30 BULLETrPT 1112, U. S. DEPARTMENT OF AGEICULTURE. 



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 

 imwounded 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 fimgi 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 tliis 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. Bui. 275, p. 59, 1916. 



