A SERIOUS LETTUCE DISEASE. 
131 
among the roots of a plant that had just died. The root was always 
still perfectly sound, even after the aerial portion of the plant was 
entirely dead and in no case were sclerotia found in the soil until the 
root had had sufficient time to rot. 
General Relation of the Fungus in Nature. 
From the facts above adduced experimentally and from field ob¬ 
servation it is seen that Sclerotinia libertiana, which is the actual] 
cause of the disease under discussion, propagates and spreads by means 
of its ascospores produced from the germinated sclerotia and by its 
mycelium. 
The ascospores are comparatively short-lived, even under the con¬ 
ditions most favorable to their longevity. In condition of nature, sub¬ 
ject to alternate dryness and atmospheric humidity they would invari¬ 
ably germinate and in the absence of favorable nutrient pabulum, die. 
Thus the ascospores can not function to any large degree as a means 
of carrying this fungus over periods of time of any considerable dura¬ 
tion. 
The mycelium has also been shown to be of comparatively short-life 
and to retain its infecting power hut a short time in the absence of 
nutriment. This, too, can not be regarded as a means of carrying the 
fungus over long time intervals. Any possibility that the mycelium 
may remain alive in soil that has borne sclerotinized plants is very 
remote, probably non-existent. The sclerotium is long-lived and well 
adapted to perpetuate the fungus. It alone, of all the structures of the 
fungus, is able to live for sufficiently long periods to bridge over 
seasons adverse to the growth of the fungus or long periods of time 
when no food is available. 
The fungus may, therefore, he likened to an annual plant, all parts 
of which except the seed die at the approach of winter, the sclerotium 
in this instance acting the role of the seed. 
The sclerotia rest during the period of inactivity of the disease, that 
is from the harvest of one lettuce crop until the next crop is present 
under suitable weather conditions to permit of infection. They then 
germinate and produce apothecia with a crop of myriads of ascospores. 
These ascospores have been shown unable to directly infect healthy 
lettuce leaves. They may germinate in the film of dew upon the plant 
but can not force entrance into its tissue and can only perish. Neither 
are mere wounds ordinarily sufficient to allow infection. The asco¬ 
spores must at first sustain a period of saprophytic existence until the 
mycelium developing from it attains a certain degree of vigor. To do 
this the ascopores must fall upon and germinate upon organic matter, 
dead moist leaves or other plant parts or particles of manure or some 
other similar dead organic substance. A dead lettuce leaf or torn 
fragment of leaf serves its purpose admirably. If the saprophytic 
existence of the sporeling be upon organic matter which lies very 
near a lettuce plant the vigorous mycelium may reach over to it and 
parasitize it. 
