H.D. GERHOLDH.D. GERHOLD is associate professor, School of Forest Resources, Pennsylvania State University, United States.
THE EMERGENCE of substantial evidence that resistance breeding offers a practical means of controlling forest diseases should be recognized as one of the main scientific advances of this decade. At a time when mankind is becoming acutely aware of the dangers in contaminating our environment, it is urgent that this alternative to pesticides be exploited as quickly and fully as possible. DDT is the best-known example, and the manifold ill effects of its persistent residues on many organisms, including man, have been convincingly established (Wurster, 1969). Although DDT and other chemicals have been very useful, greater precautions are clearly needed, and less hazardous control measures would be preferable. In addition to safety for humans, inherent disease resistance can offer economy and long-lasting effectiveness for protecting planted or artificially seeded forests of many species.
This paper reviews some accomplishments since 1960, and looks ahead to prospects in the next decade for further improvements in the disease resistance of forest trees. The definition of disease resistance as used in this paper is broader than that commonly employed by pathologists and entomologists, but is consistent with the true meaning of disease. Thus, disease resistance may include any trait that decreases damage caused by any pathogenic organism. Progress made in finding and using resistance to fungi, insects, viruses, and other damaging agents has been impressive. This is especially true as regards the difficulties caused by complex interactions between two organisms and the time restrictions associated with long tree generations and commercial rotations.
E. J. Schreiner (1960) provides a convenient reference point for charting subsequent progress. He pinpointed many research results indicating the possibilities of obtaining improved disease resistance and 'outlined several important principles, but he was able to cite very few examples of the actual use of resistant tree varieties. The accuracy of his prophetic statement is therefore especially striking: " If the last decade's tremendous upsurge of forest genetics research can be combined with effective international cooperation during the next ten years, pest-resistance control may be expected to have a major role in the forest practices of the 1970s." Probably few foresters or pathologists would have fully subscribed to this optimistic viewpoint expressed at the Fifth World Forestry Congress, but plans for further progress were already being formulated there. A notable consequence was the World Consultation on Forest Genetics and Tree Improvement held at Stockholm in 1963, where two excellent summary papers were presented (E. Björkman, 1964; B. Soegaard, 1964).
The need in resistance breeding for effective cooperation among geneticists, pathologists, entomologists, and physiologists was recognized by a recommendation issued by the 1963 consultation (FAO, 1964). A year later scientists in these disciplines were able to discuss this subject in greater depth at a two-week Advanced Study Institute on Genetic Improvement for Disease and Insect Resistance of Forest Trees (Gerhold et al., 1966). Once again the need was expressed for improving communication among personnel engaged in resistance breeding.
Subsequently IUFRO'S Sections 22 and 24 established a Working Group on Genetic Resistance to Forest Diseases and Insects, which met for the first time at Munich in 1967. Within this Working Group smaller committees have been formed. Committee members share a common interest in a particular resistance breeding problem. These problems include Dutch elm disease, poplar insects, poplar diseases, white pine blister rust, pine twisting rust, and Peridermium stem rust of pines. Other committees will be added as needs and opportunities develop. They can meet occasionally to exchange information, to make plans for obtaining plants or pathogens, and to facilitate collaboration in research, breeding, and resistance testing. This type of cooperation among poplar breeders has been stimulated for some years by the International Poplar Commission, although its focal point has not been disease resistance alone. These activities stimulate the " effective international cooperation " called for by Schreiner a decade ago.
The activities of the White Pine Blister Rust Resistance Committee illustrate some of the particular ways in which cooperation can be helpful. The chairman of the committee, R.T. gingham, organized an Advanced Study Institute on Biology and International Aspects of Rust Resistance in Forest Trees, which was held in 1969 at Moscow, Idaho. This provided opportunities to review the biology, pathology, and genetics of rust resistance; to assay the world's white pine breeding materials; to discuss progeny-testing procedures and breeding schemes for mass-producing resistant pines; and to develop recommendations for procuring breeding materials and for establishing international resistance testing facilities. Over 100 persons from 18 countries participated. Improved human relations among participants are just as important a consequence as the scientific aspects of such a programme. Mutual respect and confidence are essential lubricants for the human machinery in a cooperative tree improvement venture, especially at the international level.
