Dangers From Diseases Pests
by J. S. BOYCE - Professor of Forest Pathology, Yale University, U.S.A.
The Seventh Session of the Conference of FAO urged extension of the facilities afforded by FAO for the international exchange of forest seeds and planting material. It is as well that the dangers attaching to the introduction of exotic species should be kept in mind. Some of these are discussed in this article, which is an extract from a study, later to be published, prepared by Professor Boyce for FAO in connection with the Organization's program of work on afforestation and reforestation.
In large-scale planting programs the general tendency is to establish pure plantations of exotic conifers because the keenest demand throughout the world has always been for softwoods. North America has been the only continent with sufficient volume and variety of species to fulfill all requirements for such exotics. Furthermore, certain North American conifers grow so much more rapidly than conifers in other parts of the world, that it has been only natural to attempt to establish them wherever there is need for softwoods. Exotics should, however, be introduced only if they fulfill some definite need not met by native species, and it must be remembered that many years are required to learn enough about an exotic to determine whether or not it can be successfully established and perpetuated.
The introduction of exotics continues to be based largely on trial and error. Inherent difficulties are first, the problem of securing seed of correct provenance; second, the difficulty of proper site selection; third, the question of whether an exotic will be silviculturally and commercially desirable, even on the best sites, in its new home.
By means of measurements of climate and soil it should theoretically be possible to ascertain the exact ecological habitat of any species or of its races, if the species is not racially uniform, and then to find suitable equivalent habitats elsewhere, no matter how geographically remote. In practice, the time and money necessary to do this may be considered prohibitive. Moreover, it may be desired to plant an introduced species in pure stands, even though the tree naturally grows in mixture. A species normally growing in mixture is more or less dependent on the other species for its welfare, probably in some instances to such an extent that it cannot succeed without them or their equivalents; and finally it is impossible so far to evaluate pathogenic factors, which can cause the failure of an exotic even though climatic and soil factors are favorable.
There are three great hazards from parasites in the use of exotics:
1. a parasite of minor significance on the exotic in its native habitat may be introduced along with it and become extremely severe under new conditions;
2. the exotic will encounter a parasite in its new environment to which it has no resistance so that the tree will be valueless;
3. a parasite of the exotic may be introduced which will be exceedingly destructive to a native tree.
Experience with exotics in Europe
Tree species, particularly conifers, have been introduced from all over the northern hemisphere into western Europe, and this is an unsurpassed region in which to obtain information on the behavior of exotics. It is notable that, after more then two centuries of experience, the majority of foresters in western Europe arc inclined to be pessimistic regarding exotics, because they have not yet found a completely successful introduced tree, even though certain species showed great initial promise.
There is justification for the introduction of exotics into countries with a few commercially valuable tree species. Europe, for example, has relatively few species of value; parts of it do not have any native conifer that attains timber size, and in the whole of Europe there is not, for example, a single pine yielding softwood of such highly desirable quality as eastern white pine (Pinus strobus L.). The only white pine in Europe is Balkan pine (P. peuce Griseb.) which occurs to a limited extent in the mountains of the Balkan Peninsula, but the wood is almost as heavy as that of Scots pine (P. sylvestris L.). Furthermore, the native European trees grow slowly, so there has been a search for faster growing species. Eastern white pine grows more rapidly than the native European conifers with which it has been associated. In northern Germany near Eberswalde, the writer has seen 45-year-old Douglas fir (Pseudotsuga taxifolia (Lam.) Br.) of the same size as 100-year-old Scots pine. Near Tharandt, 55-year-old Douglas firs were from 14 to 20 inches (36 to 51 cm.) in diameter at breast height, exactly twice the size of Norway spruce (Picea abies Karst.) of the same age mixed with them. In southern Germany, near Munich, 45-year-old planted Douglas fir was the same height as 77-year-old naturally reproduced silver fir (Abies alba Mill.).
The first exotic to be extensively planted in continental Europe was eastern white pine, introduced in 1705. At first of great promise and hailed with enthusiasm, it finally encountered an unpredictable pathogenic factor - white pine blister rust caused by the rust fungus, Cronartium ribicola Fisch., a migrant from Asia - with the result that the tree was all but abandoned throughout Europe. Of late years it is again finding some favor because in certain localities there are no, or few, susceptible currant and gooseberry bushes (Ribes) so that the tree is able to survive. In fact, where the rust fungus does not occur, or where it is no more serious than a tolerable nuisance, the tree is completely fulfilling the true test of a successful exotic, which is that it maintains itself by natural reproduction in competition with the native species. The wide range of soils on which white pine will grow vigorously in Europe is impressive. Both in Upper Silesia near the former Polish border and in Switzerland, the author has seen it flourishing on pure peat. In Switzerland also he saw superb mature trees which had developed in mixture with beech (Fagus sylvatica L.), Scots pine and a little Norway spruce. In southern Germany the tree was seen in its fourth generation, the first two generations having been planted and the second two naturally reproduced.
