Assessing the risk of fungal stem defects that affect sawlog quality in Vietnamese Acacia plantations

Acacia hybrid clones (Acacia mangium x A. auriculiformis) are widely planted in Vietnam. An increasing proportion of the Acacia hybrid plantations established (now standing at 400,000 ha) is managed for solid wood, mainly for furniture. Silvicultural practices such as pruning and thinning ensure the production of knot-free logs of sufficient quality for sawing. However the wounds that such practices involve may lead to fungal invasion which causes stem defects and degrade. In order to assess the extent of fungal stem defect associated with pruning, a destructive survey was conducted in a 3-year-old Acacia hybrid plantation at Nghia Trung, Binh Phuoc province, 18 months after experimental thinning and pruning treatments. A total of 177 Acacia hybrid trees were felled for discoloration and decay assessment. Below 1.5 m tree height, the incidence of discoloration and decay in the pruned and thinned treatments was significantly higher than in the unpruned and unthinned treatments, respectively. The percentage of decay area was significantly greater in pruned than in unpruned trees and it was similar between thinning treatments. There was a significant pruning and thinning interaction for percentage of healthy wood and discoloration area but not for the discoloration and decay rating and the percentage of decay area. Above 1.5 m, all wood quality variables were unaffected by thinning and thinning x pruning interaction; discoloration and decay rating, and percentages of discoloration and decay areas were significantly greater (and consequently decreased the percentage healthy area) in pruned than unpruned trees. Having established that there were significantly higher levels of discoloration and decay associated with pruned and thinned treatments compared to unpruned and unthinned treatments, the question is whether, as a tree grows, fungal decay and discolouration established at an early stage also increase in volume. It was decided to investigate the type of fungi associated with discoloration and decay as their identity may be indicative of the threat posed to solid wood production. Such fungi were examined in two Acacia hybrid plantations in southern Vietnam (Nghia Trung and Phan Truong Hai) by isolation into culture and direct sequencing from wood. A total of 231 fungal isolates were obtained from discoloured wood samples harvested from Nghia Trung and Phan Truong Hai. The DNA-based identification of isolates showed a wide diversity of fungal groups with three times as many ascomycetes as basidiomycetes. Approximately four years after the sampling for isolation and at harvest age, discoloured and decayed wood samples were taken from the same Acacia hybrid plantation at Phan Truong Hai. DNA was extracted directly from discoloured and decayed wood samples and associated fungi identified by 454 sequencing of fungal PCR products. Again there was a predominance of ascomycete fungi associated with discoloured and decayed wood with 68 fungi identified as Ascomycota and 41 as Basidiomycota and a low incidence of wood rotting basidiomycetes. Different silvicultural regimes did not influence the fungal communities associated with discoloured and decayed Acacia wood and there was no significant difference between fungal communities in discoloured wood compared to decayed wood. The low prevalence of wood decay basidiomycetes indicates that the risk of fungal stem defects at harvest is not as significant as previously considered. Observations of the levels of discolouration and decay from Phan Truong Hai in the trees sampled at harvest support this hypothesis (data not shown in thesis). Greater incidence and severity of vascular wilt diseases is commonly associated with mechanical wounding. Ceratocystis manginecans, a canker and wilt pathogen, was identified from wood samples at Nghia Trung and Phan Truong Hai. In SE Asia, the productivity of Acacia plantations is being severely threatened by this aggressive pathogen. Apart from growing a less susceptible plantation species, there are few opportunities in Acacia for the management of this disease apart from the use of tolerant clones. Acacia is typically singled at 6 months to ensure a single stem and, as previously mentioned, must be pruned and thinned for solid wood production. In addition, Acacia plantations are subject to wounding by boring insects and browsing mammals. A pot trial was established in Binh Duong province, southern Vietnam to screen for the host response of nine Acacia genotypes (six Acacia hybrid clones, two A. auriculiformis clones and mixed provenance seedlings of A. mangium) to artificial inoculation with three isolates of C. manginecans. Lesion lengths as measured on the inner bark indicated that the two A. auriculiformis clones were relatively more tolerant to C. manginecans than the A. mangium genotype. In contrast, the lesion lengths of all six Acacia hybrid clones fell between the A. auriculiformis and A. mangium genotypes. The study suggests that it is possible to select amongst Acacia hybrid clones for tolerance to C. manginecans. However the main focus of the study was to explore the chemical basis to tolerance and the potential for using host chemical response to early infection as a rapid bioassay for tolerance selection. Chemical analysis of crude sapwood extracts sampled from the lesion provided some evidence that induced phenolic compounds, particularly tetrahydroxyflavanone, and condensed tannins may have a defensive role in the Acacia - C. manginecans pathosystem. However, these results were not consistent across individual Acacia hybrid clones and A. mangium genotypes and therefore not reliably indicative of the host susceptibility. In summary, the research in this thesis investigated two major biotic threats to the productivity of Acacia hybrid especially when grown for solid wood products. From the Acacia hybrid plantation investigated at Nghia Trung there appeared to be significant levels of discolouration and decay associated with early pruning and thinning operations. The fungi associated with this type of stem defect are however very diverse and, even at harvest age, do not comprise those wood decay basidiomycetes commonly associated with heart rot. Ceratocystis manginecans is the most serious threat to productivity but there appears to be differential levels of tolerance across Acacia species and hybrids that can be exploited, although no clear chemical host response indicative of tolerance was demonstrated.