9 
joiy i, jji8 True Nature of Spinach-Blight 
6 , A. The cloth used to cover the cages is known as light-weight sheeting. 
For aphid experiments it is necessary to use cloth with a close weave and 
yet not so close as to prevent a free circulation of air. 
A type of small cage was developed which could be used to cover indi¬ 
vidual plants, or pots of plants on which insects were placed within the 
larger cages. These are shown in Plate 6, B. They consist of 12-mesh 
wire screen, rolled to a cylinder, and covered with a fine grade of sheet¬ 
ing or surgeon’s gauze. Those with a diameter of 4, 6, or 10 inches 
were found most convenient. For use with spinach a convenient height 
is 12 inches, although for taller growing plants, as peppers and tomatoes, 
a height of 24 inches is preferable. The cloth is allowed to extend 7 or 8 
inches beyond the top of the cylinder. It may then be drawn toward 
the center, gathered, and tied with a string, which permits the cylinder 
to be set over a pot and the plants examined through the opening made 
by loosening the string at the top. The cylinders were forced about 1 
inch into the soil to prevent insects entering from below. 
All individual plants or pots of plants within the larger field cages, on 
which aphids were placed, were covered with the small individual cages. 
At the end of the time the aphids were allowed to r emain 0 n experimental 
plants, the cages were fumigated with nicotine to kill the aphids. The 
plants were then carefully examined to insure that all the aphids had 
been killed and removed. Control plants were grown in each field cag e, 
in order that the controls on each series might be under similar conditions 
of temperature, humidity, and soil. Records of conditions of tempera¬ 
ture and humidity were made both within and without the field cages. 
The humidity was determined by means of wet and dry-bulb thermome¬ 
ters kept about 3 inches above the surface of the soil. 
As no specific microscopic organism had been found associated with 
spinach-blight, the only means of proving infections was by further in¬ 
oculations from the artificially infected plant, either by aphid transfers 
or by needle pricks. As certain forms of chlorosis (PI. 10, B) and mal¬ 
nutrition, produced either by the lack of fertilizers or by the excessive 
applications of them, may cause the spinach plants to become similar i n 
appearance to those affected with blight, the appearance and color of the 
plant could not be taken as positive evidence of the presence of the 
infectious disease. Two to five healthy plants were inoculated with the 
virus. When one or more of these plants developed a mottled appear¬ 
ance of the leaves characteristic of the disease, the original plant was 
credited with positive symptoms at the time the first color changes 
became visible in its leaves. Most of the secondary inoculations were 
performed in the greenhouse. As the season advanced and larger series, 
with higher percentages of infection occurred, only 5 to 50 per cent of 
the plants in each series were checked out in this manner. In the ma¬ 
jority of cases the prick inoculations were made in the true leaves, but 