Following World War I, Douglas fir found high favor and was planted extensively, although enthusiasm diminished somewhat in time because of the low quality of wood produced. The first pathogenic factor to threaten the tree was Phomopsis canker caused by a European fungus, Phomopsis pseudotsugae Wilson. This turned out to be not as serious as first feared, being largely connected with frost. Next was Rhabdocline needle cast caused by Rhabdocline pseudotsugae Sydow - the causal fungus coming from the native home of the tree, but with its virulence apparently increased by the damper European climate during the growing season. Fortunately, only the intermountain and Rocky mountain or blue forms of the tree were attacked, the coast or green form, the really valuable kind for Europe, being unmolested. At about the same time an aphid, Adelges cooleyi Gill., attacking the coast form was causing some concern, but it proved to be tolerable. Adelopus needle cast, now a severe disease, apparently caused by the fungus Adelopus gäumanni Rohde which seems to attack all three forms of the tree, has made the future of Douglas fir in Europe dubious. Planting of the species has been given up in much of southern Germany, while in Switzerland it is being used in mixture only, not with the idea that the mixed stand as such will check the disease, but merely to have other species to take the place of Douglas fir in the stand should the disease prove as catastrophic as is now feared.
In view of the difficulties besetting Douglas fir, it was natural that Sitka spruce (Picea sitchensis (Bong.) Carr.) should increase in favor and by 1935 it was thought that this species might supplant Douglas fir for extensive planting. However, in 1939, in Holland, the author saw the last of what had been a fine 40-year-old stand of Sitka spruce. In 1935, when this stand comprised nearly 300 trees per acre (741 per ha.), a few began to die from attack by a bark beetle, Dendroctonus micans Kugel., which soon reached epidemic proportions, and by 1939 only 30 trees per acre (74 per ha.) remained. These were clear cut within a few years. Similar beetle attack has occurred elsewhere, and it now seems doubtful whether Sitka spruce can be grown to a greater age than 40 to 50 years in western Europe. D. micans occurs naturally on Norway spruce (Picea abies Karst.) but causes little damage.
Sitka spruce is also vigorously attacked by a louse or aphid, Elatobium (Aphis) abietinum, which causes considerable defoliation in some plantations, but whose seriousness cannot yet be predicted.
Plantations of the same species which have failed at 20 to 45 years old with no pathogen responsible have been seen in Germany, Switzerland and Great Britain, and it would seem that failure has clearly resulted from an unsatisfactory site. Sitka spruce is exacting in its site requirements, and too often has been planted on unsuitable soil, where the annual precipitation is too low, where temperatures are too severe or where some other adverse factor occurs. Unfortunately, the bad effects of an unsuitable site frequently do not appear until some years after a stand has been established, growth of the earlier years being vigorous. Sitka spruce on the whole is better adapted to Great Britain, where the climate is nearer that of its native habitat, than to most of continental Europe. Even so, care must be exercised in selecting the areas where it is to be established.
Grand fir (Abies grandis Lind.) is now being most favorably regarded by many foresters in western Europe because of its rapidity of growth and, up to the present, freedom from disease. It is thought that it may ultimately take the place it was first hoped that Douglas fir would occupy, and next Sitka spruce, neither of which are fulfilling their original promise. For example, in Great Britain grand fir will grow more rapidly than any other conifer. However, unless the site has a mean annual precipitation of 50 inches (1,270 mm.) or more, growth declines at about the age of 50 years and the trees are no longer vigorous, although no definite disease may have developed. Of course, since grand fir occurs over a wide geographical range in its native habitat with marked variations in temperature and precipitation, and since consequently there are climatic races within the species, it would be possible to find a race adapted to drier conditions, but such a race would probably grow so slowly that it would have no advantage over native European conifers. The grand fir which the author saw in Europe was all of the fast-growing coastal form.
