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Review

Managing Macadamia Decline: A Review and Proposed Biological Control Strategies

by
Xiaofang Yao
1,2,3,
Qiumei Liu
1,2,
Yongxin Liu
4,* and
Dejun Li
1,2,*
1
Key Laboratory of Agro-Ecological Processes in Subtropical Region, Institute of Subtropical Agriculture, Chinese Academy of Sciences, Changsha 410125, China
2
Guangxi Key Laboratory of Karst Ecological Processes and Services, Huanjiang Observation and Research Station for Karst Ecosystems, Chinese Academy of Sciences, Huanjiang 547100, China
3
University of Chinese Academy of Sciences, Beijing 100049, China
4
Shenzhen Branch, Guangdong Laboratory of Lingnan Modern Agriculture, Genome Analysis Laboratory of the Ministry of Agriculture and Rural Affairs, Agricultural Genomics Institute at Shenzhen, Chinese Academy of Agricultural Sciences, Shenzhen 518120, China
*
Authors to whom correspondence should be addressed.
Agronomy 2024, 14(2), 308; https://doi.org/10.3390/agronomy14020308
Submission received: 13 November 2023 / Revised: 7 January 2024 / Accepted: 26 January 2024 / Published: 30 January 2024
(This article belongs to the Section Pest and Disease Management)
Browse Figures
Versions Notes

Abstract

:
Macadamia decline poses a serious economic threat to the macadamia industry. It exhibits either a slow decline due to infection by Kretzschmaria clavus or Ganoderma lucidum, or a quick decline caused by pathogens like Phytophthora spp., Lasiodiplodia spp., Neofusiccocum spp., Nectria rugulosa, Xylaria arbuscula, Phellinus gilvus, Acremonium recifei, and Rosellinia spp. Chemical strategies, resistant cultivars, and agronomic measures have been widely adopted to control macadamia decline, but effective biological control measures have rarely been applied. This paper proposes two key steps for implementing biological control strategies, i.e., the isolation and selection of biological control agents from healthy plants, or from the disease-suppressive soil for the construction of synthetic microbial communities, and the integration of synthetic microbial communities with various strategies, including seed coating, root dipping, seedling substrate, soil drenching, foliar spraying, and application as a bio-organic fertilizer. By adopting these strategies, we aim to provide proactive and efficient approaches for combating macadamia decline and safeguarding the health of macadamia orchards.

1. Introduction

Macadamia (Macadamia integrifolia Maiden and Betche) is a valuable nut tree species native to the coastal areas of northern New South Wales and southern Queensland in Australia, with a wide climatic adaptability [1]. Macadamia nuts are famous for their high nutritional value and healthcare benefits by containing abundant unsaturated fatty acids (e.g., oleic acid, arachidonic acid, and palmitoleic acid), protein, amino acids, and various vitamins (e.g., vitamin B1, B2, and nicotinic acid) [2]. As the global demand for macadamia nuts increases, the cultivation of macadamia has expanded into other subtropical and tropical regions, including Australia, China, New Zealand, South Africa, the United States, Brazil, Guatemala, Kenya, Malawi, and Vietnam [3,4], with China owning the largest plantation area of more than 260,000 hectares in 2021 according to the Ministry of Agriculture and Rural Affairs.
However, the escalating cultivation of macadamia is accompanied by the risk of various diseases, particularly macadamia decline. Macadamia decline was first observed on the east of Hawaii Island [5], but has become a major challenge to macadamia production in the major regions such as Queensland, China, South Africa, and Kenya [6]. This disease includes slow decline and quick decline, with the latter being more prevalent. Macadamia decline is caused by multiple pathogens [7,8], leading to several symptoms, including root rot, leaf blackening, branch wilt, and seedling damping-off [8], so it represents a significant threat to macadamia production and profitability. For example, the macadamia decline caused by Phytophthora cinnamomi resulted in a yield reduction of approximately 60% in Kenya [9], and an annual economic loss exceeding AUD 20 million in Australia [10].
Despite many years of efforts in suppressing pathogens through the use of agrochemicals, resistant cultivars, and agronomic measures, the prevention and control of macadamia decline are still large challenges. For instance, while Fosphite® application can alleviate symptoms of macadamia decline, it only extends the productive life of macadamia trees by approximately 700 days [11]. The cultivation of disease-resistant cultivars is both expensive and time-consuming [12], and agronomic measures cannot entirely prevent disease occurrence [13]. Consequently, macadamia decline remains a critical global issue that necessitates the adoption of an environmentally friendly control strategy.
The microbiome plays a key role in plant health and disease [14]. The utilization of beneficial biological control microbes presents a promising alternative to combat soil-borne diseases [15,16]. Numerous biological control agents, including Bacillus, Pseudomonas, Trichoderma, Streptomyces, Flavobacteria, Enterobacter, Actinomycetes, Serratia, Alcaligenes, and Klebsiella strains, function as disease antagonists, rhizosphere colonizers, and plant growth promoters [17,18]. Many of these have been commercially exploited for the control of plant diseases [19]. Synthetic microbial communities (SynComs) have demonstrated greater efficacy than single strains in long-term colonization and functionality within the rhizosphere soil [20,21]. These SynComs can provide antibiotics, secondary metabolites, enzymes, and other compounds with pathogen inhibitory effects [22]. Nevertheless, the role of biological control agents, particularly SynComs, in preventing macadamia decline remains poorly understood [23]. This review aims to (i) synthesize current knowledge on macadamia decline and its control strategies, and (ii) explore the potential application of biological control in decline disease prevention.

