ERIE BJÖRKMAN
ERIK BJÖRKMAN is Professor of Forest Botany and Pathology in the Royal College of Forestry, Stockholm Sweden. He was assisted in the preparation of the final version of this chapter by R. T. BINGHAM (United States). Other members of the drafting team consisting of E. J. Schreiner (United States), Bent Søegaard (Denmark), H. D. Gerhold (United States) and O. Fugalli (FAO).
Breeding for resistance to disease involves recognition of the disease organism and an understanding of the relation between hod tree and parasite, the genetics of both tree and parasite and the environmental conditions under which the attack develops.
Oak wilt, Ceratocystis fagacearum, is a major disease affecting oaks but resistance breeding has not yet been started. By contrast, breeding for resistance to chestnut blight, Endothia parasitica, and the elm disease, Ceratocystis ulmi, has demonstrated the feasibility of producing resistant cultivars but reveals how complex is the task. The work being done in Sweden on the strains of Valsa nivea which causes severe damage to the aspen hybrid Populus tremula x tremuloides provides an example of the studies required on the parasite.
Increasing resistance to Cronartium ribicola, the Blister rust disease of white pines, is a major preoccupation of northern American tree breeders. Canker-free individuals of Pinus monticola are rare but do exist. Most of these resistant phenotypes have been shown by clonal and progeny testing to be inherently resistant, and one fourth to one fifth exhibit general combining ability for transmission through seed of a fairly high level of rust resistance. One important finding from the work done on the related Pinus strobus is that resistance to Cronartium appears to intensify with increasing age.
Other cankers on conifers that are receiving attention are Peridermium pini, Melampsora pinitorqua and Dasyscypha willkommii, the last named providing a well-documented and tested example (in Larix decidua) of variation in resistance associated with provenance of seed. Another example concerns the resistance of certain Swedish provenances of Pinus sylvestris to the snow blight disease, Phacidium infestans.
So far bacterioses and viruses have not received much attention but extensive testing of clones of Populus in Europe has led to the selection of several possessing resistance to Pseudomonas syringa f. populae.
In discussing the basis of resistance, it is postulated that there is a special biochemically definable relation between host tree and pathogen which determines the degree of parasitism and resistance. The role of the water content of the host tissue, nutrient status pH of the host tree and osmotic pressure have all been studied and shown to be important in specific circumstances.
Next, attention is given to the methods used in breeding for resistance to disease and the vital role of progeny testing under a range of site conditions is emphasized. Encouraging results in producing disease-resistant cultivars have been obtained from selection within species and from hybridizing closely related species. In particular, the F1 hybrids Pinus echinata x taeda and P. echinata x elliottii have been shown to be resistant to the devastating Fusiform rust in the southern United States.
Finally, the great need for international co-operation through FAO and IUFRO is stressed.
Resistance to infection is influenced for the most part by two factors, the genetic qualities of the host tree and parasite, and the environmental conditions which for various reasons can be more or less propitious for the occurrence of an attack by parasites. Generally speaking, an invasion of dead tissues such as heartwood is not governed by the same genetic factors as those that determine resistance. The hereditary occurrence in the heartwood of growing trees of substances which have a toxic effect on decay fungi and increase wood durability is, however, an important branch of resistance research. In some cases the same rot fungus may appear as saprophyte in one tree species and as a parasite in another. An example of this the root-rot fungus Fomes annosus, which causes serious damage to the heartwood of species of Picea and also occurs as a parasite destroying the roots of species of Pinus. Another example of fungi of the same type is Armillaria mellea.
The definition of resistance sometimes includes resistance to certain extraneous factors such as extreme drought or cold. In this chapter, however, the concept of resistance is restricted to those diseases caused by the parasitic attack of micro-organisms and viruses.
Earlier phytopathological writings are often limited to descriptions of the attack and of the organism responsible for the disease. Later, the relationship between the host tree and the parasite was examined chiefly on the basis of the host's genetical and physiological status. In recent years great attention has also been paid to the nature of the parasite. When the infectious agent is present, the hereditary qualities of both organisms and the environmental factors will determine whether the disease develops. Since research into resistance biology is also the basis of breeding for resistance, it involves genetical, plant physiological and ecological factors (Allen, 1954).
