July i, 1918 
True Nature of Spinach-Blight 
27 
siphum solanifolii were more abundant than Rhopalosiphum persicae. 
Two other pots of seedlings were inoculated by mashing diseased tissues 
from the blighted plants into the leaves of the seedlings. Nineteen days 
after inoculation one of the inoculated plants had developed characteristic 
symptoms of the blight. One mottled leaf was removed from this plant, 
and with it three seedlings in another pot were inoculated by mashing 
the diseased tissues into their leaves. With another leaf from the 
blighted plant inoculations were made in one of the field cages by mash¬ 
ing the blighted tissues into the leaves of the plants. Twenty-five days 
after the inoculation typical symptoms of blight appeared on eight 
plants in the field cage, while the control plants remained healthy. 
Farm F is located a considerable distance from the majority of the farms 
where the blighted plants used in the other experiments had been 
obtained. The fact that virus from the diseased plants on this farm 
produced blight in both greenhouse and field-grown plants indicates that 
the disease is the same as that occurring on the other farms. 
COMPARISON OF SCATTERED, INDIVIDUAL BLIGHTED PLANTS* WITH 
BLIGHTED PLANTS FROM AREAS 
Further experiments were begun with the object of determining any 
variation in the nature and transmissibility of spinach-blight occurring 
in individually diseased plants and plants in diseased areas. Both 
species of aphids were used to make the transfers, and the material was 
checked each time by needle-prick inoculations. When transfers were 
made, an average of four insects were placed on each plant. Twenty- 
four hours later the plants were fumigated to kill the aphids. The 
healthy plants were 24 days old at the time of inoculation. The diseased 
material used for inoculation purposes was collected on various farms in 
the spinach-growing region of eastern Virginia. The results of this 
experiment are given in Table m. These show that the percentages of 
infection are similar for the individually diseased plants and for those 
collected in the diseased areas. This indicates that the factors produc¬ 
ing the disease from both of the above sources are the same. 
Since spinach-blight occurs both in scattered plants in the field and in 
distinct areas, it led to a study of the cause of these variations. Briefly 
stated, the results of this study indicated that under certain conditions * 
favorable to the development of aphids there often occur heavy local 
infestations. During such periods the apterous females are particularly 
numerous and active, passing readily from plant to plant. Provided the 
favorable conditions persist, the infested area may be enlarged from a 
few plants to an area many yards in diameter. If a blighted plant 
occurs near the center of infestation, infection may be carried by the 
aphids to numerous other plants. When the conditions change and the 
aphids are greatly reduced in numbers, the spread of infection is like¬ 
wise checked. Thus is an area of blighted plants formed. The infec- 