The organization of such meetings costs money and, as in other sectors of tree breeding, financial and administrative support is not easily obtained. Existing sources of funds are at present inadequate to support a meaningful level of activity by the various disease resistance committees. Reasonable progress toward their goals cannot be expected unless more adequate provisions can be made to enable regular biennial or triennial meetings of each committee.
Occasional visits to other institutions engaged in tree improvement research can be invaluable for impressing the practical significance of research and the possibilities of wide-scale application. New insights and broader perspectives can be gained. by both the visitors and their hosts.
At national and agency levels, great needs exist for better organized cooperation in resistance breeding projects. Administrators of forestry research agencies and of tree breeding programmes are encouraged to review the working relationships among geneticists, pathologists, entomologists, and physiologists. Opportunities should be provided for meaningful collaboration among those working on projects related to resistance breeding. Financial support of travel to professional and working meetings, both intra- and international, is especially important in this respect.
Research related to the breeding of disease-resistant forest trees has been so extensive during the sixties that a complete review is not possible here. Numerous literature references may be found in the citations given earlier, and in several recent review articles (Beck, 1965; Gerhold, 1967; Hare, 1966; Heimburger, 1962; Kozlowski, 1969; Stark, 1965; and Toole, 1966). The following examples have been chosen as illustrating points particularly relevant to the practical theme of this consultation.
The finding of useful levels of genetic resistance against pathogens of many tree species has become commonplace. A list of these would include not only fungi and insects but also bacteria (Ridé, 1967), viruses (Bigornia and Infante, 1965; Castellani, 1966), mice, hares, and voles (Chiba, 1963; Takahasi and Nishiguchi, 1966), and air pollutants (Rohmeder and von Schönborn, 1967). Even if resistance is rare, as in blister rust resistance in Pinus monticola (Bingham, 1963) or Dutch elm disease resistance; in Ulmus americana. (Ouellett and Pomerleau, 1965), it; may be uncovered through diligent searching under ideal conditions. Provenance experiments or species trials that sustain damage may provide unexpected but very useful opportunities for selecting resistant trees or populations.
Some ways of coping with the complications that may arise when a variable host interacts with a variable pathogen in variable environments are illustrated in Baker and Norris (1968) on elm bark beetles, Hattemer et al. (1969) on Neodiprion sawflies of pines, and of Smith (1969) on pine bark beetles. An even more complex situation may occur when two or more pathogens influence each other, for example air pollutants and bark beetles. as reported by Cobb et al. (1968). These studies and many others emphasize the need for a thorough understanding host-parasite relationships, especially the variations in pathogenic and genetic aspects, in order that adequate selection and testing procedures may be designed.
The most effective methods for evaluation and selection expose large numbers of young trees to a pathogen at an optimum level under controlled conditions, with replication in time and space. For parasitic microorganisms, artificial inoculation techniques are usually employed. Typical are techniques developed for Aplanobacterium populi (Ride, 1967), Ceratocystis ulmi (Tchernoff, 1965), Cronartium fusiforme (Jewell and Mallett, 1967), Cronartium ribicola (Patton and Riker, 1966), and others. Insects pose greater problems because of their mobility and discrimination among hosts, but similar selection techniques that involve forced attack are being developed. Insects may be confined in cages on individual trees, such as Dendroctonus on pines (Smith, 1966), or on groups of trees, such as Pissodes strobi on white pines (Soles et al., 1969). Field grafting of selected white pines for natural exposure to Pissodes strobi has been employed by Heimburger (1967). The use of aggregating pheromones for attracting natural populations of bark beetles has been suggested by Gara et al. (1965) and Smith (1966). Several of these techniques have been developed sufficiently for them to be tried out now on a larger scale. The remarkable success in cases where such procedures have been applied on a practical basis suggests that research on inoculation or forced attack should be extended to additional pathogens. As a precaution, provisions should be made for checking that resistance genes selected under partly artificial conditions will actually be useful in forest plantings.
Knowledge about the genetic control of resistance to some tree diseases has advanced considerably, but in no case can it really be considered adequate for long-term progress. Evidence of monofactorial control was found for resistance against Didymascella thujina in Thuja hybrids (Soegaard, 1966), but there are indications of multigenic inheritance for nearly all other cases that have been studied. Perhaps the convenience of single. rating scales that are commonly used for quantifying genetic variances and gains has tempted us to avoid the more painstaking work of defining different components or types of resistance that could help in finding single gene effects. Just recently, three major genes for resistance have been tentatively identified in western white pine (Bingham, 1969; McDonald, 1969).