As recently as three years ago, breeding for resistance and tree improvement was dealt with in a comprehensive manner at the Fifth World Forestry Congress in Seattle, U.S.A. The findings of prominent research workers were collated and their contributions were published in Volume 2 of Proceedings of the Fifth World Forestry Congress, 1960, which includes papers by Schreiner, Righter, Hepting, Peace, Krstic, Imazeki, Riker, Kalandra and Murray. At that time, many theoretical and practical questions were reviewed. After a period of only three years, therefore, there is little new material to report. Some diseases of international importance, will, however, be reviewed here; the emphasis will be on American literature (Spaulding, 1956, 1968, 1961) although some reference will also be made to the latest results of the increasingly intensive resistance research being done in Europe.
Diseases of Quercus Castanea and Ulmus
Oak wilt, caused by Ceratocystis fagacearum (Chalara quercina) is regarded by some as a major disease affecting oaks (Fowler and Bretz, 1951). The disease appeared in the 1940s and has been studied in detail, particularly by Hepting (1955) and by True et al. (1960). Aircraft were used to make initial, quick surveys of the range and intensity of the problem. Inoculation experiments in the United States of America revealed that all the fifty species of Quercus tested were susceptible, as also were several other broadleaved trees (Hepting, 1955; Bretz, 1952 a, b; Hoffman, 1953). Warning regulations have been issued by FAO and other organizations. It is possible that insects serve as pathogen vectors for long-range transmission. Short-range spread is primarily from tree to tree by natural root grafts and this has been demonstrated by the use of radioactive isotopes (Kuntz and Riker, 1955). The distribution biology has, however, not been fully clarified (Fowler, 1958), and the chief method of combating the disease is to remove the affected trees, sometimes also in conjunction with the disinfection of stumps and timber. Through chemotherapy, attempts are being made to isolate centers of infection in stands (Himelick, 1959); although some compounds show promise, none is yet in practical use. Resistance breeding has not yet started. The occurrence of the fungus may depend to a considerable extent on temperature and humidity conditions (Houston and Kuntz, 1960).
Chestnut blight, caused by Endothia parasitica, typifies a disease which, during the present century, has practically exterminated a valuable tree species in America, and since 1918 has - in conjunction with the ink disease, caused by Phytophthora cambivora and P. cinnamomi - diminished considerably the amount of Castanea sativa in Europe (Krstic, 1960). A relatively small number of trees seems to have survived the disease in America and plans have been made to utilize these as the basis of a resistance breeding program (Andersen, 1960). In the long run it should be possible to replace some of the lost stands, but for the present the only choice would appear to be to change the tree Species and aim at a vegetative increase of the most resistant individual trees. In many of the affected countries the question of protecting the ground from erosion also arises.
Moreover, it has been observed that Endothia parasitica can attack Quercus species, such as Q. stellata and Q. virginiana (May and Davidson, 1960) in America, and several species including Q. ilex, Q. sessiliflora and Q. pubescens in Europe (Gravatt, 1951). Krstic et al. (1959) also report that Quercus robur is attacked in Yugoslavia. Furthermore, the fungus has shown an ability to form different strains (Andes, 1961).
In many countries various provenances of Castanea have been introduced for eventual crossing with indigenous types. This is, of course, a long-term program and can already be regarded as having produced results. Several hybrids have been produced in which Castanea dentata, C. crenata and C. mollissima are represented, as also are C. pumila and C. senguini (Nienstaedt and Graves, 1956; Arretini, 1959). Fischer (1959) has given an account of experiments with 71 clones of C. sativa and C. mollissima and established that there are very considerable differences between the clones. Only one clone was completely resistant, but, according to Fischer, much can nevertheless be expected from resistance breeding. Work of this nature is in progress in many countries (see, for example, Bazzigher and Schmid, 1962). In the north of Spain a few small areas of the disease have been detected recently but it has not been reported from Portugal or Greece (Taveira Fernandes, 1957; Krstic, 1960). In areas where E. parasitica does not occur or is of minor importance, ink disease of sweet chestnut is often present. This disease is fairly wide spread in Spain and Portugal but control is anticipated. The disease is also present in France. There appears to be a certain risk of its spreading from these countries to other parts of Europe (Anonymous, 1958; Molina and Vieitez, 1962).
