46 
Journal of Agricultural Research 
VoL XIV, No. i 
There are, then, two groups of diseases: One affecting animal life, the 
other affecting plants. The pathogens in the groups have several points 
of similarity. First, they are evidently parasitic; second, they are caused 
by an infectious virus; third, their manner of transmission from diseased 
to healthy hosts is usually through the agency of animal parasites infest¬ 
ing the hosts—that is, insects, mites, or ticks; fourth, students of both 
groups of diseases generally believe the definitive parasites to be ultra- 
microscopic organisms. Among animal diseases there are several which 
are transmitted by insects, mites, or ticks. Rocky Mountain spotted 
fever, a disease caused by an infectious virus is carried from diseased to 
healthy hosts by several species of ticks. Ricketts 1 found that not only 
was infection carried by the adult tick but a percentage of the offspring 
of the ticks from diseased animals inherited the ability to produce the 
disease. This case is similar to spinach-blight in which the causal factor 
of the disease is transmitted by the parent aphids to their offspring. The 
infectious entity of yellow fever has not been definitely proved to pass 
from adult mosquitoes to their offspring, yet the later experiments by 
Finlay 2 indicated the probability that the infectious entity was hereditary 
in certain species of Calopus. 3 
The well-known Texas fever of cattle was found by Smith and Kil- 
boume to be caused by a definite organism. The organism is transmitted 
by ticks on affected cattle to their offspring. 
SUMMERING OF SPINACH-BLIGHT 
ALTERNATE HOSTS 
If the disease survives the summer on plants other than spinach, it 
is probably carried by aphids both to and from the other species of 
plants. Certain insects feed on spinach only in the spring and fall; 
hence, it is possible that it may be carried by one of these species instead 
of by those commonly inhabiting the spinach during the winter as well 
as in the spring and fall. The combined known food plant list of Macrosi - 
phum solanifolii and Rhopalosiphum persicae comprises more than ioo 
species, representing a wide range of botanical groups, any of which might 
possibly be alternate host plants of the disease. So far as time would per¬ 
mit, those species closely allied botanically to spinach have been carried 
through series of inoculations the results of which were negative. Many 
species appeared to be affected with mosaic diseases, and these were 
used to inoculate healthy spinach plants, but without success in pro¬ 
ducing infectious blight. Only by a great amount of systematized work 
entailing thousands of inoculations can any definite proof be obtained 
relative to the question of alternate hosts of the inciting factor of spinach- 
1 Ricketts, H. F. spotted sever report, no. 1/2. In 4th Bien, Rpt. State Bd. Health Mont. 
1907-8, p. 87-191. 1908. 
* Finiay, C. J. trabajos SELEcros (selected papers). 657 p. Habana, 1912. 
3 Smith, Theobald, and Kilborne, F. L. investigathons into the nature, causation, and pre¬ 
vention op southern cattle PEVER. In U. S. Dept. Agr. Bor. Animal Indus. Sthlgth Ann. Rpt., 1891/9 2 , 
p. 177-304. 10 pi. 1893. 
