592 HENRY D. GERHOLD 



blister rust (Cronartium ribioola J. C. Fisch. ex Rabenh.) or by white 

 pine weevil (Pissodes strobi Peck) is so severe that improved resistance 

 to one or the other pathogen obviously deserves primary emphasis. Else- 

 where these pathogens cause little or no damage, so that timber yield may 

 be improved more directly through traits such as growth rate, adaptation 

 to climatic factors, or wood quality. Although the ultimate objective 

 of a program may be to improve the economic yield of timber, a single 

 trait, in practice this is commonly pursued by selection for multiple 

 traits that are known or assumed to be closely correlated with yield. 



If disease resistance is one of the important objectives in a tree 

 breeding program, rather severe restrictions may be placed on the selec- 

 tion method. Certain selection systems for resistance to white pine 

 blister rust (Bingham et al.> 1969; Patton and Riker, 1966) and to white 

 pine weevil (Soles, Gerhold, and Palpant, 1969) obtain genetic informa- 

 tion and discard large proportions of populations at early ages and in 

 atypical environments. Resistance to both of these pathogens would be 

 useful in parts of Wisconsin, Michigan, Ontario, New York, and New England 

 Consequently, it may be useful to question whether selection for both 

 types of resistance, together with other traits determining yield, would 

 be compatible. 



So that we may have some concrete examples in mind, let us consider 

 two hypothetical selection schemes as outlined in Table 1. Both employ 

 progeny testing to evaluate genetic variances of resistance to white pine 

 blister rust, of resistance to white pine weevil, and of other traits 

 related to yield. The term "growth" is used loosely to refer to yield- 

 related traits such as height, diameter, and wood quality. In Scheme 1 

 all traits are evaluated in forest plantings; in Scheme 2 the two types 

 of resistance are evaluated under artificial conditions in a nursery and 

 growth is measured later in forest plantings. In the nursery, seedlings 

 under shelters are inoculated with rust spores in August at age 2 and/or 

 3, then exposed to weevils in cages during May at ages 3 and 4. About 

 15% of the original population may survive for outplanting at age 5. The 

 percent of healthy and unweeviled survivors may approach at age 20 in 

 Scheme 1 and at age 15 in Scheme 2. A high selection intensity for resis- 

 tance may therefore be achieved in both schemes before there is sufficient 

 flowering to proceed to the next generation. Parents with superior 

 resistance breeding values may be identified by age 4 in Scheme 2, how- 

 ever, while about 12 to 15 years would be required in Scheme 1 . The 

 degree of contrast between the two models obviously could be altered 

 without foresaking biological realities. 



PROBLEMS RAISED BY MULTIPLE TRAIT SELECTION 



The selection schemes that we are contemplating are rather complex. 

 Many problems would be encountered in judging their utility and in 

 developing detailed procedures for them. It will be possible to consider 

 just a few of the major problems here, and to make only a start in 

 seeking solutions. The matters that will concern us are the efficiency 

 of selection, and the possible interference between evaluation methods 

 for rust, weevil, and growth, which might introduce bias. 



The traits under selection and the genetic parameters of interest 

 are listed in Table 2. Growth rate, blister rust resistance, and weevil 

 resistance are viewed as component traits, each correlated with the 

 complex trait, yield. Selection for any one or several of the correlated 