During the discussion in Stockholm, A. Biraghi (Italy) remarked upon a curious phenomenon in respect to chestnut blight in Italy. Sprouts from chestnut trees or stumps attacked by blight have what appears to be high resistance to the blight; in some cases they have remained alive up to 25 years. This sprout resistance seems to be so general that Biraghi saw no need for continuation of breeding for blight resistance in Castanea sativa.
Elm disease (Ceratocystis ulmi) is another example of a disease which has spread over Europe, adjacent Asia, and wide areas of America during the present century. Most Ulmus species are susceptible, and in Europe a large number of trees were felled in an attempt to arrest the disease (Krstic, 1960) but there is some doubt as to the necessity for some of this sanitation cutting. In the United Kingdom, Peace (1960) considers the disease to be less malignant than before and, because of a rather high percentage of recovery of attacked trees, he warns against too drastic measures. In America, however, the disease is still gaining ground (Holmes, 1956, 1961).
The pathogen is carried by different bark beetles (Scolytus spp., Hylurgopinus rufipes). Generally the limiting factor in the spread of the disease is the distribution and rate of multiplication of these vectors. Through them climatic factors influence the spread (Liming et al. 1951; Mathiesen-Käärik, 1953; Käärik, 1960). Drought and other factors may also directly influence the severity of disease expression. Direct control is primarily against the vectors. Much research is aimed at the development of chemotheraputants that act against the pathogen itself (see Holmes, 1955; Beckman and Howard, 1957). Though some progress has been made, the chemotheraputants are not yet ready for general use. Difficulties in application and a high phytotoxicity have still to be overcome.
Breeding for resistance remains of primary importance. Use is being made of the wide variation in susceptibility within some species, as well as of the high degree of crossability between the species (Heybroek, 1962). Arisumi and Higgins (1961) have demonstrated that certain clones, notably a clone of Ulmus hollandica x carpinifolia x U. pumila, are resistant. Some degree of success may be achieved by utilizing germ plasm of subtropical elms (Smalley and Riker, 1962). In so far as trials have been made, crossings to related more resistant genera such as Celtis, Zelkova, Trema and Holoptelea have not yet been successful. In the uniformly susceptible tetraploid Ulmus americana, mutation breeding may be a relatively successful approach. Resistance seems to be governed by many minor genes. Though small seedlings generally react less than grafts or older plants, a true juvenile resistance does not seem to exist (Tchernoff, 1963).
In the breeding of Ulmus, ornamental qualities must also be considered. Further complications are caused by other diseases. In European elm breeding the use of material of southern provenance with a low frost resistance causes much concern about susceptibility to Nectria cinnabarina. This disease has rendered some otherwise good clones worthless (Heybroek, 1957).
In North America a virus disease called phloemnecrosis (Swingle, 1942) is in some areas a greater threat than the Dutch elm disease. Trees resistant to this virus disease can be found in areas where the disease has been epiphytotic and the resistance seems to be governed by major genes. Even #0, the need to combine two kinds of resistance in one tree (Riker, 1954) will inevitably delay the development of a resistant American elm.
Phloem-necrosis has not yet been reported from Europe. Fortunately the European elms, with the exception of Ulmus laevis, belong to a section of the genus which does not seem to be susceptible to the virus (Heybroek, 1962).
Cankers on poplars
Septoria musiva is one of the most injurious fungi affecting a number of hybrids of Populus in America. In conjunction with the introduction of American poplars, and with regard to the fact that it is prone to attack new species and previously unaffected hybrids, there seems to be a serious risk of the fungus also gaining a firm foothold in Europe. Dothichiza populea has proved its ability to inflict great damage on a number of Populus species and hybrids, not least in Europe.
Valsa (Cytospora) nivea has shown itself able to occur as a dangerous parasite on the hybrid between Populus tremula and Populus tremuloides, the so-called Hybrid aspen, while it appears only as a saprophyte on P. tremula (Persson, 1955, 1962). Simple big-assay with the fungus on agar plates containing ground bark of the hybrid and the parent species, indicated differences of biochemical origin. Extractions from bark showed markedly different carbohydrate contents. The hybrid aspen contained sucrose which was absent in the parent species. Attempts were made to classify 13 strains of Valsa nivea according to their ability to digest sucrose enzymatically. Resistance tests are being carried out in field experiments at the Swedish Match Company's research station at Mykinge in southern Sweden. The results of a series of laboratory and field experiments made in recent years are to be found in the paper by A. Hüppel (A. Persson, 1963) (Figure 16). A detailed account of damage to poplars is also given by Schreiner (1963).