Grand fir planted in 1894 near Lausanne, Switzerland, has been dying steadily since 1945 because of shoestring root rot caused by the honey mushroom (Armillaria mellea (Vahl.) Quel.) and by 1950 it appeared likely that the remaining trees would die in a few years. Basically the cause seemed to be drought, since precipitation in this locality averages only 40 to 50 inches (1,016 to 1,270 mm.) annually and, in addition, for the 7 to 8 years prior to 1950 there had been an annual precipitation deficit as high as 30 percent in some years. Severe attacks on conifers by Armillaria mellea following drought are characteristic.
Western red cedar (Thuja plicata D. Don.) is little seen in continental Europe, probably because in Great Britain the tree had so much difficulty in even getting a start that it was quickly discredited. The cedar leaf blight fungus (Keithia thujina Durand), which came with the trees from North America, finding the moist climate of Great Britain so much to its liking has reduced the cedar to a slow-growing tree, at least during its younger stages.
In southern Sweden a flourishing plantation of 12-year-old jack pine (Pinus banksiana Lam.) was ruined by a fungus (Dasyscypha sp.) which ordinarily occurs as a harmless saprophyte on the native Scots pine. The fungus caused flattened cankers on the main stem just above ground level, and many trees were completely girdled.
Caucasian fir (Abies nordmanniana (Steven) Spach.), introduced from the western spurs of the Caucasian mountains largely as an ornamental, was followed by an aphid (Adelges nüsslini Börner). In the Caucasus this insect is not significantly damaging to its host tree, but in southern Germany under relatively drier climatic conditions the parasite has become so severe that Caucasian fir can no longer be grown there. However, this is of little consequence compared with the fact that the aphid is now attacking silver fir so severely that foresters fear that this fir, which is the principal species in the famous Black Forest, will have to be largely abandoned, particularly on those sites below the natural lower altitudinal limit for the tree.
Exotics in the United States
In the United States, because of the large number and variety of native species there is little need for exotics, and so far they have been tried in a small way only. In Michigan, Scots pine was attacked by a gall rust (Peridermium sp.); Himalayan pine (Pinus excelsa Wall.) was so damaged by canker with which Valsa superficialis Nitschke was associated that ultimate destruction of the plantation was certain; Japanese red pine (P. densiflora Sieb. and Zucc.) was considerably affected by dieback of the leaders and branches probably caused by Cenangium abietis (Pers.) Rehm, and Austrian pine was completely destroyed by sweet-fern blister rust, Cronartium comptoniae Arth. Ornamental Austrian pines in the northeastern United States are so susceptible to dieback of the twigs caused by Sphaeropsis ellisi Sacc., also known as Diplodia pinea (Desm.) Kickx., that this factor alone, apart from other pests, probably makes the tree valueless for forest planting. Scots pine plantations in Pennsylvania grew vigorously for 20 years and then disintegrated quickly from various factors including attacks by fungi. In the State of New York, Scots pine plantations have been extensively damaged by the spittle bug (Aphrophora parallela Say.) which has killed practically all trees over 20 years old. Elsewhere in the state this tree has been severely attacked by the Woodgate gall rust.
Both Norway spruce, which has been planted frequently, and European larch (Larix decidua Mill.) have shown promise in the northeast, but these species have not yet been established long enough for a conclusive test. Some plantations of spruce have been destroyed by the white pine weevil (Pissodes strobi Peck), a native insect on Pinus strobus L. Although eucalyptus in California is sometimes cited as a successful exotic, this species has never fulfilled the purpose for which it was introduced, namely, timber production, because of the unsatisfactory technical qualities of the rapidly grown wood. However, it has become invaluable for shade and ornamental purposes.
Some successful exotics
The foregoing paragraphs present some of the unsatisfactory results of experiments with exotics, but there have also been successes.
The greatest success with exotics in western Europe has been attained with Sitka spruce and Douglas fir in Denmark and Great Britain, both countries having few tree species. Scots pine (Pinus sylvestris L.) for example, is the only native commercial conifer in Great Britain which, with its normally mild and moist growing season, is even more favorable to conifers from western Oregon, Washington, and British Columbia than is the climate of their native habitat with long dry summers. Mild wet winters are the rule in both regions.
However, it is significant that Douglas fir, regarded by British foresters generally as the most valuable exotic in the decade beginning in 1920, yielded its place to Sitka spruce during the next decade because of attacks by certain tree pests on Douglas fir and the somewhat unsatisfactory qualities of its coarse, rapidly grown wood. Difficulties with some Sitka spruce plantations, probably largely the result of their establishment on unsuitable sites, had by 1950 given rise to the idea that grand fir (Abies grandis Lind.) might be the best conifer for the future.
