Internationally dangerous tree diseases

A. J. RIKER
Professor of Plant Pathology and Forestry, University of Wisconsin, U.S.A.

THE maintenance of sustained yields in high quality timber is one of the prime aims of research in forestry. Among the enemies of sustained yields are the following: fire, insects, animals, inroads of "civilization," and diseases. In the disease category one finds not only the pathogenic agents that destroy foliage and kill trees but also those that cause root, butt, and trunk rot. In the United States, the total growth impact of forest diseases is estimated to be about 45 percent of the losses caused by all destructive agents.¹

¹ UNITED STATES DEPARTMENT OF AGRICULTURE, FOREST SERVICE, 1958. Timber resources for America's future. Forest Resource Report No. 14, 713 pp. ill.

In North America, the invasion of foreign forest tree diseases has caused tremendous losses. Examples are chestnut blight, white pine blister rust, and Dutch elm disease. The latter is insect borne. The importance of injurious insects that might come from abroad also is clearly recognized. However, the insects are outside the scope of the present consideration.

Epidemic diseases are much more serious for forest trees than for comparable agricultural crops. If a field of wheat should be destroyed, something else could be planted the next year. But if the trees are damaged, then 10, 25, or 50 years of the productivity of the land may be reduced or lost. Furthermore, with some of the root and butt rot organisms another danger appears. Such pathogens may develop in an apparently innocuous way in the nurseries. Thus, dangerous disease organisms may be distributed with the nursery stock throughout the areas used for reforestation.

A program has been initiated to investigate:

1. what the chances are that certain internationally dangerous diseases might be moved from one country to another;

2. how this movement could be prevented, or at least slowed down;

3. how quarantines could be made more effective;

4. how eradication might be accomplished if a foreign disease should appear;

5. how its damaging effects might be avoided or reduced if it appeared and could not be eradicated;

6. how research might supply badly needed information about the host, the pathogen, and disease development.

To facilitate such a program, as a member of the Working Group on Foreign Tree Diseases in Section 24 (Protection) of the International Union of Forest Research Organizations, the author visited 16 different countries to consult with leading foresters, quarantine officials, forest geneticists, and pathologists. His trip was financed by the University of Wisconsin.

Numerous suggestions on what to do have come from many competent people. A summary appears below.

Directory of critical tree diseases

In order to have available information about the epidemic diseases and their importance in each country, an annotated list of them has been or is being prepared in a number of places. From such lists, distribution maps can be made. At the same time, the maps may show the distribution of reporting pathologists rather than that of the disease. So negative reports may be significant only if a qualified person has made a diligent search.

Such lists may not disclose diseases which appear innocuous in one country but which might be dangerous in others (such as the chestnut blight in east Asia). However, research, as discussed later, may bring these to light. An example of such an annotated list is given by Dr. P. Spaulding for the United States (see table on the following page).

Much can be learned by a study of surviving exotic trees in plantations already made, as Spaulding has done.

List of key men in each country

A list is being prepared of men active in forest pathology, one or more for each country. These men will exchange critical information, arrange for research on an international basis between suitable individuals, and advise in case of an outbreak.

Strengthening quarantines

Plant quarantines are the accepted bulwark against the entry of dangerous tree diseases. Modern air travel greatly increases the chances for distributing pathogens from one country to another. Many foresters are concerned about the quarantines, no matter how well they are administered, for fear dangerous pathogens may get through. Forest pathologists have an obligation to supply to quarantine officials a list of the most dangerous pathogens and to describe the diagnostic symptoms by which they may be recognized. At the same time interferences with commerce, travel, and the exchange of scientific materials need to be held to a minimum compatible with protection. With many questions, more research is needed if quarantines are to achieve maximum dependability.

AMERICAN FOREST TREE DISEASES POTENTIALLY DANGEROUS To FORESTS OF FOREIGN COUNTRIES

Causal agent	Disease
Virus	Pierce's virus of grape
Cephaloeporium diospyri	persimmon wilt
Ceratocystis fagacearum	oak wilt
Ceratocystis fimbriata platani	canker stain
Cercospora thujina	Thuja leaf spot
Clitocybe tabescens	root rot
Coryneum berckmanii	Coryneum blight
Cronartium coleosporioides	western pine stern gall rust
Cronartium comptoniae	sweetgale blister rust
Cronartium harknessii	western globoid stem gall
Diplodia pinea	pine twig blight
Endothia parasitica	chestnut blight
Erwinia amylovora	fire blight
Erwinia nimipressuralis	elm wetwood
Fusicladium saliciperdum	willow scab
Gloeosporium ulmicolum	elm anthracnose
Guignardi aesculi	Aesculus leaf blotch
Gymnosporangium libocedri	Libocedrus-Pomaceae rust
Gymnosporangium nootkatensis	Alaska cedar rust
Melampsora abietis-canadensis	hemlock-poplar rust
Melampsora occidentalis	Douglas-fir-poplar rust
Peridermium coloradense	spruce witches'-broom rust
Phomopsis juniperovora	cedar blight
Phyllosticta minima	maple leaf spot
Phymatotrichum omnivorum	root rot
Rhizoctonia ramicola	silky thread blight
Scirrhia acicola	brown spot needle blight
Sclerotium bataticola	damping-off
Septoria musiva	leaf spot, canker
Trichoscyphella willkommii	larch canker
Thyronectria austro-americana	hardwoods canker
Uncinula flexuosa	powdery mildew
Xanthomonas corylina	filbert blight