Cankers on conifers
In his paper submitted to the World Consultation, gingham (1963) reports on the present state of research on resistance to tree rusts in North America. Work is in progress on 4 of the 9 rust diseases that cause serious losses, and increasing resistance to Cronartium ribicola, the blister rust disease of white pines, has become a major preoccupation of North American tree breeders (Heimburger, 1956, 1962), as elsewhere in the world.
Since 1950 gingham and his colleagues have sought blister rust resistance in Pinus monticola and three results of their work are relevant here.
First, canker-free individuals of P. monticola are rare but nevertheless persist in stands otherwise decimated by rust (Bingham et al., 1953). Secondly, among such resistant phenotypes clonal and progeny testing proves that most are also genotypically resistant and about one fourth to one fifth exhibit general combining ability for transmission through seed of a fairly high level of rust resistance (Bingham et al., 1960). Thirdly, both foliage and bark types of resistance are recognized and, although conclusive evidence concerning the mode of inheritance of blister rust resistance is lacking, it does appear that a number of genes are involved and a simple additive system controls their net effect.
In Europe the non-host-changing Cronartium flaccidum (Peridermium pini) is important (see Peace, 1962), chiefly on Pinus sylvestris (Figure 17). Much has been written about the differences in resistance between various individuals or provenances (Klebahn, 1938; Liese, 1936; Rennerfelt, 1947; Bolland, 1957), but the experiments described are seldom particularly extensive and nothing is known for certain about the cause of resistance.
In Pinus strobus the age of the trees would appear to be of some significance in determining resistance to the white pine blister rust (Cronartium ribicola) since resistance apparently increases with age (Patton, 1961) and, although the mode of inheritance is far from having been elucidated, the evidence of gingham and others suggests that the resistance is inherited. This information has been applied to the problem of Peridermium and in many quarters it is the practice to eliminate, as far as possible, those individuals of Pinus sylvestris which are affected by Peridermium species and also to avoid using them as seed trees. Despite the increased importance of seed orchards, systematic checks on susceptibility to this disease are still comparatively rare. The actual method of inoculation should not present any problem in the case of Peridermium pini (Bolland, 1957); the chief drawback is that two years must elapse before the result of the inoculation can be determined. The material used for progeny testing requires observation during protracted experiments, which is naturally a practical disadvantage. It appears, however, that attacks of Peridermium pini can be successfully reduced by resistance breeding.
The rust seen on Pinus species, and caused by Melampsora pinitorqua (Figure 18) can occasionally lead to serious in terminal shoots and to deformation of the stem. The fungus occurs throughout Europe (Gäumann, 1959) and may have been observed recently in Canada (Ziller, 1961, 1962) but during the discussion in Stockholm it was brought out that Ziller's diagnosis is open to some question. The disease has previously been described at length by Sylvén (1917, 1918) and Regler (1957). Pinus contorta appears to be wholly resistant to this fungus. The first intimation of differences in the attack frequency between various clones of Pinus sylvestris was made by Rennerfelt (1953) and Bergman (1954). Observations made by Klingström (1963, a, b) would appear to justify the hope that resistance breeding will be possible.
Cankers on species of Pinus, Picea, and Abies caused by various species of Dasyscypha are often closely connected with different provenances of the host trees. Dasyscypha (Trichocyphella) willkommii is thus far more injurious to the European species of Larix than it is to Larix sibirica, and the east Asian species such as L. leptolepis. Long-term experiments have been established in which the same provenances have been planted in several places in Europe with different climatic conditions (Schober, 1958). Hybrids with other species never showed the same degree of resistance as Larix leptolepis. In many areas, certain Korean and Polish larches displayed good resistance but on exposed northerly and maritime sites the resistance was not quite so strong. Needless to say, a decisive part in disease susceptibility can also be played by the local climate.
Scleroderris lagerbergii (Crumenula abietina, C. pinea) has, particularly in recent years, attracted a great amount of attention since the fungus has shown its ability to develop in nurseries in purely epidemic form and destroy great numbers of Pinus sylvestris seedlings (in one instance, in the north of Sweden, 12 million seedlings were killed in one nursery). Preliminary tests have revealed that different provenances show different susceptibility also in this case (Björkman, 1961 a).
