Biology of botryosphaeria dothidea and sphaeropsis sapinea as endophytes of eucalypts and pines in South Africa

[摘要] English: Botryosphaeria dothidea and Sphaeropsis sapinea both very important pathogens inthe South African forestry context. These fungi are well established in the countryand contribute substantially to annual losses incurred. Currently very little can bedone to control the fungi and the damage they cause. The understanding of theirrespective disease etiologies is thus of great importance to develop relevant countermeasures. The overall aim of this dissertation was to investigate various poorlyunderstood aspects of these two fungi and to try and relate the results to practicalcontributions towards controlling the impact the two pathogens have. Studies havebeen conducted during the course of five years and each study represents anindependent research investigation.The introductory chapter presents a review of the literature pertaining to all aspects ofbiology, history and taxonomy of B. dothidea and S. sapinea. The two fungi areclearly very similar in all these aspects and perhaps the only clear difference is that S.sapinea is restricted to pines in South Africa. Many other similarities and somedifferences between these two important pathogens are highlighted and many of thesehave provided the background for further investigations.In chapter two the presence of B. dothidea and S. sapinea lS demonstrated assymptomless endophytes in healthy, pine and eucalypt tissue. Botryosphaeriadothidea was found to be common in all the Eucalyptus spp. tested, occurring at highpercentages in symptomless leaves of Eucalptus smithii, E. camaldulensis, E. grandisand E. nitens. Sphaeropsis sapinea was, in contrast, only present in young, greenPinus patuIa and P. radiata cones, but virtually absent from the cones of P. elliottiiand P. taeda.Botryosphaeria dothidea is associated with die-back and canker diseases of eucalyptsin South Africa. Despite this fact, little is known about the infection process. Thefungus is known to occur endophytically in leaves of various Eucalyptus species inSouth Africa. In chapter three I consider the ability of B. dothidea to infect apparentlyhealthy Eucalyptus leaves and the subsequent location and structure of theseinfections once inside leaf tissue. Scanning electron microscopy revealed that conidiaof B. dothidea can infect healthy leaves through stomata. These infections ultimatelyreside amongst mesophyll cells and constitute a number of individual infections perleaf.Two morphologically similar fungi are associated with die-back and canker ofeucalypts in South Africa. The one was identified as part of the Botryosphaeriadothidea-complex. In chapter four, the identity of the second fungus was determinedby comparing morphology, pathogenicity and DNA sequence analysis of isolates ofboth taxa. Based on results obtained, Botryosphaeria eucalyptorum, and its anamorphFusicoccum eucalyptorum, are described as a new species. I found that theteleomorph is morphologically similar to other taxa in the B. dothidea-complex, butconidial characteristics of the anamorph are distinct, as well as the sequences of thenrDNA internal transcribed spaeers ITS 1 and ITS2. As is the case with B. doth idea ,the fungus is pathogenic to Eucalyptus, there do not, however, appear to bedifferences in pathogenicity between the two.Sphaeropsis sapinea is the most important pathogen of pines in South Africa. Thefungus, which reproduces only asexually, occurs only on exotic pines. In chapter five,I investigated the diversity of the S. sapinea population in South Africa and comparedit with a population from Northern Sumatra. Both populations were obtained fromexotic P. patuIa plantations. The phenotypic diversity of these populations wasassessed using vegetative compatibility tests. The percentage maximum genotypicdiversity, based on Stoddard and Taylor's index, for the South African population wasmuch higher than the Northern Sumatran population, thus indicating that the SouthAfrican S. sapinea population was more diverse than the Northern Sumatranpopulation. These results support the hypothesis that the population of S. sapinea inSouth Africa has been introduced from various parts of the world, during the lastcentury.In chapter six, I investigated the role that latent S. sapinea infections in seed cones ofP. patuIa, play in post-hail associated die-back. Pinus patuIa seed cones were foundto be infected during the second year of development, with extensive colonizationonly occurring m the third year when cones mature, prior to seed discharge.Vegetative compatibility tests revealed that the presence of S. sapinea in individualthird year seed cones is confined to a single genetic entity. Sphaeropsis sapineacolonisation of third year seed cones thus, apparently results from a single successfulinfection per cone. The probable role of latent infections by S. sapinea indicated thattree age and by implication, increased numbers of attached seed cones, contributes tomore severe die-back after hail damage.The control of damage caused by S. sapinea is highly dependant on a dynamichybridisation programme. Alternative species of pines is thus constantly evaluated forpotential. In chapter seven, 65 families representing both the northern and southernpopulations of P. greggii were evaluated for their tolerance to infection andsubsequent die-back caused by S. sapinea. Families were evaluated following naturalinfection after hail damage, as well as by artificial inoculation. Variation in toleranceoccurred and was highly significant between the two provenances, with the northernprovenance proving to be very tolerant. Pinus greggii trees of the southernprovenances were comparable with P. patula. The potential of the families fromnorthern origins has to be investigated further.Cultures of Cytospora isolated from Eucalyptus trees in South Africa, Congo,Thailand, Venezuela, Mexico, Uganda and Australia, as well as Cytospora-likeisolates from Indonesia were compared in chapter eight. Comparisons were based onthe homology of the internal transcribed spaeer regions and the 5.8S ribosomal DNAof the nuclear ribosomal DNA repeat unit. Isolates clustered into at least threeunrelated groupings, with a fourth grouping that included isolates thatmorphologically resembled Cytospora. Results from this chapter indicated that thecurrent description of Valsa ceratosperma encompasses several distinctly differentspecies and needs to be further refined.Botryosphaeria dothidea and S. sapinea are two of the most important pathogens ofeucalypts and pines in South Africa. The fact that they exist as symptom lessendophytes in trees has added a fascinating aspect to our understanding of their role intree diseases. In the past, they have generally been considered to be wound infectingopportunistic fungi. Results of these studies have shown that this is not so and thatthey are clearly able to infect healthy trees. They are unlikely to be able to infect deador moribund tissue. The investigations presented in this dissertation have addedconsiderable knowledge to our understanding of B. dothidea and S. sapinea and willalso promote efforts to reduce disease caused by them. However, there are manyquestions that remain to be answered pertaining to them and it is my hope that thisstudy will provide a foundation and stimulus for further work.

[发布机构] University of the Free State