44 
Journal of Agricultural Research 
Vol. XIV, No. i 
in nearly every case. The fourth generation produced no infection in 
the insectary experiments, although in the field one infection was 
obtained. The incubation periods of the disease in every instance were 
shorter than in the M. solanifolii series. Both lots of plants were kept 
under similar conditions and treated alike, and the infectious entity 
which the insects carried was derived from the same source. Observa¬ 
tions of the feeding habits of the two species reveal that individuals of 
R. persicae were less disturbed by transference and not as liable to leave 
the plants as M. solanifolii. The former also fed considerably more 
during the 48-hour period of inoculation than M. solanifolii. It is possi¬ 
ble that these slight differences in the feeding habits of the species would 
result in the transmittal by R. persicae of the more active infection, 
thereby shortening the period of incubation. 
Macrosiphum solanifolii (Alabama strain) produced infection with all 
but the second generation under insectary conditions and the first and 
fourth generations on the field plants. The reason for not obtaining 
infection with the first and second generations in the instances cited may 
be due to several causes. These can not be explained until more is 
learned concerning the relationships existing between the insects and the 
causal factor of spinach-blight. The incubation periods for this series 
correspond closely to those obtained with the Norfolk strain of M. solani¬ 
folii. The inoculations with R. persicae (Louisiana strain) produced 
about the same number of infections as did M. solanifolii (Alabama 
strain) with the exceptions that no infections were obtained with the 
second generation under insectary conditions or with the fourth genera¬ 
tion either in the field or in the insectary. The incubation periods of the 
disease were similar with both species. 
It has been shown that certain supposedly non-virus-bearing aphids, 
collected locally on plants other than spinach and their offspring for sev¬ 
eral generations may occasionally produce infections of blight when 
allowed to feed on healthy spinach. No infections were obtained in any 
case where the aphids from other States were placed on healthy spinach 
plants, unless they had previously fed on diseased plants. The offspring 
of virus-bearing aphids when transferred to healthy spinach in previous 
experiments have occasionally caused infections of blight. This ability 
on the part of the aphids is confirmed by the results obtained in this ex¬ 
periment. The preceding data indicate that the inciting factor of 
spinach-blight is transmissible by parent aphids to their offspring for 
several generations. How this is accomplished is not known and prob¬ 
ably will not be fully understood until the nature of the inciting factor of 
this disease has been discovered. There are undoubtedly several factors 
which exert a controlling influence on the ability of aphids to transmit 
the disease, when the causal factor is inherited from ancestors which have 
fed on blighted plants. From the data collected on the conditions which 
limit this factor the following appear to be the more important: The con- 
