Diseases of Acacia mearnsii in South Africa, with particular reference to ceratocystis wilt

[摘要] English: The Acacia mearnsii industry is a relatively small, though very profitable industry in SouthAfrica. Wood derived from A. mearnsii is currently in greater demand than that of eitherpine or eucalyptus in South Africa. Despite the importance of this industry, very littleattention has been given to the genetic improvement, disease tolerance or generalimprovement of A. meamsii as a forestry species. The result has been that, during the lastfew decades, pathogens have become adapted to, and spread through plantations of thistree. Although relatively little research has been conducted on the impact of pathogens onA. mearnsii, this situation has changed during the past nine years, and particularly sincethe identification of Ceratocystis wilt.The planting of exotics has many advantages over native plants. In South Africa, exoticforestry species, such as Eucalyptus spp., Pinus spp. and A. mearnsii were introduced tohalt the uncontrolled logging of native forests. These native forests were logged mainlyfor furniture and building material, but also for fuel wood, resulting in the near completedestruction of South Africa's native forests. The introduced exotics prevented the furtherdestruction of these forests and soon became a large industry. This was particularly dueto the fact that it was found that they also had a superior growth rate when compared tonative species. This accelerated growth rate brought rapid results from breeding trialsand, thus, a relatively rapid improvement of the material planted. Because they had beenseparated from their natural enemies, these trees were also initially disease free.The A. mearnsii industry has, and will continue, to face many problems and challengesfrom pests and diseases. After the initial phase in which the tree was removed from thepathogens affecting it in it's native range, it faced attacks by native South African pestsand diseases. These can spread from native Acacia species, or from any other nativeplants in the same, or even different families. Exotic, mono culture industries are alsoconstantly under threat from the introduction of pathogens from other countries, includingthe country of origin. This can be done by the introduction of new germ plasm or on anyother plant species or plant material brought into a country. Because A. mearnsii is nowplanted as a monoculture, in contrast to it's native situation, diseases and pests canpotentially be much more severe and will spread more rapidly and widely throughout evenaged and genetically uniform stands.Propagation of A. mearnsii has, recently, advanced considerably and this is concurrentwith increased demand for this wood on world markets. Lessons learned from eucalyptand pine forestry need, however, to be heeded to save unnecessary losses and time. Withthe advent of vegetative propagation of A. mearnsii in South Africa, it is important toinclude disease screening trials at the early stages of the development of clones. In orderto do this, a knowledge of all possible pathogens of A. mearnsii is needed. This includespathogens known in South Africa and those that occur beyond the borders of the country.It is also necessary to have a detailed knowledge of the biology and population structureof these pathogens in order to gain an impression of the possible success of controlmeasures.This thesis is a compilation of work conducted on some of the known pathogens of A.mearnsii in South Africa. It also includes a large component dealing with theidentification and clarification of previously unknown pathogens of A. mearnsii. It,therefore, does not focus only on diseases of A. mearnsii, but includes a chapter on adisease of Eucalyptus. The causal agent of this disease has, however, also recently beenfound on A. mearnsii in South Africa and this chapter aims at elucidating the possibleorigin of the isolates from South Africa. It also illustrates the potential threat of thispathogen to the A. mearnsii industry.South Africa is a semi-arid country that regularly suffers from severe drought. Forestryactivities in the country are also mainly restricted to areas with poorer soil and whereagriculture cannot be pursued on a profitable basis. Factors such as drought, hail, frostand sub-optimal soil conditions can all contribute to increased stress on trees. Under theseconditions, many fungi can act as opportunistic pathogens, causing large scale losses.They often live as endophytes within their hosts, not causing any negative affect until theonset of stress. At this stage, they spread throughout trees, preventing them fromrecovering from the stress condition and leading to cankers and tree death. Carefulmanagement, particularly site/species matching, is required to minimise losses caused bythese pathogens.This thesis provides a basis for future research on the development of managementstrategies to control diseases of A. mearnsii in South Africa. Information, however, alsoprovide valuable knowledge for forestry industries outside South Africa by highlightingthe threat of exotic pathogens and the importance of strict quarantine measures to preventthe spread of pathogens. This is true for the movement of not only A. mearnsii material,but as was seen here, the movement of any forestry products, since many pathogens havea wide host range. Although the thesis is comprised of a series of individual entities, theseall provide information regarding the hygiene of A. mearnsii plantations. This thesis thusaims at identifying future focus points for intensive research, while at the same timefocusing on those pathogens that have been known to the South African industry for alonger period of time.Chapter one provides a review of the available literature on