* SOURCE: SPAULDING, P. 1958. Diseases of foreign forest trees growing in the United States. U.S. Dept. Agr. Handbook No. 139, 118 pp. (see pp. 113-118).

Organizations now operating

Various organizations deal in one way or another with certain aspects of those foreign tree disease problems. The names of some important ones follow: European and Mediterranean Plant Protection Organization; Moscow-Peking Convention; Plant Protection Committee for the Southeast Asia and Pacific Region; Inter-African Phytosanitary Commission; Organismo Internacional Regional de Sanidad Agropecuaria (Central America). FAO in Rome is interested in reducing the spread of forest pathogens from one continent to another and can supply the name and address of the quarantine services in all member countries.

A symposium on critical diseases

The International Union of Forest Research Organizations is planning a symposium on critical forest diseases at its next meeting in Vienna, 10-19 September 1961. Pathologists will select and describe those forest pathogens considered to be of greatest potential importance if they are moved from one continent to another. Publication of the proceedings will provide a useful handbook.

Co-operative international research

For a number of critical problems no one knows the answers. To secure them co-operative research is essential between men in different countries. Obviously, one cannot move either the pathogens or living trees that might carry them from one country to another except with extensive precautions. However, disease-free selected or treated seed can go from one country to a suitable place in another country with relatively little danger. Some important topics for research follow:

(a) Host ranges. The different kinds of trees attacked by critical pathogens need exploration. In some cases related ornamentals carry tree diseases. Such research may best be done in nature at a location where the pathogen is active.

(b) Alternate hosts. Especially with the rusts (e.g., white pine blister rust has one critical stage on currants and gooseberries) the various plants necessary for the development of certain Stages of the fungus need to be characterized with precision.

(c) Environmental influences. Temperature, moisture, light, and mineral nutrition have important influences favoring or discouraging epidemics. The study of microclimate is essential for knowing how the pathogens work and how their damage may be prevented.

(d) Disseminating agents. The potentialities of different carriers of pathogens need investigation, especially insect vectors.

(e) Seemingly innocuous pathogens. Attempts should be made to disclose dangerous pathogens that are seemingly unimportant in their native country. Chestnut blight, was considered of no consequence in east Asia. But when the pathogen reached America, it practically eliminated the American chestnuts. Suitable experimental plantings might disclose such wolves in sheep's clothing. (The technique for such research might follow the work with the world collection of wheat.)

(f) Improved methods for eradication. If a dangerous disease should appear in a limited area of a country, eradication might be tried. Fire and chemicals have been useful, but certain other procedures may be valuable. To be most effective, a technique for eradication should be developed in the country where the disease is active. The results should be available for immediate use to countries where the disease might appear as a new invader.

(g) Disease resistance. In many countries native trees fail to meet the needs for reforestation or afforestation. Trees from other countries have been widely and advantageously used, but sometimes with disastrous result from disease. Furthermore, tree breeders are continually developing improved trees. In both cases tests for resistance to dangerous diseases are essential. The selection and development of disease-resistant trees hold great promise.

For many kinds of research involving pathogens and isolation, several locations are needed. An island with a mountain might provide many desirable conditions. Changes in moisture occur from the rainy to the dry side. Changes in temperature appear at higher or lower elevations. In some mountain valleys the length of day would be shortened. In such isolated places pathogens might be studied effectively with little danger of spreading the diseases.

Relations with agricultural research stations

In many countries the forest pathologist works in relative isolation from men trained and experienced in handling various field, forage, fruit, fibre, and plantation-crop diseases. Such men usually have a background of information and "know-how" as well as equipment that the forest pathologist could use with advantage. For the most part, all this would be available merely for the asking. Active co-operation between forest pathologists and agricultural pathologists deserves not only encouragement but also the active removal of any physical and administrative barriers that prevent their working together.

In conclusion, the possibilities are dismaying that internationally dangerous tree diseases may move from one country to another and upset sustained yields. However, international understanding, co-operation, and research can do much to slow down or even to prevent more catastrophes like that caused by chestnut blight in America.