These are thought to account for substantial portions of resistance. It is possible that effects of single genes have been overlooked in other cases where polygenic inheritance has been hypothesized. Such information can be extremely valuable to breeders in selecting, making matings, and studying pathogenicity, and it may have serious implications for the permanence of resistance (see gingham, 1969 and also Heybroek, 1969 for further details).
The degree of risk caused by genetic changes in pathogenicity will not be known in the immediate future. Hattemer (1967) has given a well-balanced discussion of this subject. The problem deserves much greater attention in our research efforts, as emphasized by Heybroek (1969) and gingham (1969). Although different strains of Aplanobacterium (Steenackers, personal communication), Ceratocystis (Holmes, 1965), Cronartium (Anderson and French, 1955), and Valsa (Hüppel, 1966) have been identified, it is not clear whether any differences in virulence might have serious consequences. However, Heybroek (1969) has called attention to the severe losses in Hevea and Coffea that resulted when resistant varieties encountered different races of pathogens. Until this matter has been studied more fully it would be safer for breeders to employ wide-spectrum resistance testing and to strive for multigenic bases of resistance. The development of international resistance testing programmes could be especially helpful in this connexion.
Several significant advances in the methodologies of breeding and propagation may profoundly influence the strategies of resistance breeding programmes. Stettler (1968) has overcome barriers to species hybridization in Populus by using irradiated mentor pollen. If this technique should prove to be effective in other genera, it would open many new avenues for introducing genes that confer resistance. Backcrossing is often required after hybridization, and this process could be greatly accelerated if precociously flowering varieties were available. Indications that precocious flowering is under strong genetic control (Gerhold, 1966; Teich and Holst, 1969) encourage the exploration of this possibility. The development of successful methods for the vegetative propagation of many more species will also have considerable impact on both mass-production and resistance testing procedures. The ability to grow aspen plantlets by tissue culture reported by Winton (1968) is a significant breakthrough, though additional time will be required for refining the method. The solution to the problem of rooting cuttings is closer at hand, however, than may be generally realized. At Escherode, Federal Republic of Germany, several conifers regarded as difficult to root are being rooted from cuttings on a commercial scale with excellent success, as a result of earlier research (Kleinschmit, 1961).
Knowledge and experience gained from research and other sources must be carefully integrated and synthesized in devising a well-designed breeding program-e e. A breeder must therefore have both scientific and creative abilities. If improved resistance to an organic pathogen is one of his main objectives, the breeder is likely to be severely taxed to cope with the complexities mentioned previously. A possible solution is to organize a team of experts who can work together at one institution, or to arrange for collaboration among persons doing related work at different locations. This type of cooperation appears to be characteristic of some of the resistance breeding programmes that have been under way for several years. An excellent example is the cooperation among various European breeders and pathologists working on bacterial canker resistance of poplars (Ride, 1967).
If one wishes to gain a thorough understanding of the design and procedures of any comprehensive resistance breeding programme, many papers must be consulted. For several programmes fairly complete series of publications are available, including those dealing with Dutch elm disease, various poplar diseases, and white pine blister rust. Even after reading papers such as these, however, it is difficult to ascertain precisely how much practical progress has been made.
In order to get a more complete picture of recent progress in developing and mass-producing resistant varieties, a questionnaire was sent in October, 1968, to 85 persons in 32 countries. At the time this was written, 71 persons had replied, and information covering 21 countries was obtained but may not be quite complete (Table 6). Limited time prevented a more thorough survey. The author assumes responsibility for any omissions or errors that may have occurred in condensing the replies. The intention was to limit the table to direct progress tin creating resistant varieties and to exclude related research that supports these efforts. In some cases it was difficult to make this distinction. Accordingly, provenances selected for greater resistance were not included, even though they may represent early stages of creating resistant varieties. Furthermore, only organic pathogens were included for reasons of tradition. In the future it may be more realistic and more useful to include also inorganic types of pathogens, recognizing an even broader definition of disease resistance.
