the mycelium with which they are 

 connected. These are reproductive organs of the fungus and will 

 soon be described. 



The characters of the mycelium alone are not in all cases sufficient 

 for the correct determination of the plant. Let then this prepara- 

 tion on the glass slip lie free in an abundance of water, and place 

 the slip in a small, moist chamber sufficiently protected so that the 

 air in the chamber will not become dry by evaporation at the point 

 of contact of the two vessels. This can be avoided by placing a. 

 sheet of wet filter paper between the cover and the edges of the 

 bottom vessel. A Petrie dish, such as is used in bacteriological 

 work, is excellent for the purpose. Some wet filter paper should 

 also be placed in the bottom and on this the support for the glass 

 slip can l)e placed. For hasty examination the material can be.- 