Red oak (Quercus borealis Michx.) has done well so far in Germany, growing on somewhat poorer soils shall native oaks. Most of the stands are still young, the only old trees being individuals or small groups. In France there is a stand planted in 1929, which is now reproducing naturally. Red oak is also proving quite valuable in Belgium. Black locust (Robinia pseudoacacia L.) so far has been successful in Romania where about 70,000 acres (28,300 ha.) have been planted, and is also said to be doing well in Hungary.
Japanese larch (Larix leptolepis Gord.) has done remarkably well in certain localities in southern Germany where the annual precipitation is sufficient. During the dry summer of 1934 the tree was severely damaged in other parts of Germany, so that its usefulness is restricted. It is not yet known whether this species will reproduce naturally in Europe. Within the past few years, a bark lesion caused by Phomopsis pseudotsugae Wilson has been found in several localities to be prevalent on, and damaging to, Japanese larch; it does not kill trees but causes large scars on the lower portion of the bole which later reduce the quality of the wood. Infection occurs through wounds during the dormant period of the tree. Since the bark lesion commonly follows artificial pruning, infection can be largely presented by pruning in May, June, July, when the trees are most active.
The introduction of European larch (Larix decidua Mill.) into Great Britain from continental Europe has proven relatively successful, despite the damage to many plantations by canker caused by Dasyscypha willkommii (Hart.) Rehm. On good sites the canker is of little consequence.
The most outstanding success with exotics has been obtained in Australia, New Zealand and South Africa, but of course sufficient time has not elapsed for a conclusive test. Monterey, ponderosa, Corsican and maritime pines (Pinus radiata D. Don., P. ponderosa Laws., P. laricio Poir., and P. pinaster Sol.) have been the species most widely planted in Australia. Along the east coast in the summer rainfall area, loblolly, long-leaf and slash pine (P. taeda L., P. palustris Mill., and P. caribaea More) are most promising; loblolly pine in particular is doing well. Monterey pine, Douglas fir, Corsican, ponderosa and maritime pines have been most used in New Zealand; and Monterey, maritime and spreading-leaved pines (P. patula Schlech. and Cham.) have been doing well in South Africa, the last-named tree coming from Mexico. The species enumerated have been arranged roughly in the order of their importance in each country. The growth of all has been superb and of Monterey pine phenomenal, saw timber commonly being produced in 25 years.
All these trees are native to the north temperate zone where the seasons are diametrically opposed to those in the southern hemisphere. Consequently it has been practically impossible to introduce these species as living plants which, together with a considerable degree of caution against such practice, has resulted in the exclusion of the pathogens occurring on these trees in their native habitat. Monterey pine in particular is affected by a number of fungi and insects in its California home. Monterey and spreading-leaved pines possess an additional advantage in that they are relatively homogenous species, so that the vexing problem of the most suitable strains or races within the species does not obtrude itself, as it does in such variable species as maritime, Corsican and ponderosa pines, and Douglas fir. Although such extensive planting of pure stands is extremely risky, less financial risk is involved than in most artificially created pure stands, because of the short rotations in Australasia and South Africa. Serious as a killing epidemic would be, the loss would still be much smaller than would result from the same damage to stands on a 50-year or higher rotation.
However, conifers introduced from the northern into the southern hemisphere have not remained entirely free from insect pests or pathogens. A wood wasp (Sirex noctilio F.), native to Europe, has been established in New- Zealand for at least 50 years where it is attacking Monterey pine and other exotic conifers. The insect usually infests suppressed, dying and dead trees, but it is damaging because it hastens the death of trees that could be utilized. It also creates conditions favorable to the breeding of a European bark beetle (Hylastes ater Payk.) that is also established in New Zealand, this beetle being particularly injurious to pine seedlings. Not until the wood wasp has reached its possible range distribution, so that the population density can increase rapidly, will the complete extent of the damage this insect is capable of causing be realized. Although as yet unknown on exotic pines in Australia, living Sirex wood wasps in all stages of development were intercepted at several Australian ports during 1951 in nine cargoes of timber, mostly spruce and fir, originating from European ports.