Foliage diseases of forest trees
The pine needle cast, caused by Lophodermium pinastri, is among the best known of all needle diseases. This very common fungus attacks Pinus seedlings and, particularly in very damp years, can cause great damage, especially in nurseries (cf. Jahnel, 1953). Literature dealing with Lophodermium disease has been summarized by Bolland (1957) among others. Different provenances and also certain individuals show varying resistance to attack. Northern and eastern provenances seem to fare best, while seedlings of southern origin run great risk of being attacked even when moved short distances. Dengler (1955) has compared Finnish and German provenances of Pinus sylvestris, and established that the Finnish material was appreciably more resistant to Lophodermium pinastri than was the German. Schütt (1957) has, with the help of inoculation, obtained plus and minus individuals of Lophodermium pinastri and has been able to establish distinct individual differences. During the discussion in Stockholm, Dr. W. Langner (Germany) reported the findings of his colleague, H. Hattemer, that among clones of 20 different Pinus sylvestris provenances planted on 10 different sites and artificially inoculated, no correlations were found between resistance to Lophodermium pinastri and cell sap concentration or pH of the needles. Provenances differ in their susceptibility on the different test sites.
Attention has often been drawn to the similarity in resistance to frost injury and needle cast. However, there is no exact knowledge of the causes of resistance. Strohmeyer (1938) illustrates differences in the pH of cellsap. There appear to have been no intensive and long-term projects dealing with resistance genetics which have endeavored to solve the needle cast problem. One of the chief difficulties is that the fungus is so destructive that there is often not one unaffected seedling. Even though appearances hardly seem to inspire much hope, the ability of seedlings to withstand and limit an attack, and by that means to recover, is very different in various individuals. Resistance breeding can perhaps lead to some results (Bolland, 1957).
Phacidium blight, caused by various species of Phacidium, occurs in areas where there is deep snow in winter. Phacidium infestans, the biology of which has been investigated by Björkman (1948, 1961), is the best known. This fungus is very prevalent in the northern parts of Scandinavia, where it often constitutes the most serious threat to the regeneration of Pinus sylvestris. The fungus can develop under cover of snow and spread 15 to 20 centimeters from the point of attack each winter (Figure 19). Observations made by Schotte (1923) indicated the probability of there being great differences in the intensity of attack on different provenances and the recently published results of investigations by Björkman (1963) have clearly confirmed these observations. Seedlings of Pinus sylvestris from the upper north of Sweden (Lapland) have shown themselves to be very resistant to attack, while in identical conditions seedlings of more southerly origin have perished, practically without exception. When planting is carried out in these areas, therefore, an appreciable improvement in the percentage of survivors can be obtained by using Pinus sylvestris seed of more northerly origin. The degree of resistance to Phacidium infestans of crosses between various provenances of Pinus sylvestris is now being studied.
In America, and more particularly in Canada, other species of Phacidium occur on species of Picea and Abies and cause a certain amount of damage (Reid and Cain, 1962). Here too, depth of snow is an important ecological factor affecting the extent of the attack.
Devastating diseases having a similar form of attack are also caused by other fungi which, however, do not depend on the presence of snow for their development. An example of this is the American brown-spot needle blight caused by Scirrhia acicola which, particularly for Pinus palustris in nurseries, can assume significant proportions in the southern states of America.
Rhabdocline pseudotsugae and Phaeocryptopus gäumanni are two ascomycetes which, because of their occasionally very severe attacks on Pseudotsuga taxifolia, have been dealt with in great detail in books on forest pathology. These fungi were introduced into Europe from North America in the early 1900s and have often played a decisive part in limiting the usefulness of Douglas fir (Vienot-Bourgin, 1949). The green or coastal Pseudotsuga taxifolia seems to be better able to withstand Rhabdocline pseudotsugae than do blue and gray Douglas fir. On the other hand, Phaeocryptopus gäumanni appears to attack all three varieties with equal intensity. Variations of a genotypic nature are said to exist regarding attack by both pathogens (Fuchs, 1958). Resistance breeding can undoubtedly play a great part in ensuring the greater use of Pseudotsuga taxifolia in European forestry.
