Breeding for disease resistance is a very wide topic. This presentation will deal specifically only with certain aspects of breeding for disease resistance in forest trees, meaning disease caused by parasitic fungi. Very little has as yet been done in this field generally; therefore this paper will draw rather heavily on our own work at Maple, Ont. with ibreeding of white pines (Pinus L., subgen. Haploxylon Koehne) for resistance to blister rust (Cronartium ribicola Fischer). The results may not all be suitable for generalization but should contribute to a better understanding of the problems and procedures involved in this particular field.Diseases are, of course, also caused by ecological or environmental factors affecting the health of forest trees, such as, for instance, the effect of extreme weather conditions, of smelter fumes and so on, and will not be dealt with at presen't. Thus by disease in the above narrower sense is meant an abnormal condition in a tree caused mainly by a fungus attack, and its manifestation is also influenced to a varying degree by the environment. Breeding involves manipulation of the genotype and thus has a direct bearing on the genotypic component of the reaction of forest trees to fungus attacks.Breeding for disease resistance in forest trees is encouraged by the promising results with breeding for resistance in agricultural and horticultural crops; the introduction of parasitic fungi (and insects) from other continents to North America; the very serious economic losses following such intro-
Forest Seruicr, D r p n r t l n~n t of the Interior While there is plenty of poplar in the forests of .Eastern Canada and elsewhere, there is a el-owing scarcity of sound poplar wood of good quality.I n spite of the rapitllj~ increasing areas stocked with aspen poplar as a result of recent fires, such wood is very scarce, ant1 is finding many uses for match stock and for veneer. T h e Swedish match factories have for a long time relied on Russian wood, but it is now 'becoming very scarce, and Canadian match stock is rapidly entering the world market. Recently it has been found economical in Get-many (LVettstein, 1935 ) ant1 in Englantl (Lotbiniet-e, 1935) to raise aspen poplar untler forest Inanaeenient in order to obtain aspcn wootl of gootl (111~.lity in sufficient iunounts to satisfy the growing demands for it. Under natural conditions, gootl aspen wood is found growing only on the very best soils, and only a very small proportion of the immense areas at the present time stocked wit11 asp:-n in America and in Europe will ever yield good, sol~nd wood. T h e reason for t!lis is the Freat su5ceptibility of aspen to heart-rot, Irhich attacks it at an early age, in most cases before it has reachcd merchantable size. Only a sm;lll proportion of the aspen stands of niel-chantable size yield sound wood. I t is tllel-efore consitlered good fol-estl-y PI-;~ctice to find such methods of managing aspen stands as will retard the tlevelopm~nt of heartrot and will yield an increasing pl-oportion of sound wood of the size suitable for match stock and for vencer. T h i s can he accomplishetl through methods of si1vicultu1-a1 treatment of existing aspen stands and by the use of suitable varieties or strains of aspen that are more resistant to heart-rot :ind reach larger sizes tht-o~1g11 being longer-lived ant1 more vigorous than the existing 'aspen specles. T h i s report deals with this second phase of the problem, namely, the efforts leading towards the development of strains and of varieties yielding good, sound wood ~~n d e lforest management. I t has been found that a natural cross between the European silver poplar (Popult~s nlbn L.) and the European aspcn (Popult~s tremr~ln L . ) combines some of the good qualities of its parents. Silver poplar is a healthy vigorous tree, I-eaching larger sizes than the aspen. I t IS l e s~ subject to 1le;u-t-rot, and it grows faster than the aspen. I t has been found to yield wood of a q~~a l i t y suitable for match stock, although this wood is inferior to aqpcn wootl of the highest grade. T h e tree can easlljr be grown and pl.opaThe Forestry Chronicle Downloaded from pubs.cif-ifc.org by 52.183.12.225 on 06/07/19For personal use only.
Interspecific hybridization in various forest tree genera gave the following results: Populus, 43 crosses; Picea, 12; Pinus, 4; Betula, 14; Fraxinus, 5; Ulmus, 7; and Tilia, 9. The hybridity of most of these materials has been proved or strongly indicated by various criteria; but for some of the materials, hybridity is assumed on the basis of seedling production under conditions that largely precluded the possibility of self- or chance-pollination.
The results of further progress in a poplar hybridization project initiated in 1935 with the aim of producing valuable hardy and disease resistant material of rapid growth for the production of wood of high quality for industrial purposes, and of value for shelterbelts in the Prairies are reported. It has, been possible to cross the native aspens of Eastern and Western Canada with several exotic aspens and their hybrids with silver poplar, and to obtain hybrid material of great promise for these purposes. Some western balsam poplars have also been crossed with exotic species and have yielded hybrids of promise forshelterbelts. An attempt to cross an aspen with a cottonwood has so far yielded indifferent results. The cress of a basket willow with a cottonwood wan not successful. Hardiness. disease resistance, and good propagability from stem: or root cuttings are a t present the most importa n t characters used in evaluating the young hybrids.T H E OBJECT of this report is to outline the further development of the poplar hybridization project initiated in 193 5 and described previously (Heimburger, 1936). I t is an attempt to produce new varieties of aspen poplars, suitable for reforestation on upland soils and yielding good, sound wood for the manufacture of match stock, veneer, and pulp. The new varieties should be fully hardy, resistant to several important diseases damaging the native aspen poplars, and should possess superior growth characters as compared with the native asFcns. Since hybridizatioil and selec. tion are the methods most commonly employed with such outstanding results in breeding work with other plant material, these methods are here applied to the development of superior aspen poplars. In inore recent years the desirability of producing suitable tree varieties for shelterbelts in the Prairies has become increasingly apparent and some of the efforts are there. fore directed towards the breeding of aspens and other poplars for these purposes also.In 1935 and 1936 it was found that the European silver poplar (Populus alba L.) and its natural hybrid with the European aspen (P. tremula L.), the so~called gray poplar (P. canescens Sm.) could be crossed successfully with the native amens (P. tremuloides Michx. and P. grandidentata Michx.). The u cross between the two native aspens, P. tremuloides x grandidentata was also successful. HYBRIDS OF 1936In the fall of 1936 the hybrid seedlings described in the previous report were carefully dug out and their roots prepared into cuttings, care being taken to keep the cuttings from each seedling separately. Each bunch of root cuttings thus represented a singe clone (Schreiner, 1939). The material was then trans, ferred to the nursery of the Petawawa Forest Experiment Station and planted. The possibility of propagating aspen poplars by means of fall-planted root cut, tings had been ascertained in previous tests with root cuttings of native aspens.all planting of root cuttingsbf aspen poplars as compared with spring plant, ing has the advantage of affording b...
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