132 
N. C. AGRICULTURAL EXPERIMENT STATION. 
In cases where the ascospores fall upon dead lettuce leaf fragments 
still in contact with the live lettuce plant the infection bridge is open. 
If the ascospores fall upon organic matter separated by some centi¬ 
meters from the lettuce plant the organic matter there available will 
be consumed and the mycelium will perish without any infection re¬ 
sulting. In some instances when the food supply is fairly large new 
sclerotia may be formed. Migration of the mycelium through the 
soil in efficient infecting condition, for any considerable distance, does 
not occur, even in soils bearing a large amount of organic matter as 
reckoned by the horticulturist. 
Infection from plant to plant is governed by these same conditions. 
If the infected plant lie sufficiently near to the uninfected one, the 
distance can be bridged by the mycelium, but this does not occur at 
any great distance, usually not over a few centimeters. 
That infection from plant to plant does not bridge over any large 
distance is shown by many observations of healthy plants standing, 
surrounded on all sides, by dead sclerotinized plants. 
Attempts to Control the Disease. 
According to Stone 9 the lettuce sclerotiniose became so bad in many 
green-houses in Massachusetts that many growers lost practically their 
whole crop from this cause. Disinfection of the soil, however, proved 
practicable under green-house conditions. 
Two principal methods of soil disinfection were early used (1) by 
means of chemical solutions; (2) by means of heat. The first of these 
methods has been proved to be unsatisfactory and impractical in most 
cases. The second has been of a very great benefit. 
One of the earlier methods for heating the soil was to sprinkle it 
with hot water. This was of Some value but not entirely effective 
and moreover it possessed the disadvantage that it left the bed so wet 
that it had to remain idle for considerable time in order to become dry 
enough to be worked. 
The substitution of steam as a disinfectant followed and it is today 
used with considerable success under green-liouse conditions. There 
are three ways in which steam may be used: (1) by placing 2-inch 
drain tile permanently under the soil in rows 16 inches apart, more 
or less as circumstances dictate, and flooding them with steam; (2) 
by laying perforated steam pipes upon the soil in the middle of the 
bed and throwing the soil from the sides upon them, then heating by 
steam, and after steaming pulling out the pipes to use elsewhere. The 
soil is then covered with canvas for several hours; (3) by a harrow¬ 
like arrangement, of pipes. Tig. 29. The teeth of the harrow are 
perforated on all sides to allow the steam to escape into the soil. The 
teeth are driven into the soil to a depth of perhaps 10 inches and 
heated to 208 degrees T., and this temperature maintained as long as 
is desired. 
The disinfection of green-house soil in some of these ways has been 
used in Rhode Island, 20 Vermont, 21 Ohio, 22 and some other States, 
nearly always with beneficial results. 
