**4. Discussion**

This is the first baseline data of disease incidence (DI) of the current as well as emerging pathogen threats to plantation forests in Sarawak. It comprised about 5 years of field surveys as well as monitoring the health on a yearly basis. PSPs were

*New and Emerging Disease Threats to Forest Plantations in Sarawak Borneo, Malaysia DOI: http://dx.doi.org/10.5772/intechopen.107027*

established randomly across all the four regions of Sarawak and were representative of the 521702.27 ha of exotic tree species. Overall, seven species of plant pathogens were recognised as either current or emerging threats to the forestry sector. *Ceratocystis fimbriata* is of the greatest concern due to its significant increase in geographical range, the successive rotation and fear of spread or 'jumping host' from Acacia to other species such as *Eucalyptus*.

#### **4.1 Diseases of Acacia**

#### *4.1.1 Root rot disease*

This disease is considered one of the most important diseases of *A. mangium* plantations in Indonesia and Malaysia [15–17]. The pathogen builds up in every successive rotation and caused tree mortality in trees as young as 3-month-old in particular in the 2nd and 3rd rotation trees based on observation. Tree death can exceed 50% in some areas within less than 20 years of establishing the first rotation [18]. The trees with symptoms of root rots were adjacent to one another. It is a known fact that the root systems of trees are entangled with one another, and thus, one infected root will have higher possibility of infecting the roots of the tree next adjacent. As such, the trees infected with root rot appeared to be centralised and clustered in patches that were roughly circular. The nearer the planting distance, the probability spread of infection will increase exponentially.

Currently, the spread of root rot diseases is control by excavating and destroying the infecting trees as well as construction of drainage trenches filled then with fungicides, which will probably minimise the roots contact of infected trees to healthy trees adjacent. However, mostly all plantation owners commented that such practices is not economically viable and opted to just leaving the infected trees as it are. From general overview, root rot disease will always be a menace as the pathogen fungi, which are present in native forest left in infected stumps and roots when native forest is cleared for plantations, and these then act as inoculum sources and it will definitely increase with successive rotation [16]. Land preparation and silviculture management are the only way to minimise the inoculum build up as the wood debris left after planting will act as the food source of the fungi.

#### *4.1.2* C. fimbriata complex-*Ceratocystis wilt disease*

The pathogen emerged as a force to be reckoned with, as with the concerns about the vitality and survival of Acacia plantations in Indonesia, Vietnam and Sabah since 2010. It had created havoc on the tree plantation industries in Indonesia and resulted in the replacement of approximately 600, 000 ha of *A. mangium* plantation with *E. pellita* and its hybrid [3, 19, 20]. The disease was first observed in Sarawak in early 2010 in the Bintulu region, infected not more than 10 trees adjacent to the roadsides and trees in the vicinity of the conservation area and buffer zone. Over the years, based on observation and feedback from plantation company personnel who worked on the ground, the number of infected trees is increasing and spreading to the region of Sibu, Miri and Kuching as well. Compared to root rot pathogen modes of spread through root contact, the spread of Ceratocystis pathogens through air and water could be considered rapid especially when the trees were inflicted with wounds made by pruning, animal damages such as squirrel observed in the Bintulu region. However, one most important discovery in some of the surveyed plots in Bintulu region was

some infected trees showing sign of recovery from the infection with new sprouts growing out from the trees.

This could indicate resistance and/or tolerance of some of the individual trees. It was later found out that the plantation company has planted improved clones in their area. This justifies that breeding of disease should be actively pursued as a tool to help with the growing problem of invasive pests and pathogens that threaten our forests. However, resistance breeding is sometimes viewed as being too long term (5 years) and too expensive to be practical as lack of scientific understanding between planters and scientists in the matter concerned [21].

There are some assumptions that wood-boring insects could have the possibilities of harbouring the pathogen in their gut and acting as a vector of the disease; however, no concrete studies have been able to prove this theory. The attempted isolation of Ceratocystis from the carcass of the ambrosia beetle collected during the survey using the carrot bait method but none bore any traces of the pathogens, thus concluding that it did not harbour the fungus and thus not the vector in this specific study. Therefore, we presume the wood-boring beetle found is most probably due to secondary infestation caused by abiotic tree stress.

