DAMPING-OFF OF SEEDLINGS 
The name damping-off is applied to a type of disease affecting 
seedlings rather than to any one specific disease. Many species of quite 
different groups of fungi are known to cause damping-off, but as here 
used the name will be restricted to the diseases produced by certain 
phycomycetous fungi. 
SYMPTOMS 
The most striking symptom and the one which gives to this type of 
seedling-disease the name damping-off, is the peculiar effect of a necrotic 
lesion in the stem at or near the surface of the ground. Often the 
lesion originates just below the surface of the soil. A wilting and then a 
disintegration of the tissues affected, results and the plantlet falls over and 
dies. Root-rot may accompany damping-off and may prove fatal except 
where adventitious roots are put out above the lesion and remain healthy. 
Study the damped-off seedlings provided and photographs 1, 2, 3, 4 
and 5. OBSERVE :— 
1. The withering of the stem-tissues at or near the surface of the 
soil. 
2. The sharp angle made by the fallen stem. While affected 
plants usually fall over, they may remain erect and wilt if the vascular 
tissues are sufficiently rigid. ~ 
3. The dying of all the plants in affected areas. These damped- 
off spots are quite characteristic of infested seed-beds. 
DRAW to show these characteristic symptoms. 
ETIOLOGY 
A rather large number of fungi are now known which produce the above 
symptoms in seedlings. (See text.) As indicated above, only phycomyce- 
tous forms will be here considered. Of these it is one of two species that 
is usually responsible, either Pythium de Baryanum Hesse or Phytoph- 
thora Cactorum (Cohn and Lebert) Schroeter. These two species are 
very much alike in structure and habits so that the following outline 
should serve for either. 
Life-history. These pathogenes exhibit in their life-history both primary 
and secondary cycles, with a short period of pathogenic activity and a well- 
marked saprogenic phase, especially in the case of P. de Baryanum. 
The Primary Cycle is initiated at or shortly after the germination 
of a host. Sometimes the seedlings are killed before they get above 
ground. 
; Pathogenesis. Both fungi pass their saprogenic existence in the 
soil, probably always in association with host-debris. Both produce 
resting sexual spores,—the oospores, which carry them through the winter. 
These germinate and produce the inoculum for the primary cycle. Mount 
and examine some of the mature oospores provided (in pure cultures). 
OBSERVE :-— 
4. The form, size and color of the sexual bodies. 
5. The old oogonial sac, the outer covering enclosing the single 
oospore. The thick smooth wall of the oospore and its densely granular 
oily contents. DRAW. : 
28 