diseases affecting not only A.mearnsii, but also other Acacia spp. important to the forestry industry, world wide. Italso highlights some of the uses of these species in the countries where they are planted.The multi-purpose use of Acacia spp. is an important aspect emerging from this review.In many countries, Acacia spp. are not only planted as forestry species but are also usedfor soil reclamation, nitrogen fixation and fodder. The main focus of the chapter,however, is on the A. mearnsii industry in South Africa, with a brief discussion on all thediseases currently known to occur in the country. It is concluded that much research isstill needed to reduce the impact of these diseases and to ensure that the Industryfunctions optimally.Ceratocystis albofundus must be considered as one of the most important pathogens ofAcacia spp., world-wide. Currently this pathogen occurs only in South Africa, but if it isto spread to other countries, large scale losses will be incurred. It may also affect, notonly A. mearnsii, but most likely many other plant species. Breeding programmes for A.mearnsii in South Africa focus strongly on this pathogen. In Chapter two, the populationdiversity of C. albofundus was investigated and compared with data for other Ceratocystisspp., using nuclear and mitochondrial DNA fingerprinting. It was found that the C.albofundus population has a greater genetic diversity than any of the species with which itwas compared. This will thus mean that intensive breeding programmes will be necessaryto ensure durability of disease tolerance. It also supports previous hypotheses that C.albofundus is native to South Africa and may be a temperate species, not found in tropicalareas where its close relative, C. fimbriata, commonly occurs.The first unequivocal report of C. fimbriata and Ch. elegans from A. mearnsii ispresented in Chapter three. Both these fungi were isolated from dying trees with typicalsymptoms of Ceratocystis wilt caused by C. albofundus. Both were shown to be capableof causing disease to seedlings under green house conditions. It was, however, found thatC. albofundus is more virulent than either Ch. elegans or C. fimbriata. Both isolates wereidentified using molecular and morphological approaches. Unfortunately only one isolateof each exists and surveys to obtain additional samples continue to be a priority.The first report of a wilt disease of Eucalyptus, caused by Ceratocystis fimbriata in theRepublic of the Congo in West Africa is recorded in Chapter four. This is not only thefirst report of C. fimbriata as a pathogen of Eucalyptus in Africa but is also one of the fewunequivocal reports of this fungus from the continent. Pathogenicity of C. fimbriata onEucalyptus spp. was confirmed in glass house tests. In this Chapter, C. fimbriata and C.albofundus from A. mearnsii, and C. fimbriata from Eucalyptus in Brazil were alsocompared to the C. fimbriata from the Congo. Comparison of the lTS region of therRNA operon showed that isolates from all three areas grouped together in a clade of C.fimbriata, separate from European isolates. Sequence data showed that C. fimbriata fromA. mearnsii in South Africa is nearly identical to the fungi from Eucalyptus in Brazil andCongo, suggesting that they may have a common origin. These findings stress theimportance of sound quarantine measures to prevent the introduction of potentiallydevastating pathogens to South Africa. It is not yet known why C. fimbriata has notcaused more diseases on A. mearnsii or Eucalyptus spp. in the country, but the situationwill need to be monitored closely.Apart from C. albofundus, there are many other fungi that cause disease of A. mearnsii inSouth Africa. Chapter five reports on a species of Seiridium that was isolated from stemcankers on A. mearnsii. Morphological and molecular comparisons, as well aspathogenicity studies have shown that the species from A. mearnsii is similar to thosespecies responsible for Cypress canker in many parts of the world. It also confirmsprevious reports that the taxonomy of the three Seiridium spp. causing cypress cankerneeds re-evaluation, since molecular data support the view that the three species, representa single taxon. Pathogenicity trials on mature Cuppressus lusitanica and on A. mearnsiitrees showed that both the cypress and A. mearnsii isolates are capable of causing lesionson both hosts.Many of the fungi isolated from diseased A. mearnsii during the current and previousstudies of diseases resulted in the isolation of fungi, commonly found as latent pathogenson other forest trees. Chapter six encompassed a survey of the endophytic fungi of A.mearnsii, with the specific aim of identifying possible pathogens. Thirty different fungaltaxa were found as endophytes of the xylem and rachi. These included F. graminearumand Botryosphaeria dothidea, which are known pathogens. During periods ofenvironmental stress, these fungi can apparently cause disease. This is especially truebecause A. mearnsii is often planted on marginal sites in South Africa.Chapter seven represents the first report of Fusarium graminearum from A. mearnsii andpresents evidence for the fungus being involved in disease of A. mearnsii. This pathogenwas first isolated during 1994-95 disease surveys, but was not identified due to the factthat cultures on artificial media did not sporulate. In the current study, additional isolateswere obtained from stem cankers and die-back symptoms and the fungus was identifiedbased on β-tubulin gene sequences. Field inoculations using F. graminearum showedextensive lesion formation in the xylem. Previously, this Fusarium sp. was known only asa pathogen of maize and wheat in various parts of the world. Results of this study are,therefore, enigmatic and intriguing.

[发布机构] University of the Free State