July i, 1918 
True Nature of Spinach-Blight 
3 i 
periods of inoculation varied from 50 to 75 per cent, but apparently 
had no direct relationship with the length of time the aphids had 
been on the plant. Little difference could be observed in the infectivity 
of the two species. The length of the incubation period of the blight 
showed but slight variation in the various plants affected, 14 to 18.8 
days, and there was no indication of a relationship between its length 
and the length of the inoculation period. The shortest average incu¬ 
bation period was obtained with individuals of Rhopalosiphum persicae 
placed on the plants for 2 hours, while the longest was when Macrosiphum 
solanifolii were placed on the plants for 14 hours. When a number of 
M. solanifolii were allowed to remain on the plants for 5 minutes 4 plants 
out of 8 became infected and showed positive symptoms on the twenty- 
fourth day. Where R. persicae were used, 2 plants out of 9 showed 
positive symptoms in an average time of 27.5 days. Of 10 plants inocu¬ 
lated with the virus of the original diseased plant from which the aphids 
were taken, 8 became infected and developed positive blight symptoms 
in an average time of 16.1 days. Ten untreated plants used as controls 
remained healthy; hence, it will be seen that virus-bearing aphids pro¬ 
duce infections when they feed on healthy plants for only a few minutes. 
INACTIVITY OF MATURE AND IMMATURE APHIDS 
The following series of transfers were made in order to determine the 
relative infectivity of aphids in their various stages of development. 
These were performed with both Macrosiphum solanifolii and Rhopalo¬ 
siphum persicae . The insects were reared and allowed to feed, repro¬ 
duce, and develop on typical fourth-stage-diseased spinach plants. 
Individuals representing each of the five instars were transferred to 
series of healthy plants. Transfers were also made with adult alate and 
apterous females. A series of 10 plants were inoculated with insects 
in each instar, four insects to each plant. They were allowed to remain 
on the plants for a period of 48 hours. As will be seen from Table VI, 
those individuals of M. solanifolii of the first instar which had been 
transferred directly from the diseased to the healthy plants produced 1 
infection in 10 inoculations, positive symptoms appearing on the twenty- 
second day (PI. 9, B). The remaining plants were healthy when the 
experiment closed, 54 days later. The second instar of M. solanifolii 
produced 4 infections out of 10 inoculations, positive symptoms appear¬ 
ing in an average time of 19 days. The third instar of M. solanifolii 
produced 4 infections in an average time of 18.5 days. The fourth instar 
of M. solanifolii produced 8 infections in an average time of 17.6 days. 
The fifth instar, alate form, produced 9 infections in 17.2 days, while 
the fifth instar, apterous form, produced 7 infections in an average time 
of 14 days. It will be seen that when healthy plants are inoculated by 
transferring to them adult aphids from diseased plants, the incubation 
period of the disease is several days shorter than it is where aphids in 