Monterey cypress (Cupressus macrocarpa Gord.) in East Africa exemplifies two dangers to an exotic: (1) a native fungus of no consequence becoming pathogenic on an exotic, and (2) a fungus of no significance on the exotic in its native habitat causing loss to the tree in its new environment. Monochaetia unicornis (C. and E.) Sacc., found in a harmless form on a native juniper (Juniperus procera Hochst.), apparently has become pathogenic on various cypresses, including Monterey, causing severe cankers on young trees in plantations. Polystictus versicolor, a wood-destroying fungus relatively world-wide in its distribution but restricted to dead trees in the native habitat of Monterey cypress in California, causes a serious white rot of the wood of living trees in East Africa.
Although exotic conifers have been planted widely in Latin America, notably in Chile, little is known about their development. Several needle cast fungi native to the northern hemisphere have been reported on exotic pines.
The spread of pathogens over great distances is almost invariably brought about by man, and as methods of transportation increase in number and in speed, the chance of spreading pathogens likewise increases.
Pathogen carriers are many, but the most important are described in the following pages.
Experience has shown that exchange of living plant material is extremely hazardous; for example, the chestnut blight fungus, which came to the United States from Asia on nursery stock, has proved so uncontrollable that one of the most valuable hardwoods will be commercially extinct before long, unless resistant trees can be developed by cross-breeding or selection. Even if successful, it will probably be impossible to restore chestnut to more than a small portion of the forest land it once occupied. The loss of American chestnut (Castanea dentata (Marsh.) Borkh.) is the outstanding example in forests of the world of the loss that can be caused by an introduced pathogen, and is an appalling lesson on the danger of introducing nursery stock from other countries. Europe now faces the same future with its valuable native chestnut (C. sativa Mill.).
The white pine blister rust fungus was introduced into the United States and Canada on seedlings from German and French nurseries. Although controllable by eradication of Ribes, this constitutes a perpetual addition to the cost of growing white or five-needle pines in North America.
The brown-tail moth, Nygmia phaeorrhoea (Donov.), a destructive defoliator of various broadleaf trees, was introduced from Europe to the United States on nursery stock. The insect was discovered in 1897. It is now permanently established, and in areas where severe infestation continues year after year has to be controlled by expensive spraying with DDT.
Although not proven, it seems likely that the fungus causing Rhabdocline needle blight of Douglas fir was first introduced into Scotland on seedlings dug from the forest in North America, was then spread over Great Britain by shipments of nursery stock and windborne spores, carried from Great Britain to Germany on nursery stock, and from Germany spread over much of western Europe on nursery stock and also by natural means. The fungus responsible for Adelopus needle blight of Douglas fir probably has followed much the same pattern. Both these pathogens have also been introduced into the eastern United States from their native habitat in the west, presumably on living plants, since the distance seems much too great for wind-borne, rather thin-walled spores to travel and remain viable.
In addition the above, other pathogens probably carried on living plants into Europe include the Douglas fir aphid (Chermes cooleyi Gill.), the silver fir aphid (Adelges nüsslini Börner), the cedar needle blight fungus (Keithia thujina Durand), and the chestnut blight fungus (Endothia parasitica (Murr.) A. and A.). The persimmon wilt fungus (Cephalosporium diospyri Crandall), the mimosa wilt fungus (Fusarium perniciosum f. oxysporum, (Hepting) Toole), and several insects have probably been introduced on living plants into the United States.
Cuttings, although not quite such dangerous carriers as complete living plants, are also hazardous. There is no way to sterilize plants or cuttings effectively without severe injury or death to the tissues. With the large amount of poplar breeding and selection that is now underway in Europe, South America and to a lesser degree the United States, resulting in a considerable interchange of poplar cuttings between some countries, there is great danger that destructive poplar diseases may be widely distributed. Recent attempts to sterilize poplar cuttings carrying the organisms causing leaf blotch (Septotinia populiperda Waterman and Cash), Septoria canker (Septoria musiva Pk.) and Dothichiza canker (Dothichiza populea Sacc. and Br.) have not been successful. However, it is not by means of living plants only that pathogens can be spread. Even dried plants can be dangerous. A needle rust of Scots pine (Pinus sylvestris L.) was brought into Wisconsin on leaves of sow thistle (Sonchus asper (L.) Hill.) with imported Norway spruce (Picea abies Karst.) from Europe. The rust (Coleosporium sonchi-arvensis (Pers.) Lev.) spread to living sow thistle plants and then to planted Scots pines in the immediate vicinity. Fortunately, this needle rust proved of no consequence but the manner of its introduction is important.