There are also examples of several other ascomycetes which have the character of needle cast fungi and which are most serious at the nursery stage and during early stand formation. Only one will be mentioned here, namely Didymascella (Keithia) thujina, which occurs chiefly on Thuja plicata but to some extent also on T. occidentalis. The disease can be very troublesome in nurseries and there are conflicting reports as to the possibilities of inhibiting the spread of the disease with the help of, for instance, Bordeaux solution. The disease is indigenous to North America, but today is also found in Europe. Pawsey (1957, 1960) has described the biology of the fungus, and Søegaard (1958) has made experiments with hybrids of Thuja plicata x standishii, which proved to be resistant. Søegaard claims that attacks by the fungus on seedlings are much more severe than they are on cuttings derived from the older mother trees.
Muhle Larsen (1983) has shown that in Populus deltoides resistance to Melampsora leaf rust is inherited as a dominant character. He also reported at Stockholm that the resistant P. deltoides individuals may be of constitution RR or Rr which when crossed with each other in various combinations, give the expected proportions of resistant and susceptible individuals in the F1 and F2.
Bacterioses
So far bacterioses have been touched on quite superficially because of their relatively slight importance. Considerable importance attaches, however, to the canker of poplar caused by the organism usually known as Pseudomonas syringa f. populae Sabet. Extensive testing of clones of poplar in Europe has led to the selection of several which possess resistance to this damaging disease.
A few other examples of the bacterioses encountered in forestry can be mentioned. Species of Ulmus can sometimes be affected by "wetwood", caused by Erwinia nimipressuralis, which is of some consequence in America.
Several important tree species, including Castanea, Eucalyptus and Populus, have been seriously damaged by a pathogen which was probably introduced from the United States called Agrobacterium tumefaciens (Imazeki, 1960). Bacterial blight of walnut trees (Juglans species), caused by Xanthomonas juglandis, occurs in Europe and the United States but has not been found in Asia. The bacterium is spread by pollen.
Viruses
It has long been known that viruses occur in forest trees, but only incomplete research into the question has been made. A few examples will be mentioned here (see Kristensen, 1960).
Several types of virus diseases occur on Ulmus. The most important is elm phloem necrosis, which has already been referred to, but elm mosaic and elm zonate canker can also be mentioned. The results of the work done so far suggest that the viruses might be combated by resistance breeding.
Birch dieback is a typical disease in Canada and the northern parts of the United States. The problem has been discussed at great length and, although the disease has been attributed to both fungi and viruses as well as to climatic conditions, there is much to suggest that a virus is the most important causal factor (Clark and Barter, 1958).
Poplar mosaic was described by Atanasoff as early as 1935 together with a great number of virus symptoms. Individuals afflicted with poplar mosaic do not thrive as well as healthy trees. The virus particles have recently been identified (Brcák and Blattny, 1962).
Several notes deal with viruses on conifers, such as Picea abies and Pinus mugo (Blattny, 1948; Smolák, 1948; and Cech, Králik and Blattny, 1961). It has not yet been established, however, whether or not these are of significance.
It is probable that between each host tree and pathogen there is a special, biochemically definable relationship which determines the degree of parasitism and resistance. In only a few cases has intensive research been made on fungal attack on forest trees. It has been suggested that the presence in Castanea dentata of tannins, such as pyrogallol tannin and catechol tannin, influences Endothia parasitica (Nienstaedt, 1953; Bazzigher, 1953). Generally speaking, it could be regarded as a big step forward if resistant provenances or clones could be found. However, the biochemical nature of resistance is seldom known, and only a few very brief comments will be made here.
The water content of the host tissue has on several occasions been connected with susceptibility to pathogens; this is made clear in a number of works by Bier and others (Bier, 1959 a, b, c, and 1961 a, b, c; Bier and Rowat, 1962 a, b). The vigor of plants and all the factors which affect it, such as nutrition, water and light, are also included in the relationship between host and pathogen (see, for example, Barnett, 1959). The relationship is extremely complicated and varies with different types of parasites. Fast-growing, healthy and well-balanced individuals often have the best chance of outgrowing the disease. In the case of rust fungi, however, the more vigorous individuals are usually attacked. From the point of view of resistance, therefore, it appears that the application of fertilizers could produce opposing effects. Phosphorus and nitrogen often seem to have different effects (Böning and Böning-Seubert, 1933). The phosphorus requirement of the parasite can influence the host tissue and, in conjunction with this, polyphenols can form toxic quinones (Humphrey and Dufrenoy, 1944). The enzymatic system affecting the nitrogen metabolism of the host tree and the parasite can also affect resistance (Grecusnikov, 1936; Farkas and Király, 1961). Rust fungi, for instance, are often sensitive to nitrogen metabolites and may very likely depend on the ability of the host tree to remove them (see Farkas and Király, op. cit.).