Several conclusions can. be drawn from the survey. In at least 35 breeding programmes substantial work on improving disease resistance is under way. Several have already released resistant varieties or clones, and over half of them expect to reach this stage of accomplishment within the next ten years. A start has been made in at least 29 additional programmes. One programme has been discontinued, which can serve as a reminder that not all of the listed projects will necessarily be completed. In the case of hybrid Jeffrey-Coulter pine and the pine reproduction weevil, a silvicultural control method was substituted. Other breeding programmes in which freedom from disease is one of the selection criteria are not listed because the effectiveness of selection is not yet known. The work by Zobel and his colleagues on resistance of Pinus taeda to fusiform rust might not have been included ten years ago, when the degree of expected improvement would have been underestimated.
Resistance of forest trees to pathogenic plants has been sought and found more often than resistance to insects. According to this survey, resistance to a botanical pathogen involves roughly 4 times as many programmes, 11 times as many selected trees, and 7 times as many resistant trees. These figures might be interpreted in different ways. Smith (personal communication) points out that there are in general more alternative control methods available for insects than for diseases. The survey also indicates the greater difficulty of improving resistance to insects. This situation should be regarded as a challenge rather than a deterrent to further progress.
The effectiveness of resistance breeding as a protective measure has been clearly established for several forest and ornamental species. Both experimental evidence and reduced damage to improved varieties in use support this conclusion. The success of several of the programmes that pioneered resistance breeding undoubtedly has stimulated the formation of others. The breeding of elms resistant to Dutch elm disease has spread from the Netherlands to Canada and the United States. Six programmes seek resistance to white pine blister rust in three species; four of the programmes are of very recent origin. Similar encouraging statements probably could be made about resistance breeding of poplars and other species.
TABLE 6 - REALIZED AND EXPECTED PROGRESS IN TREE BREEDING PROGRAMMES THAT ARE DEVELOPING DISEASE RESISTANT VARIETIES ¹
|
Taxon being improved |
Pathogen (s) against which resistance is sought |
Number of trees |
Resistant varieties available pre-1979 |
Respondent and country |
|
|
Selected |
Proven² |
||||
|
Abies alba y A. nordmanniana |
Chermes abietina |
|
30 |
|
Soegaard, Denmark |
|
Castanea, hybrids |
Endothia parasitica |
Many |
9 |
|
Jaynes, United States. |
|
Castanea sativa e hybrids |
Phytophthora cinnamomi |
|
200 |
1970 |
Landaluze, Spain |
|
Cocos |
Cadang-cadang viris |
533 |
|
|
Bigornia, Philippines |
|
Larix leptolepis |
Physalospora laricina |
36 |
|
|
Yanagisawa, Japan |
|
L. leptolepis spp. |
Mycosphaerella larici |
92 |
7 |
|
O. Chiba, Japan |
|
L. leptolepis e hybrids |
Clethrionomys rufocanus |
|
Many |
1970 |
Takahashi, Japan |
|
Picea abies |
Fomes annosus and rost |
49 |
22 |
|
Frölich, Germany, Fed. Rep. |
|
P. abies |
P. annosus |
5 |
|
|
Hyppel, Sweden |
|
P. abies |
Pissodes strobi |
100 |
4 |
1975 |
Holst, Canada |
|
Pinus banksiana |
Scleroderris lagerbergii |
90 |
|
1979 |
Teich, Canada |
|
Choristoneura pinus |
5 |
|
1979 |
Teich, Canada |
|
|
P. canariensis |
Thaumetopoea pytiocampa |
67 |
3 |
|
Franclet, Tunisia |
|
P. contorta |
Tortrix bouliana |
70 |
|
|
Soegaard, (Soegaard), Denmark |
|
P. echinata |
Rhyacionia frustrana |
7 |
|
|
Coyne, United States. |
|
P. elliottii |
Cronartium fusiforme |
153 |
|
|
Blair, United States. |
|
P. elliottii |
C. fusiforme |
5 |
4 |
|
Dinus, United States. |
|
P. elliottii |
