A Eucalyptus clonal test stand that illustrates genetic variability in commercially grown Eucalyptus.

Exotic trees and plantations
The greatest damage, by far, has been from exotic fungi and insects, rather than from native pathogens and insects. It is reasonable to speculate that various soilborne pathogens were introduced with early plant introductions. For instance, Rhizina undulata results in serious losses to pine plantations after fire. Also, recent evidence has shown that the Northern Hemisphere tree pathogen Armillaria mellea was introduced into South Africa about 300 years ago, but it appears not to have escaped from its introduction point in Cape Town. Available data suggest new records of pests and pathogens of pines, eucalypts, and wattles are increasing in frequency. This must clearly increase the costs associated with plantation forestry and, likewise, have a negative impact on the trade and development of the people living in this region. For instance, the pine woolly adelgid (Pineus boerneri) appeared in Zimbabwe and Kenya in 1968, and more recently, the pine needle aphid (Eulachnus rileyi) has been found in several countries of east and central Africa, although the pine needle aphid is less damaging. In 1974 pines in South Africa were first found to be under attack by the black pine aphid (Cinara cronartii), which originates in the United States. Probably most damaging is the cypress aphid, Cinara cupressivora, which first appeared in Africa in 1986 in Malawi. By 1991 it had spread to at least eight countries, causing severe browning and sometimes death of cypress trees, particularly Cupressus lusitanica, which is used for industrial plantations but also popular in urban areas for hedging and in rural areas for fuel wood. One estimate put the economic cost in East and Central Africa at $41 million in dead trees, with an annual growth increment loss worth $13.5 million. For some time it was unclear where the insect had originated, but taxonomic and biogeographical studies eventually showed it probably originated in the Eastern Mediterranean area.

A stand with two clones of Eucalyptus - on the left a healthy disease free clone (white stems) and on the right a clone heavily damaged by Coniothyrium canker.	Cinara colony.

We attribute the success of exotic plantations to the separation of trees from their natural pests and diseases. So one approach to a pest problem is to give up growing the tree and move on to another exotic. For instance, leucaena was actively promoted as animal fodder until the arrival of leucaena psyllid (Heteropsylla cubana), which caused a severely eroded farmers’ confidence in agroforestry, and many have since switched to other species. Cupressus macrocarpa and Pinus radiata are no longer planted in East Africa due to canker, Rhynchospaeria cupressi, and needle blight, Dothistroma pini, respectively. It is ironic that C. macrocarpa was replaced with C. lusitanica, which was subsequently ravaged by the cypress aphid. This shows that giving up on a tree species because of pest attacks is an unsustainable approach. Replacing exotic trees with indigenous trees is becoming increasingly fashionable, but there is much research to do on their silviculture as they tend to grow slowly.

Leucaena psyllid (Heteropsylla cubana)	An agar plate with isolates of the exotic pine pathogen Sphaeropsis sapinea showing the great genetic diversity between isolates in South Africa, resulting from multiple introducitons.

Natural ecosystems
There are fewer examples of exotic pests attacking indigenous tree species, but the cypress aphid has been a notable recent case. The aphid attacks many members of the cypress family, and in Africa there are a number of indigenous cypresses. In East Africa, the Red Pencil cedar (Juniperus procera) is highly valued for its termite resistant wood but in recent years appears to have suffered a “decline.” A number of contributing factors have been identified, including attack by the cypress aphid. Further south the cypress aphid also attacks Mulanje cedar (Widdringtonia cupressoides), the national tree of Malawi, as well as other Widdringtonia species in southern Africa, stunting growth and sometimes killing trees. The only example of a pathogen that has had some impact on native woody plants and that is believed to be exotic is Phytophthora cinnamomi (Linde et al, 1997; 1999). P. cinnamomi is associated with the death of Proteaceae in native ecosystems in South Africa. However, the lack of attention to diseases means that virtually nothing is known of exotic forest pathogens that might have had, or are having, a serious impact on native trees in Africa. Sub-Saharan Africa is also inhabited by a very large number of species of relatively small, nonforest trees that are highly important to natural ecosystems. A typical example is the many species of Acacia that provide a crucial source of food to the widely diverse browsing antelope and other native fauna. Exotic pests and pathogens could have devastating effects on natural ecosystems.

International trade
As the economies of Africa expand, trade will increase, and a number of steps are being taken specifically to increase trade. A free-trade area was established in 2000 under COMESA, and quarantine issues are on the agenda. The Southern African Development Community (SADC) and the just formed East African Community (EAC) also aim to promote regional as well as external trade. Outside Africa, attempts are being made to stimulate international trade with the continent-for example, the United States recently passed the Africa Growth and Opportunity Act (AGOA). Like the Northern Hemisphere, the Southern Hemisphere has many related families of trees and shrubs growing in geographically isolated areas. Good examples are the Myrtaceae, Proteaceae, and Mimosoideae. Southern Hemisphere pests and pathogens that are currently separated by oceans, deserts, and mountains are primary threats to these woody ecosystems. Trade between Southern Hemisphere countries is increasing significantly, and as trade increases, there will be increased risk from exotic pests and, in turn, increased impact on trade.

A common feature of exotic insect pests that attack trees in Africa is that they often spread over large distances very rapidly. This emphasizes the need for coordinated international action, both in reducing the impact of uninvited guests once they arrive and in attempting to prevent their immigration in the first place. Unfortunately many countries in Africa do not have quarantine services that are adequately resourced to operate with a high level of effectiveness. Similarly, the regional body for Africa concerned with quarantine, the Inter African Phytosanitary Council, part of the Organisation for Africa Unity, is unable to provide the level of services and coordination it would desire. Thus, although the guidelines for implementing quarantine are available from the IPPC, in many parts of Africa it is almost impossible to put them into practice. Ultimately, the analogy of the "weakest link in the chain" must hold true for such regions, where the country with the poorest quarantine will define the risk for the entire region. For countries battling to cope with health care, education, and nutrition it is clearly difficult to mobilize funds for significant quarantines. The inability of a country to implement acceptable quarantines is ultimately a matter of global concern, not exclusively a matter of national interest. Therefore, an argument for global funding to limit the spread of exotic pests and pathogens can be made.

Improved technology will ultimately lead to enhancement of quarantines and the reduction of risks due to exotic pests and pathogens. Understanding risks requires an extensive knowledge of the taxonomy and biology of pathogens. With modern tools such as those linked to DNA analysis, it is already becoming possible to detect pathogens in germplasm shipments, which would not previously have been possible. Furthermore, our ability to recognize new pests and pathogens, or even elements of populations of these threats, is rapidly increasing. Thus, strategies to understand risk and to develop meaningful risk-abatement programs are improving. This trend is likely to continue in the future.

Wingfield, M.J. 2000. Pathogens in exotic plantation forestry. International Forestry Review: 163-168.

Wingfield, M.J., and G.H.J. Kemp. 1994. Diseases of pines, eucalypts and wattle. Forestry Handbook ed. 3. South African Forestry Institute, Pretoria.