The pH of the host tree can affect the pathogens or the enzymatic activity and, consequently, also the question of the suitability of the substrates for the pathogens concerned (Barnett, 1959). The auxin content of the tissue has also been associated with parasitism and resistance (Straw and Hawkins, 1958). In the same way, changes in permeability in conjunction with the penetration of fungal hyphae have also been recorded (Thatcher, 1943).
Differences in osmotic pressure have often been considered to lead to differences in the degree of resistance (Bier, op. cit.). The reason for the appreciably higher resistance to snow blight of northerly provenances of Pinus sylvestris, as opposed to that of southerly provenances, has also been linked by Björkman (1948, 1961 a, 1963) with Langlet's (1936) proof of higher dry matter content - and subsequently the sugar content of the needle cell plasma and higher osmotic pressure in winter.
Exosmosis has also been shown to affect resistance; both inorganic and organic substances can be exuded and can affect the germination of spores (cf. Suchorukow, 1958).
The weaker a certain resistance is, the more it can be changed by external intervention. The really resistant forms therefore appear to be the least modifiable (Gäumann, 1951).
The study of a parasitic attack in the natural environment of the host tree can be taken to represent both the beginning and the end of an investigation into resistance biology. By employing genetic and physiological methods, valuable information about the etiology of disease can be gained. To obtain conclusive information about the so-called field resistance, however, the experience thus obtained must be tested under natural conditions where site factors are affected by the competition for nutrition, light, etc., offered by other plants.
In some countries where research into forest genetics and forest tree breeding has been in progress for some time, work has generally been concentrated on making inventories of plus trees and collecting grafting material from phenotypically superior plus trees. Grafts raised from these plus trees have been brought together in seed orchards for intercrossing.
To establish whether the material chosen for its phenotypic qualities is also inherently good, it is necessary to test the progeny. In Sweden, for example, the time is approaching when both clones and the products of crossbreeding can be the subject of assessment from various pathological points of view. The breeding program must include the assessment of the resistance of various progenies to different fungus diseases. Resistance to disease is a primary requirement. Indeed, testing for resistance to disease should be the first phase of progeny assessment since high yields of wood or good qualities of stem form, knottiness, and so on, are of secondary importance compared with the primary demand on the ability of the plant and tree to survive violent attacks by different parasitic fungi.
In a resistance test one must also pay great attention to changes in the genetic status of the parasite as it is known that new physiological strains sometimes appear which are able to attack previously resistant host trees (see Savile, 1952). Research into resistance biology and resistance breeding must consequently be planned ON a long-term basis.
It is of greatest importance to devise and employ methods of testing which demonstrate resistance and separate it clearly from failures of inoculation or other techniques.
Examples have already been given of intraspecific breeding and the use of species hybrids in an effort to obtain resistance to pathogens. In some cases research is quite advanced. Several provenance tests on Larix and Picea can be regarded as classics, and knowledge of the possibilities of moving different provenances of Pinus sylvestris with regard to the risk of attack by needle cast fungi has grown considerably in recent years (Björkman, 1963). By selecting resistant individuals, intraspecific breeding work has been able to make great advances.
In many cases encouraging results have been obtained by hybridizing closely related species. At best, marked hybrid vigor and a good combination of the qualities of the parents can be obtained. It is not possible here to go into the details of hybrids which have been produced using coniferous species. Regarding the degree of resistance which such hybrids have attained, it can be mentioned that the F1 hybrids, Pinus echinata x taeda and Pinus echinata x elliottii, have been shown to be immune to the devastating fusiform rust under field conditions (Henry and Bercaw, 1956; Jewell and Henry, 1959). Later experiments made by Jewell (1961) have shown, however, that the resistance is not absolute and that the most resistant individuals do not always show the best increment (see also Henry and Jewell, 1963). In broadleaved trees there are even more examples of interspecific breeding employing species of Quercus, Alnus, Betula, Fraxinus, Populus and Eucalyptus.