#### *4.1.3 Pink disease*

Currently, the pink disease caused by the pathogen *U*. *salmonicolor* was never been considered a major threat to Acacia plantations as compared to root rot and *Ceratocystis* wilt [22] disease as the reports of the pink disease were very few and it assumed that Acacia species particularly *A. crassicarpa* seems to be resistant to the fungal pathogen of *U. salmonicolor* [8, 23]. In Sarawak, the earliest occurrence of pink disease on *A. mangium* was recorded in 1979 and it reached epidemic proportion in 1987 [24]. The mortality inflicted by this pathogen observed in 2-year-old *A. mangium* indicated that this disease should be taken seriously. As the spores could travel by wind and water, it will be a threat to nurseries and early establishment of seedlings planted.

#### **4.2 Disease of Eucalyptus**

#### *4.2.1 Canker disease*

Several fungi are known to cause stem cankers in eucalyptus, among them *Botryosphaeria* spp., *Chrysoporthe deuterocubensis* and *Teratospaeria zuluense* (syn. *Coniothyrium zuluense*) [25]. *Chrysoporthe deuterocubensis* is a widespread and important pathogen of plantation eucalypts in the tropics. Basal cankers caused by *C. deuterocubensis* can extend several meters up the stem and have the ability to kill young trees. Where stems have been girdled, young trees may wilt and die suddenly during hot and dry weather [26–28].

Older trees of 11 to 12 years in the Bintulu region at the first rotation that exhibit symptoms of canker in this assessment are growing well and without any concern for mortality. The wood under bark seems to be healthy without any signs of lesion or discolouration although gummosis is present on the canker stem. This might be that trees normally could have survived the initial infection and only will develop basal swellings and severe bark cracking over brown necrotic sapwood [29]. However, younger trees of 2-year-old in this study recorded mortality. The trees are currently in 2nd rotation planting; thus, it could be concluded that DI and severity, as well as tree

*New and Emerging Disease Threats to Forest Plantations in Sarawak Borneo, Malaysia DOI: http://dx.doi.org/10.5772/intechopen.107027*

mortality in especially young *E. pellita* plantations, can thus only be expected to increase after the species has been grown for several rotations on the same site [16].

### *4.2.2* R. solanacearum—*bacterial wilt of Eucalyptus*

Bacterial wilt disease of Eucalyptus will pose a difficulty to contain as *R. solanacearum* sensu lato is known to be a destructive bacterial phytopathogen that is able to cause bacterial wilt in over 50 plant families growing in tropical, subtropical and some temperate areas globally [30]. Bacterial wilt arises from the blockage of the xylem tissue by bacterial growth, thereby resulting in wilt symptoms in the aerial parts of infected plants that ultimately die. The symptoms are very much similar to Ceratocystis wilt disease with the inner stem exhibiting blackish-brown. However, to distinguish between the latter and bacterial wilt disease, examination of the plant stem using a hand lens will be required to observe bacterial oozing (**Figure 7**). As with many pests and diseases, the technology of breeding and selection of disease and insect cultivars should be the key to minimising damage and ensuring the productivity of tree plantation industry [30, 31]. However, the process requires scientist to weigh many factors that include pathogen diversity, prevailing environmental conditions and the availability of material for breeding as well to make the management level understand that process is lengthy that requires at least 1 cycle rotation (8�9 years) planting and costly to begin with.

#### **4.3 Disease of** *F. mollucana*

Gall rust was found as the predominant disease of *F. mollucana* in part of Miri and Bintulu regions. It was first reported by some plantation owners back in 2012; however, the disease only affected the seedlings in the nursery. The source of infection was deduced from the seeds imported from the island of Jawa Indonesia. This could be related that Indonesia had experienced the major outbreak of the disease, which affected much of *F. molluccana* plantation estate in Java in 2010 [32].