Pathogens can be carried in soil, so that the movement of living plants is doubly dangerous; a pathogen may be in the plant or in the soil attached to its roots. A nursery in New Hampshire was free from damping-off for many years until transplants were brought in from another nursery. Almost immediately a destructive damping-off fungus appeared and, since the introduced trees were too old to be affected, the fungus was evidently in fragments of soil attached to the roots. The movement of plants from nurseries is particularly likely to result in the spread of soil-inhabiting fungi causing root rots. It is more difficult to detect diseased roots by inspection than diseases of the above-ground portions of plants.
The introduction of soil from another nursery to promote the development of mycorrhiza can result in soil contamination. There had been no damping-off caused by Cylindrocladium scoparium Morg. at a nursery in New Jersey until soil was brought from another nursery. From then on the prevalent fungus causing damping-off was C. scoparium.
Other means of introducing pathogens
The elm disease fungus (Ceratostomella ulmi (Schwarz) Buisman) was introduced into Europe from Asia (presumably its native home) during World War I, probably on basket or crating material; into the United States from Europe on burl logs for furniture veneer manufacture; and into Canada from Europe probably on crating material. Sirex wood wasps have recently been intercepted at Australian ports in timber shipped from Europe.
The gypsy moth (Porthetria dispar (L.)), so destructive to broadleaf trees, first appeared in the United States in 1869 when insects which had been brought from Europe for experimental purposes escaped. Although since that time more than a hundred million dollars have been spent to keep the insect under control, the estimated average annual damage resulting from defoliation and consequent loss of growth, and even death of trees, still amounts to one-and-a-half million dollars in the United States.
Although the means by which pathogens can be introduced are so many - for example, insects in the egg stage can be carried on almost anything - as vet there is no definite evidence that any pathogen of consequence on trees, particularly on conifers, has been distributed by seeds. Furthermore coniferous seeds can be sterilized effectively by chemicals, although some seeds of broadleaved trees are not easily treated.
There are pathogens now known, and undoubtedly many more unrecognized, that could be extremely destructive if introduced into other parts of the world. Much work to increase poplar production is now underway, particularly in Europe and to a lesser degree in North and South America, and the possibilities of introducing pathogens destructive to poplars are therefore great indeed, since there is a widespread exchange of cuttings from country to country, and even from continent to continent.
The worst pathogen on poplar, in the experience of the writer, is Pseudomonas syringae f. sp. populea, also known as P. rimaefaciens Koning, causing bacterial canker. Although this bacterium is not yet known outside Europe, it has severely attacked certain North American poplars planted there. Another pathogen native to Europe but which has long since been introduced into the United States and Canada is the fungus Dothichiza populea Sacc. and Br. which causes canker. In the United States and Canada it has proved damaging in nurseries and plantations but has not yet appeared in natural stands. Two poplar diseases have recently been noticed in Europe the causes of which are unknown, while the fungus, Septotinia populiperda, causing a leaf blotch, although first found in Europe, is also present in the United States.
There are two extremely dangerous fungus pathogens of poplar in the United States and Canada. The first of these is Septoria musiva Pk., which causes a leaf spot of little consequence on native species, but produces cankers on native and exotic hybrid poplars so severe as to destroy entire plantations. The other is Hypoxylon pruinatum (Klotsche) Cke. on aspens (Populus tremuloides Michx. and P. grandidentata Michx.) causing a canker. A mortality of 70 percent has been reported in some stands. Considering the value of aspen (P. tremula L.) in Northern Europe, if H. pruinatum was introduced it could be catastrophic.
There are also certain pathogens attacking poplar foliage that can be damaging, for example, Septotinia populiperda mentioned previously. Melampsora rusts, although worldwide in distribution, are composed of various species, strains, and hybrids which differ according to region. Some poplars are so susceptible to these rusts that they cannot be used for planting. Taphrina aurea (Pers.) Fr., causing yellow leaf blister, although widespread also varies greatly in its virulence.
The bacterium, Bacterium salicis Day, which causes watermark disease of willow, actually a destructive dieback, is so far known only to occur in Europe. It is so damaging to willows in Great Britain that it seems reasonable to suppose that if it were distributed elsewhere in the world, it would cause equal or even greater damage. Another bacterium, B. savastanoi fraxini Tub., causes a cankerous gall on European ash (Fraxinus excelsior L.). Although this pathogen is known only in Europe, it is potentially dangerous to the United States and Canada since white ash (F. americana L.) native to those countries is susceptible, as has been proved by artificial inoculations.
