Botryosphaeria species on native South African Syzygium cordatum and their potential threat to Eucalyptus

Abstract

The South African commercial forest industry is almost exclusively reliant on plantations of exotic trees, of which Eucalyptus spp. make up almost 50 %. Botryosphaeria spp. are important canker pathogens in these Eucalyptus plantations in South Africa. However, exotic plantations and their pathogens cannot be viewed separately from the related native flora. This study showed the importance of extending our knowledge on pathogens that occur on related native and exotic hosts, and which can pose a threat by cross infection between these host groups. In Chapter 1, a review of the literature concerning Botryosphaeria spp. that occur on Eucalyptus in its native range and exotic plantations is presented. It is clearly shown that Botryosphaeria spp. are important pathogens on Eucalyptus in exotic plantations worldwide, causing various symptoms on this host. Botryosphaeria spp. are also important canker pathogens in Eucalyptus plantations in South Africa. Traditional identification of this group of fungi, based on morphological characteristics, led to much confusion about their identity. However, in recent studies morphological characteristics were combined with DNA sequence data to distinguish and identify these fungi. Based on these data a few revisions have been done and new Botryosphaeria spp. were described on Eucalyptus. Botryosphaeria spp. recognised as pathogens on Eucalyptus in South Africa include B. dothidea, B. parva and B. eucalyptorum. Future studies should be focused on correct identification of Botryosphaeria spp. that occur on Eucalyptus, which is the first step towards preventing the spread of this group of pathogens and developing management strategies to control disease outbreaks. During the study on Botryosphaeria spp. on Syzygium cordatum, isolates of two Botryosphaeria spp. appeared to be undescribed. One of the undescribed species was represented by only one isolate and it was not named. The other species was described as the new Botryosphaeria anamorph within Lasiodiplodia, namely L. gonubiensis. This species grows endophytically on native S. cordatum in South Africa and is the first species in Lasiodiplodia to be found on native trees in the country. Identification of the new species was based on conidial and cultural morphology and DNA sequence data of the rDNA internal transcribed spacers, ITS1 and ITS2. Identification and description of L. gonubiensis is presented and discussed in Chapter 2. In total nine Botryosphaeria spp. were isolated from native Syzygium cordatum in South Africa. These include B. parva, B. ribis, B. lutea, B. australis, B. rhodina, B. dothidea, Fusicoccum mangiferum, Lasiodiplodia gonubiensis and an unknown Botryosphaeria sp. The isolates related to B. ribis, B. parva and F. mangiferum were the most abundant, while only one isolate represented B. dothidea. Species were identified based on morphological characteristics of their anamorphs combined with ITS rDNA sequence data. Some species, such as B. parva and B. ribis, could not be distinguished based on morphology or ITS rDNA data. A PCR-RFLP fingerprinting technique was, therefore, used to distinguish isolates of these two species. Once again this technique has proven useful and reliable in identification of Botryosphaeria isolates, including cryptic species. However, isolates of closely related B. lutea and B. australis could not be distinguished using this technique. It could be of interest to develop PCR-RFLP identification system that could be used in identification of the latter species. Identification and characterization of Botryosphaeria spp. are presented in Chapter 3. Isolates of all Botryosphaeria spp., obtained from native Syzygium cordatum in this study, caused lesions on the stems of Eucalyptus camaldulensis and S. cordatum in trials conducted under greenhouse conditions. Except for Fusicoccum mangiferum, all the other Botryosphaeria spp. were more pathogenic on Eucalyptus than on S. cordatum. The most pathogenic species on Eucalyptus were B. rhodina, B. ribis and B. lutea, whileF. mangiferum and B. ribis were the most pathogenic on S. cordatum. Botryosphaeria dothidea and L. gonubiensis were the least pathogenic on both hosts. The results obtained from this trial clearly show that Botryosphaeria spp. on S. cordatum pose a potential threat to exotic Eucalyptus plantations. Future study should be conducted under field conditions to evaluate data obtained in greenhouse trials. These results were presented and discussed in Chapter 4. The results presented in this study provide the first detailed information on Botryosphaeria spp. on the native Myrtaceae in South Africa. Most of the species isolated from Syzygium cordatum are not known on Eucalyptus in the country. All of the Botryosphaeria spp. obtained in this study are pathogenic to Eucalyptus, and thus pose a threat to this host. Large number of B. ribis, B. parva and F. mangiferum isolates obtained from the native S. cordatum could imply their origin in this region. Further sampling is needed on myrtaceous trees native to the Southern African region, as well as on Eucalyptus. Population studies on the most abundant and most pathogenic Botryosphaeria spp., should provide more information on the movement and origin of these pathogens. The results from this study also highlights the need for quarantine measures to avoid the introduction of new Botryosphaeria spp. or new strains that can be more pathogenic to either native or cultivated plants. Please cite as follows Pavlic, D 2004, Botryosphaeria species on native South African Syzygium cordatum and their potential threat to Eucalyptus , MSc dissertation, University of Pretoria, Pretoria, viewed yymmdd < http://upetd.up.ac.za/thesis/available/etd-02062006-112938/ >