As Wilde (1954) among others has stressed, the site can also affect the expression of hybrid vigor and the development of the phenotype.
It has already been stated in Chapter 3, however, that it is not enough to know that a hybrid can be produced; it must also be suitable for mass production at an acceptable price to be of practical importance to forestry.
The selection of seed stands represents the first step toward better seeds and mass production, while seed orchards provide the means for controlled crosses, progeny tests and renewed selection and make possible the mass production of seed. The formation and treatment of seed stands and seed orchards is dealt with in Chapter 9 of this report.
Experience has shown that the transfer of species to new areas is fraught with great risk and that breeding work can meet severe setbacks because of unforeseen damage by fungi and other factors. There is an obvious need for international co-operation in the prevention of the spread of pathogens, in the combating of existing damage, in the testing of plant material for research and study, and not least in the dissemination of the results of research work, in regard to which the publications of the Food and Agriculture Organization of the United Nations play their part. The Working Group on Forest Diseases of IUFRO exists to examine means of preventing the spread of dangerous tree diseases and further the contacts between researchers in different countries (Riker, 1960; Fabritius Buchwald et al., 1961).
One means of reducing, or at least appraising, the risk of tree introduction would be to publish international host-parasite indices modeled after those of Spaulding (1956, 1958, 1961).
International co-operation is also necessary in other fields.
If there is an increase in the international exchange of plant material, the demand for information on origin, germination capacity, seed and plant control, and so on, will be even more accentuated. Moreover, it would undoubtedly be of great value if thoroughly tested material were maintained in special collections in different countries. These collections could later be used for breeding purposes.
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BIER, J. 1959 a, b, c., 1961 a, b, c The relation of bark moisture to the development of canker diseases caused by native, facultative parasites. Canad. J. Bot. 37: 229,238 781-788, 1140-1142; 39: 139-144, 145-154, 1555-1561.
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BLATTNY, C. 1948. Notes on the viruses of conifers. Ochrona Rostlin, 21, (516): 26-28. (Biol. Abstr., 25 (2): 1951).
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BRCÁK, J. & BLATTNY, S. 1962. Electron microscopic investigation of poplar mosaic. Phytopathology, 52: 9.
BRETZ, T. W. 1952a. New hosts for the oak wilt fungus, Chalara quercina Henry. Phytopathology, 42: 3.
BRETZ, T. W. 1952b The ascigerous stage of the oak wilt fungus. Phytopathology, 42: 435-437.
CECH, M., KRÁLIK, O. & BLATTNY, C. 1961. Rod-shaped particles associated with viruses of spruce. Phytopathology, 51: 3.
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FABRITIUS BUCHWALD, N. F., KUJALA, V., ROLL-HANSEN, F., BJÖRKMAN, E. & KÄÄRIK, 1961. A. Lists of parasitical fungi and of hosts of such fungi in Denmark, Finland, Norway, Sweden. I. Pinus, Populus, Quercus. (Compiled for IUFRO). Medd. det Norske Skogsforsøksv, 59.
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HOFFMAN, P. F. 1953. Chemicals for therapy of oak wilt. Phytopathology, 43.
HOLMES, F. W. 1955. Field and culture tests of antibiotics against Graphium ulmi. Phytopathology, 45: 1.
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HOLMES, F. W. 1961. Recorded Dutch elm disease distribution in North America as of 1957. Plant Dis. Reptr., 45: 1.
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HÜPPEL, A. 1963. Enzymatic splitting of sucrose by some strains of Valsa nivea Fr. Studia Forestalia Suecica, 7.
IMAZEKI, R. 1960. Introduced diseases with particular attention to the threat they pose forest protection in Asia. Proc. 5th World For. Congr., Seattle, U.S.A., 2.
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KÄÄRIK, A. 1960. Growth and sporulation of Ophiostoma. Symb. Bot. Upsalienses, 16: 3.
KALANDRA, A. 1960. Diseases of poplars in Czechoslovakia with special reference to Dothichiza populea Sacc. and Briand. Proc. 5th World For. Congr., Seattle, U.S.A., 2.
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