Oaks are the most valuable broadleaved species in the north temperate zone. At present, a fungus, Endoconidiophora fagacearum Bretz, better known by the name of its imperfect stage, Chalara quercina Henry, is causing a wilt that is killing oaks over an extensive area in the United States and is spreading rapidly. The origin of the fungus is unknown, but since nothing similar has been reported elsewhere, it must be concluded that this fungus is a potential threat to oaks in other parts of the world.
Powdery mildew of oak caused by Microsphaera alphitoides Grif. and Maubl., almost certainly introduced from North America where it is native and not damaging, has been extremely injurious to indigenous oaks in Europe for nearly 50 years. In the French Basque region the fungus has caused almost total destruction of native Quercus toza Gill. and Bosc. (Parrot 1948). This oak will have to be replaced by red oak (Q. borealis) and pin oak (Q. palustris Muench.) both from North America and both resistant to powdery mildew. The fungus is potentially dangerous to oaks elsewhere in the world. Since overwintering is by means of fructifications on dead leaves and mycelium in buds it can be easily distributed.
Plantations are more susceptible to pathogens causing root rot than are naturally reproduced stands. Such pathogens can be carried long distances either in plant tissue or in soil. There are two fungi known to cause root rot which are particularly dangerous, because of the great variety of hosts that they attack. Helicobasidium mompa Tanaka which occurs in Japan is known to attack 104 species of plants, including broadleaved trees and conifers. The soil-inhabitating fungus Phymatotrichum omnivorum (Shear) Duggar, which so far is restricted to the southwestern United States, causes a destructive root rot of both broadleaved trees and conifers. Although this fungus is limited in its distribution apparently by high temperature requirements, there are undoubtedly various places in the world where it might flourish - in southern Europe near the Mediterranean, for example.
The blister rust fungus, Cronartium ribicola Fischer, has largely stopped the planting of exotic five-needle pines throughout Europe and has added considerably to the cost of growing five-needle pines in North America where they are native and valuable commercially, because the removal of Ribes, the alternate host of the fungus, is essential to the production of a satisfactory stand. There are other fungi causing blister rusts that, seem potentially capable of great damage outside their native habitat. In the Himalayas, Cronartium himalayense Bagchee occurs on long-leaved Indian pine (Pinus roxburghii Sarg.), the alternate host being species of Swertia. Since Swertia is found in Europe and North America, this fungus if introduced might cause serious damage to hard pines there. In Europe, Cronartium asclepiadeum (Willd.) Fr., also known as Peridermium pini (Willd.) Kleb., causes resin top of Scots pine resulting in the deformation and death of many trees. This rust fungus is specially dangerous since no alternate host is needed, and infection can be transmitted from pine to pine. The reaction of other hard pines to it is unknown. Probably the most dangerous of all is Cronartium harknessii (Moore) Meinecke commonly occurring on ponderosa pine (Pinus ponderosa Laws.) and lodgepole pine (P. contorta Loud.) in the western United States and Canada, which causes large globose galls on the trunks and branches. The fungus has found its way to the eastern United States and Canada, where, under the name of the Woodgate gall rust, it has vigorously attacked Scots pine of all ages. Trees are killed or deformed, as many as 18,000 galls having been found on a single tree. If this rust fungus were introduced into Europe losses would be terrific, and the pathogen would be extremely difficult to control since, as stated above, infection occurs directly from pine to pine. Of course some trees are resistant to it, but it would require decades to build up resistant stock. Meanwhile, Scots pine stands would be decimated.
The rust fungus Melampsora pinitorqua Rostr. causes twisted shoots of Scots pine in Europe but is unknown elsewhere. However, the alternate stage of this rust is on poplars which occur wherever hard pines grout
Seemingly, the most dangerous insect if introduced elsewhere would be the nun moth (Lymantria monacha L.) which periodically causes heavy defoliation of conifers, particularly Norway spruce (Picea abies Karst.) and Scots pine, and to a lesser degree defoliates broadleaved trees. If introduced into the United States, for example, this insect might do as much damage to conifers as the gypsy and brown-tail moths do to broadleaved trees.
