20 
Tolohado Expeimaient Station 
))een sliown to be injurious to plants of all kinds for many miles 
around. Trees and shrubs in the larger cities are often injured by 
smoke. 
Mechanical injuries may be produced in a variety of ways. Fruit 
trees may be injured by cultivating too closely. Trees as well as 
herbaceous plants are frequently injured by hail. Young fruit trees 
are often barked by rodents or other animals. The opportunities 
offered for the entrance of parasitic fungi by mechanical injury are 
perhaps moi*e important than any direct result of the injury. 
Orchard fruits are often injured and disfigured by excessive and 
improper application of chemical sprays or by the application of 
sprays of too great concentration. 
DISEASES OF UNCERTAIN CAUSE 
Cei'tain plants are subject to a disease commonly known as 
‘‘mosaic” the cause of which is doubtful. The disease, as the name 
indicates, is characterized by a mottling or mosaic condition of the 
leaves of an affected plant. No parasitic organism large enough to 
be detected with a miscroscope can be found associated with the 
disease, yet the. disease in some cases has been shown to be highly 
infectious. If the juice from a diseased leaf is injected into a healthy 
plant this plant will become diseased after a period of eight or ten 
days. This leaf juice or virus can be passed through the finest 
filter and yet retain its power of infection. For this reason the 
diseases are commonly said to be due to a filterable virus. 
The Irish potato is subject to certain diseases known as curly 
dAvarf, leaf roll and degeneracy, the causes of which are not 
thoroughly understood. The diseases are all characterized by a 
dwarfed condition of the plant and a low yield of tubers. Attempts 
to establish definite symptoms for each disease as Avell as experiments 
to determine their cause have been more or less confusing. The 
Plant Pathologists of the country, however, have gotten together and 
a concerted effort is being made to solve this important problem. 
Curly-top of sugar beets and ])each yellows are other diseases, the 
causes of which are somewhat obscure. 
