1242 
Damping Off 
Corticuim vagunr 
This trouble 1s caused by a soil fungus 
frequently called Rhizoctonia and known 
technically as Corticuum vagum. This 
fungus, which is widespread, is capable 
of causing damping-off diseases in many 
different kinds of seedlings and plants. 
In its attack on lettuce it may either 
“damp off” the young seedlings at the sur- 
face of the ground or, if it attacks older 
plants, it may produce the rosette. This 
condition is characterized by the failure 
of the center leaves to grow. 
Control 
Hither steam sterilization of the soil 
or a treatment with formalin has been 
found effective. In sterilizing with form- 
alin, use a 5 per cent solution of the 40 
per cent formaldehyde in water and 
drench thoroughly. Allow soil to dry 
before setting plants. 
Downy Mildew 
Bremia lactucae 
This disease is caused by a fungus 
which in its life history is similar to the 
fungus causing the onion mildew. This 
disease occurs, in general, only under 
eertain conditions favorable to the 
growth of the fungus. It occurs in the 
greenhouse or in forcing frames which 
are kept too warm or moist and may 
ordinarily be controlled by properly reg- 
ulating these conditions. 
Symptoms 
It is recognized by yellow areas on the 
upper surface of the leaf and by the white 
downy growth of the fungus on the lower 
surface. 
Control 
It is advisable to burn diseased leaves 
or plants and water the beds in such a 
way that the foliage is kept dry. 
Drop 
Sclerotinia libertiana 
The fungus causing this disease occurs 
on many different kinds of plants, and 
since it is a common soil organism of a 
type that is hard to control should be 
carefully guarded against. The affected 
plant shows no definite diseased spots. 
It grows slowly, finally the stem and 
ENCYCLOPEDIA OF PRACTICAL HORTICULTURE 
lower leaves become watery and soon the 
whole plant collapses and rapidly decays. 
Control 
It is claimed that the disease can be 
eradicated from a bed in two years if all 
diseased plants are removed and de- 
stroyed as soon as they appear and the 
place is immediately drenched with Bor- 
deaux mixture or a solution of bluestone. 
This is necessary in order to prevent the 
sclerotia or resistant stage of the fungus 
from maturing. In some cases it may be 
easier to change or sterilize all of the soil 
in the bed. The disease can be held in 
check by the careful regulation of tem- 
perature and water supply. A low night 
temperature with constant day ventila- 
tion is very essential. 
Gray Mold 
Botrytis cinerea 
This disease is due to a common fungus 
and is in all essentials like the drop. 
After the plant has collapsed, a gray mold 
appears and at this stage great numbers 
of spores capable of spreading the dis- 
ease are produced. The same control 
measures apply as for drop. 
Leaf Perforation 
Marsoma perforans 
This disease has recently been found 
in greenhouses in several localities in 
the Northwest, and as it has caused con- 
siderable damage in Ohio, where it was 
first discovered, it seems advisable for 
the growers to be on the lookout in order 
to hold it in check. 
Symptoms 
On the leaves this fungus causes spots 
which die and drop out. On the midrib, 
many sunken, elongated spots are pro- 
duced. A plant once affected seldom ever 
recovers and the new leaves are mis- 
shapen and unsalable. 
The spores of the fungus are often scat- 
tered by watering or air currents. They 
are produced in great numbers on the 
spots of dead leaf tissue which fall out. 
Control 
It is claimed that Bordeaux mixture 
can be used in the seed beds and on seed- 
lings to keep the disease down, but if 
