30 BULLETIN 1112, U. S. DEPARTMENT OF AGRICULTURE. 
estimated, since this is the practicable means by which rational silvi- 
cultural management is actually secured. 
Meinecke * has suggested the establishment of pathological rota- 
tions for species of economic importance which are subject to serious 
diseases. A pathological rotation may be defined as the period during 
which the timber crop may grow and be subject to exploitation at a 
profit, but beyond which rapid deterioration is imminent and the 
product will be marketed at a great reduction in value. The patho- 
logical rotation is therefore a limiting factor; and, with regard to 
heartwood-destroying fungi entering the heartwood through open 
wounds, it is based upon the age of decline or the age at which even 
unwounded trees become subject to heavy infection because they are 
unable to throw off the parasite or to hold its growth in check. From 
the discussion in the earlier part of this bulletin it will be seen that 
the mistletoe may kill small seedlings only a few years old, as well as 
veteran western yellow pines 200 to 300 years old. It is therefore 
evident that it would not be practicable to attempt to establish a 
pathological rotation for mistletoe-infected western yellow pine. 
CONTROL OF MISTLETOE. 
Any method of control of this serious and widely distributed enemy 
of the western yellow pine forests of the Southwest must be adapted 
to existing conditions. As in the case of other diseases, prevention 
is the basis for control. The protection of healthy stands of timber 
from disease is accomplished mainly by removing the source of 
infection. : 
Mistletoe is more susceptible to control measures than many fungi. 
Infection is confined to the aerial parts of the host, while fungi may 
infect the subterranean as well. The seeds of the mistletoe, which 
are much larger than the microscopic spores of fungi, are not subject 
to such wide dissemination. Mistletoe is killed with the death of its 
host, whereas certain fungi may remain alive for an indefinite period 
following the death of their hosts. 
It is evident also that mistletoe develops rapidly after cutting, 
since many of the trees left standing on the permanent sample plots 
were killed by mistletoe during the five years following the logging 
operation; and many more, although still alive, will die in the course 
of a comparatively few years. In this study mistletoe was found to 
be directly or indirectly responsible for the greater number of deaths 
of western yellow pine. 
All of the facts at hand indicate that the cutting of mistletoe- 
infected trees is the only practicable method of control. Since 
mistletoe appears to spread more rapidly and to grow faster.on 
lightly infected trees, as a result of increased wind and light following 
11 Meinecke, E. P. Forest Pathology in Forest Regulation. U.S. Dept. of Agri. Bul. 275, p. 59, 1916. 
op ~>teal. 