C. fusiforme |
254 |
30 |
1970 |
Kraus, United States. |
|
P. jeffreyi X coulteri |
Cylindrocoptorus eatoni |
323 |
73 |
Discontinued |
Parks, United States. |
|
P. lambertiana |
Cronartium ribicola |
325 |
1 |
|
Barnes, United States. |
|
P. lambertiana |
C. ribicola |
171 |
4 |
|
Parks, United States. |
|
P. monticola |
C. ribicola |
2000 |
200 |
1970 |
Bingham, United States. |
|
P. monticola |
C. ribicola |
555 |
7 |
|
Barnes, United States. |
|
P. palustris |
Scirrhia acicola |
543 |
53 |
1969 |
Snyder, United States. |
|
P. pinaster |
Melampsora pinitorqua |
|
|
1978 |
Illy, France |
|
P. radiata |
Diplodea pinea |
30 |
20 |
1970 |
Nikles, Australia |
|
P. radiata |
Dothistroma pini |
37 |
9 |
|
Ivory, Kenya |
|
P. radiata |
D. pini |
67 |
|
|
Thulin, New Zealand |
|
Pinus sect. Strobus spp. and hybrids |
Pissodes strobi |
67 |
3 |
|
Heimburger, Canada |
|
Cronartium ribicola |
467 |
30 |
|
Heimburger, Canada |
|
|
P. strobus |
C. ribicola |
200 |
6 |
|
Patton, United States. |
|
P. strobus |
C. ribicola |
700 |
|
1975 |
Miller, United States. |
|
P. sylvestris |
Melampsora pinitorqua |
100 |
5 |
|
Klingstrom, Sweden |
|
Peridermium pini |
100 |
5 |
|
Klingstrom, Sweden |
|
|
P. sylvestris |
P. pini |
75 |
|
|
Faulkner, Scotland |
|
P. sylvestris |
Fomes annosus |
6 |
|
|
Hyppel, Sweden |
|
P. sylvestris |
F. annosus |
50 |
|
1978 |
Siwecki, Poland |
|
P. taeda |
Dendroctonus frontalis |
112 |
5 |
1979 |
Coyne, United States; van Buijtenen, United States. |
|
P. taeda |
Cronartium fusiforme |
167 |
|
|
Blair, United States. |
|
P. taeda |
C. fusiforme |
22 |
|
|
Dinus, United States. |
|
P. taeda |
C. fusiforme |
163 |
5 |
1970 |
Kraus, United States. |
|
P. taeda |
C. fusiforme |
1500 |
|
|
Zobel, United States. |
|
Populus spp. |
Phyllocnistis suffusella |
22 |
1 |
1970 |
Istvan, Yugoslavia |
|
P. spp. |
P. suffusella |
3 |
|
|
Panetsos, Greece |
|
Gypsonoma aceriana |
3 |
|
|
Panetsos, Greece |
|
|
Populus spp. |
Melampsora and cankers: |
250 |
70 |
1970-75 |
Schreiner United States. |
|
Septoria, Valsa, Dothichiza |
|
|
|
|
|
|
P. spp. (aspen group) hybrids |
Melampsora larici-populina |
|
61 |
|
S. Chiba, Japan |
|
P. spp. (balsam group) hybrids |
M. abietis-populi |
|
17 |
|
S. Chiba, Japan |
|
P. spp. (aspen group) hybrids |
Valsa nivea |
|
8 |
|
Hyppel, Sweden |
|
P. sect. Aigeiros |
Dothichiza populea |
90 |
|
|
Frölich, Germany, Fed. Rep. |
|
P. deltoides |
Melampsora spp. |
4 |
2 |
1971 |
Barderi, Argentina |
|
Septoria spp. |
4 |
2 |
1978 |
Barderi, Argentina |
|
|
P. deltoides |
Melampsora spp. |
3 |
15 |
1973 |
Ragonese, Argentina |
|
P. deltoides |
M. spp. |
Many |
1 |
1970 |
Nagel, United States. |
|
P. deltoides e hybrids |
Leaf diseases |
|
Many |
1970 |
Koster and Gremmen, Netherlands |
|
P. deltoides, P. nigra, P. trichocarpa e hybrids |
Leaf disease |
|
Many |
1970 |
Steenackers, Belgium |
|
Dothichiza populea |
|
Many |
1970 |
|
|
|
Aplanobacterium populi |
|
Many |
1970 |
Steenackers, Belgium |
|
|
P. deltoides and P. X euramericana |
Pollaccia elegans |
|
Many |
1970 |
Castellani y Cellerino, Italy |
|
Marssonina brunnea |
|
Many |
1970 |
Castellani y Cellerino, Italy |
|
|
Poplar mosaic |
|
Many |
1970 |
Castellani y Cellerino, Italy |
|
|
Taphrina aurea |
|
Many |
1970 |
Castellani y Cellerino, Italy |
|
|
P. deltoides and P. X euramericana |
Phloeomyzus passerinii |
|
Many |
1970 |
Arru, Italy |
|
Pemphigus spp. |
|
Many |
1970 |
Arru, Italy |
|
|
Thecabius affinis |
|
Many |
1970 |
Arru, Italy |
|
|
P. sect. Leuce |
Pollaccia radiosa |
80 |
|
|
Frölich, Germany, Fed. Rep. |
|
P. maximowiczii x laurifolia |
Dothichiza populae |
|
|
1972 |
Siwecki, Poland |
|
P. sect. Tacamahaca |
Melampsora spp. |
70 |
|
|
Frölich, Germany, Fed Rep. |
|
P. tremula X alba |
Venturia tremulae |
5 |
|
1972 |
Siwecki, Poland |
|
Pseudotsuga menziesii |
Chermes cooleyii |
|
1 |
1970 |
Soegaard (Naess-Schmidt), Denmark |
|
Robinia pseudoacacia |