The plantations affected were those of higher elevation of more than 152 m asl with spacing of 3 m<sup>2</sup> that could be considered to be a close gap between trees. The pathogens thrive in an environment with elevations ranging from 152 m asl to 975 m asl, in trees ages 1-year-old to 9-year-old, and spacing between 6 m<sup>2</sup> to 16 m<sup>2</sup> [33] and require fog or mist as well as high relative humidity to ensure infection [34] (m).

#### **4.4 General overview**

The loss of yield due implicated by insect pests and pathogens attacks could hamper the overall maximum production yield of the plantation forest. Sarawak, in recent years, has observed the dramatic losses of *A. mangium* plantation in its neighbouring country Sabah due to the incidence of a serious canker and wilt disease caused by *Ceratocystis acaciivora* [19], the conversion of 1 million hectares of the *A. mangium* plantation estate in Indonesia to *E. pellita* Muell. and related hybrids over the past 5 to 10 years as detailed by [20]. These critical developments have since propelled the forest plantation industry players in Sarawak to be more cautious and attentive to any signs of threats especially caused by fungal pathogens.

Insect pests have not been associated with widespread mortality or failure of any acacia plantations in Sarawak so far. However, an investigation survey conducted in early 2022 to determine the cause of mortality in one of *A. mangium* superbulk

plantations in the Bintulu region revealed that termite's infestation further identified as *Coptotermes travians* appeared to be the most prevalent pest that had inflicted tree mortality in the area, which accounted for 24.1% in mortality of trees.

The tree plantations in Sarawak are currently in the second rotation planting but will soon enter the third rotation of planting, and the concerns of increasing pests and diseases attack will definitely be inevitable [35, 36]. There is growing concern that *A. mangium* may no longer be capable of producing commercial yields after three rotations [37]. Significant reductions in productivity have been reported with each successive rotation and reductions have frequently been associated with mortality caused by fungal pathogens [1].

Thus, Sarawak is looking for other alternative tree species to compliment or maybe even replace *A. mangium*. *E. pellita* and its hybrid are seen to be the next species of choice as Eucalyptus species are easy to hybridise as compared to Acacia species, which is due to the long time and the low productivity in multiplication due to the ageing effects [38]. By theory, hybrid clones combine fast growth, increased tolerance to pests and diseases, excellent rooting ability, as well as wood quality suitable for different uses [39].

Other tree species such as *F. mollucana* seem to be at a lower risk of pathogen attack; however, through observation and communication with planters, insect pests such as defoliators moth and borers will contribute to lower quality of the woods produced although mortality caused by these pests are expected of less concern. New exotic species of that Paulownia are slowly making a wave in Malaysia, and some planters in Sarawak are experimenting in planting this tree on a small scale.

The diagnosis and determination of the cause of any damage or disease on a tree can only be made when the trees are still alive. Once the tree is dead, it is very difficult if not impossible to determine the cause of the problem. Therefore, it is very important that tree health inspections and pest and disease surveys are carried out at regular intervals to enable recognition of early signs and/or symptoms of pests and disease infection [40, 41].

Once recognised, the initial stages and the development of symptoms can be followed until the death of the tree. This would go a long way towards better identifying the causes of tree mortality in the plantations. By so doing, instead of just encountering a dead tree during a random inspection, one would have detailed information about when the symptoms first appeared, their pattern of development and spread, mode and rate of spread, and thus a better idea of the agent(s) involved. It is, therefore, very encouraging that the management has decided to set up pest and disease monitoring transects in the young plantations.

As such, different strategies should be employed in better managing the threats of pests and disease, such as good silviculture, increasing the knowledge of proper identification of pathogens and pests threats among the planters, sharing of knowledge and more transparent in R&D data and information between different companies in Sarawak particularly, management consideration in acknowledging that R&D especially tree pest and disease surveillance, as basis it seems but the information gain is invaluable to predict the next outbreaks and to better contain the damages.