The foregoing pathogens are those that are well known in their native habitats and potentially dangerous to trees elsewhere in the world. There are undoubtedly many more obscure or even unknown pathogens that would be capable of causing great damage in new habitats and on tree species unaccustomed to them,
From the foregoing it can be concluded that the vicissitudes to which an exotic is likely to be subjected will at least increase the difficulty and cost of establishing the species in a new habitat, even if the establishment is not precluded entirely. One must be convinced that the introduced tree has some essential advantage over the native species and gives promise of some possibility of successful culture. Then ideally one should proceed on a properly planned, long-term experimental planting program. Early promise must not be accepted as a guarantee for the future, to be followed by extensive planting, because this way lies the possibility of unreasonable losses. In fact, not until the end of the first rotation can a reasonable prediction be made as to the success of an exotic; even this is not conclusive. It is important that all the races in a variable species have the same long-term test, because from the biotic standpoint, varietal or racial differences within a variable species can be greater than differences between species.
Exotics are not all foredoomed to failure, but for every exotic the chance of failure appears to be much greater than the chance of success.
In the course of the Seventh Session of the Conference of FAO, His Holiness The Pope received delegates in audience at the Vatican Palace. In speaking of FAO's work, His Holiness said: 'This program, so varied and so extensive undeniably raises uncommon difficulties. The most delicate, perhaps as you have indeed perceived, will consist in creating social conditions by virtue of which the workers to whom you will provide aid and direction will take an intelligent interest in their occupation and utilize to the greatest extent the resources placed at their disposal. It is useless, in fact, to send experts to a place for the purpose of instructing in new methods and of perfecting mechanical equipment, if the human conditions in which a man is living prevent him from deriving that fruit from his efforts which he has a right to expect.
To arouse personal interest and initiative, to show that the good of the community is not obtained at the cost of personal good, but rather to its profit, and ensure that it should really be so - all this is surely a capital element of success. In this way, your economic labors will have also an equally decisive social v value, which We wish to underline. Hence, in Our care to manifest Our support and to collaborate with you in this enterprise, We recently made Our contribution towards the program extending technical assistance to various countries, especially to the most neglected regions."
H. H. The Pope also mentioned "as a significant example of the cultural inter-relation of your labours, the project, now being put into operation, of increasing paper production, which has been entrusted to you by the Economic and Social Council of the United Nations".
On 21 September 1953 a new Pulp Research and Experiment Center was opened at Rome in the presence of the Under-secretary of Industry and Trade. This new establishment which is one of the many projects being carried out by the Ente Nazionale per la Cellulosa e per la Carta (National Institute for Cellulose and Paper), will as its name implies - study all plants supplying raw materials for pulp and paper. In the beginning, in particular those trees suitable for central and southern Italy will be studied, since the well known Poplar Experiment Station at Casale Monferrato is already studying and improving poplar cultivation in northern Italy. The activities of the two institutes will of course be coordinated.
The setting up of the new establishment is the direct result of the supply difficulties encountered by the pulp and paper industries in 1950-51, when the Ministry of Industry and Trade invited the Ente to work out a program for intensifying and expanding its forest and agricultural activities, in order to stimulate a larger output of pulping raw materials. Attention has also been drawn to the need for extending research and experiment in this field at the FAO pulp and paper consultation which was attended by prominent experts from all over the world.
In Italy, availability of the raw materials most widely used today (spruce, poplar, cane, straw) will never be sufficient to meet requirements and only the use of new materials on a large scale can remedy the situation. Without underestimating the possibility of higher production through better care and utilization of the softwood forests in the Alps and, on a shorter term basis, through more extensive poplar cultivation in the Po valley, it can still be said that the best opportunities are offered by the planting in central and southern Italy, and in the off-shore islands, of rapidly growing species. Monterey pine, Canary pine, Douglas fir, heat tolerant poplars, and particularly eucalypts, can certainly achieve much in this respect, provided the social, ecological and technical difficulties confronting the planting of trees in central and southern Italy are successfully overcome. The best means to this end are well coordinated experimentation, combined with research on the biological, ecological, cultural and technological characteristics of the species to be raised. This is precisely the object of the newly established center.
For the purpose of accomplishing its hard and far-reaching task, the center has been divided into the following five sections: Biology, Ecology, Cultural Techniques, Protection, and Wood Technology, which will work in close co-operation. Much research will, in fact, have to be, carried out jointly by two or more of them.
A. de Philippis, Professor of Silviculture at Florence University, has been appointed Director of the Center and the supervision of the various sections has been entrusted to other university professors, amongst whom is G. Giordano of Florence, as well as to L. Chianese, the Inspector-General of the Ente.
The results of the Center's work will be published at regular intervals.