Megacyllene robiniae |
10 |
1 |
|
Wollerman, United States. |
|
Salix alba x babylonica |
Marssonina spp. |
2 |
2 |
1970 |
Ragonese, Argentina |
|
Thuja plicata x standishii F1 |
Didymascella thujina |
|
Many |
1970 |
Soegaard, (Vigen), Denmark |
|
Ulmus americana |
Ceratocystis ulmi |
114 |
|
|
Holmes, United States. |
|
U. americana |
C. ulmi |
900 |
4 |
|
Pomerleau, Canada |
|
U. spp. |
C. ulmi |
800 |
100 |
|
Heybroek, Netherlands |
|
Nectria Cinnabarina |
800 |
100 |
|
Heybroek, Netherlands |
|
|
U. spp. e hybrids |
Ceratocystis ulmi |
100 |
10 |
|
Smalley y Lester, United States. |
¹ A summary of replies to a questionnaire sent October, 1968. Comparisons within the table should be made only with caution, as some figures are estimates, and no attempt was made to standardize selection criteria, resistance levels, or qualifications of improved varieties between entries.- ²" Proven " indicates that trees transmit, sexually or asexually, a useful level of resistance, based on progeny or clonal tests.
The success of some resistance breeding programmes does not imply that inherent resistance offers the only or even the best ultimate solution for other pest control problems of forest trees. In each case the various control methods available should be compared as to time and personnel requirements, costs, effectiveness and hazards. It seems likely that combinations of methods may be' useful in many cases. The overall strategy in a control programme, as it progresses from short-range to long-term solutions, may well call for a shift in emphasis from one control method to another.
Although disease resistance has been a focal point, in this discussion, it should be emphasized that improved resistance must be kept in proper 'perspective in relation to other goals of a breeding programme. A prudent breeder will review his goals periodically. and at such times he should reassess the potential dangers of various pathogens. Changes in disease hazards may require adjustments in selection criteria or mating designs, either by good judgement or more precisely by means of selection indices [see Gerhold (in press) and also Illy, 1966]. Even in programmes where resistance is currently of minor importance, every improved variety being developed should at least be exposed under natural conditions to pathogens that are to be part of its environment. Entomologists and pathologists should check periodically progeny and variety tests. This precaution will be especially important when homozygosity is increased or when a species is moved to a foreign environment. Routine testing of susceptibility not only provides insurance against the unexpected failure of a new variety, but it can also yield information that Will be useful for silviculture and for revising objectives of the breeding programme.
Resistance breeding of forest trees has progressed rapidly, from hopeful beginnings through a period of exploring various approaches to the realization of some tangible achievements. Now that the needs for certain types of scientific information have been clarified and genetic methods of improving disease resistance have proved to be useful, it may be time to consolidate these gains in several ways. The genetics and dynamics of resistance and pathogenicity need to he elucidated much more precisely. The effectiveness of alternative selection and breeding methods should be compared, and the best features should be incorporated in systems designed to handle large populations in shortened time periods. Better techniques of vegetative propagation need to be perfected for experimental and practical uses. Successful improvement methods can be adapted to additional species and can be utilized more widely. These are a few ways in which a sound, theoretical and practical foundation can be strengthened for future advances in developing disease-resistant forest trees.
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