Breeding and Selecting Pest-Resistant Trees 



467 



Tree hybrids usually lose vigor when 

 inbred, and the process of inbreeding 

 trees requires too long a time. When 

 the tree breeder obtains maximum re- 

 sistance in his hybrids in combination 

 with other desirable characters, he is 

 ready to plant them on appropriate 

 sites for final testing. Since his hy- 

 brids, in general, will not breed true, 

 the question arises as to the nature 

 of the progeny from these hybrids when 

 they are planted in the wood lot and 

 in the forest. Part of the progeny may 

 be resistant but not vigorous, another 

 part may be vigorous but susceptible, 

 and another part may be both vigorous 

 and resistant. The tree breeder can 

 determine the theoretical proportions 

 of these progeny types because he 

 knows the way in which characters are 

 inherited in the species with which he 

 works. 



Each tree-breeding project presents 

 problems of its own. Examples of ex- 

 perimental work will be described to 

 illustrate various methods of testing 

 trees for resistance to particular pests, 

 and to indicate the progress that has 

 been attained. However, most of the 

 selecting and breeding of trees for re- 

 sistance to pests is still exploratory in 

 only a few instances hybrids have been 

 developed to the stage that permits 

 planting them as replacements for their 

 inferior parents. 



WHITE PINES RESISTANT TO BLISTER 

 RUST: A. J. Riker and associates at the 

 University of Wisconsin, in cooperation 

 with the Department of Agriculture, 

 have tested selections of eastern white 

 pine against the blister rust. One thou- 

 sand grafts were made from 163 trees 

 selected for their resistance to heavy 

 natural infection for more than 15 

 years or for other special properties. 

 Most of the grafts resisted artificial in- 

 fections of the blister rust fungus. How- 

 ever, when 10,000 seedlings from the 

 selected trees and commercial seedlings 

 were subjected to natural and artificial 

 infections, a high percentage of the 

 seedlings were infected with stem can- 

 kers within a year. Ray R. Hirt of the 



New York State College of Forestry, in 

 cooperation with the U. S. Department 

 of Agriculture, observed eastern white 

 pines of various ages in the period of 

 1927-47 for resistance to the rust. He 

 found varying degrees of rust resist- 

 ance in a small percentage of the total 

 pines observed. Those trees showing 

 greatest resistance to rust are being 

 propagated by grafting and cuttings so 

 that more extensive tests for resistance 

 can be made. The low percentage of 

 rust-resistant seedlings reported by 

 Riker and Hirt indicates that rust re- 

 sistance is not inherited as a dominant 

 character. 



The white pine blister rust fungus 

 also attacks other five-needle species 

 of pine. Forest pathologists are keenly 

 interested in determining the relative 

 susceptibility of native and exotic 

 species of pine to the fungus. Seven 

 species of pine were tested against rust 

 by Ray R. Hirt, of the New York State 

 College of Forestry; in the Northwest, 

 nine species were tested by Thomas 

 W. Childs and Jess L. Bedwell, of the 

 Division of Forest Pathology. 



The species of pine that showed re- 

 sistance to blister rust were: Pinus 

 cembra var. helvetica, P. armandi, P. 

 griffithii, and P. koraiensis all are 

 foreign species but are not known to 

 have any timber value. Those showing 

 susceptibility in increasing degrees 

 were: P. aristata, P. pence, P. ayaca- 

 huite, P. flexilis, P. monticola, P. al- 

 bicaulis, and P. lambertiana. Several 

 trees of the latter species, commonly 

 called sugar pine, have withstood in- 

 fections from blister rust for 14 years 

 and will be used as breeding and 

 propagating material. 



RESIN MIDGE: In the blister rust 

 experiments, inherent resistance of 

 selected pines was determined by inoc- 

 ulation tests. In the case of an insect 

 parasite, the resin midge, we find 

 that an external characteristic of the 

 trees new shoots with dry, smooth 

 bark is an indicator of resistance. 

 The problem was approached by se- 

 lecting for this particular character. 



