July i, 1918 
True Nature of Spinach-Blight 
47 
blight. Thus far it has been impossible to give this phase of the problem 
more than the briefest consideration. Further data along this line are 
being accumulated as the experiments proceed. 
RELATIONSHIP OF INSECTS TO THE SUMMERING OF THE DISEASE 
As certain aphids have been found to possess the ability to transmit 
the causal factor of spinach-blight to their offspring, the question arises, 
Do aphids then serve as a means of carrying the disease over from the 
time the crop is harvested in the spring until the spinach is planted in 
the autumn? The evidence we have would indicate that succeeding 
generations from the original parent aphid from diseased spinach grad¬ 
ually become less infective and the percentage of virus-bearing offspring 
decreases with each generation, provided they do not have access to 
diseased food. The condition of the diseased plant at the time the 
parent aphids feed on it is an important determining factor in the trans¬ 
mission of blight by their offspring. Also, the numbers of the aphids 
are reduced to a minimum in July and August, which would mean that 
a very small percentage of those which have the earlier conditions entirely 
favorable would survive to produce offspring which eventually reach the 
spinach in the fall. When blight first appears in the autumn, it occurs 
usually as widely separated cases, from which infection is carried to 
surrounding plants by aphids. In fact, the first blighted plants to be 
found in the autumn are about as numerous as the aphids bearing virus 
by heredity might be expected to be. In the autumn of 1917, collections 
were made of numbers of both Macrosiphum solanifolii and Rhopalosiphum 
persicae from various cultivated plants and weeds and placed on healthy 
spinach seedlings for a few days. Positive results were obtained with 10 
adults of M. solanifolii collected on the vines of sweet potatoes ( Ipomoea 
batatas) on September 24, 1917, and with three adults of R . persicae 
collected from celery (. Apium graveolens) on October 1. A number of 
M. solanifolii were placed on five spinach seedlings in the insectary, and 
two of them developed symptoms of blight on October 30. Similarly, on 
November 1, positive symptoms appeared on one of the two plants on 
which the individuals of R . persicae were allowed to feed. At the time 
the insects were collected, healthy spinach plants were inoculated with 
the juice of the plants from which the aphids had been collected. These 
inoculations gave, negative results. 
On June 4, 1917, in the potato field on farm A, which had been in 
spinach during the winter and early spring, a number of Macrosiphum 
solanifolii were collected on wild mustard plants growing near the 
center of the field where blight had been serious during the previous 
winter. These aphids were placed on pots of healthy spinach plants in 
the greenhouse; similar pots of plants were kept as controls. Seven 
days Mter some of the spinach plants showed doubtful symptoms of 
blight. Six days after this some of the plants developed the characteris- 