2. Macadamia Decline

2.1. Symptoms and Pathogens

Macadamia trees face two distinct forms of decline diseases, i.e., slow decline and quick decline. The slow decline, caused by Kretzschmaria clavus [5] or Ganoderma lucidum [24] is characterized by a progressive onset of symptoms such as leaf discoloration, leaf drop, and branch dieback. These two pathogens induce slightly different symptoms, with K. clavus producing small, mushroom-shaped lesions on the roots and basal trunks of the infected trees, marked by obvious black lines [5], but G. lucidum producing large brown basidiocarps on the lower trunk or above decaying roots [8].
The quick decline is more detrimental to macadamia trees [25], often resulting in swift tree death within a month with the canopy turning to dry brown (Figure 1a,b). This type of decline is caused by various pathogens (Figure 1d), including Phytophthora spp. (e.g., P. tropicalis, P. capsici, P. cinnamomi, P. heveae, P. palmivora, P. multivora) [10,26,27,28,29,30], Nectria rugulosa [31], Xylaria arbuscula [25], Phellinus gilvus [32], Acremonium recifei [29], and Rosellinia spp. [33]. Among these pathogens, Phytophthora is not only responsible for macadamia decline [10], but also causes diseases in other perennial tree crops like avocado, chestnut, apple, plum, mango, and pistachio [18,34]. Although P. tropicalis, P. cinnamomi, P. gilvus, and P. heveae belong to the Phytophthora spp., they exhibit slightly distinct symptoms of decline disease. For instance, both P. cinnamomi and P. tropicalis can induce gum exudation in the infected macadamia trees (Figure 1c) [35]. P. gilvus generates large, dark yellowish-brown basidiocarps on the trunks of infected trees (Figure 1c) [32], a characteristic unlike those produced by other fungal pathogens. N. rugulosa produces small, reddish perithecia on the trunks, accompanied by symptoms of drying bark and grayish wood [25]. X. arbuscula triggers quick decline, evidenced by a shift in canopy color from dark green to brown along with numerous fruiting bodies [36] (Figure 1c). While macadamia decline may be caused by a single pathogen, it is typically caused by two or more different pathogens. For instance, fruiting bodies of A. Recifei, and P. tropicalis were isolated from the same macadamia tree with quick decline symptoms [29].

2.2. Pathogenesis

2.2.1. Infection Sources

Investigations conducted in the forests adjacent to macadamia orchards in Hawaii first revealed the presence of fruiting bodies of K. clavus on both dead and diseased trunks of melochia and trumpet trees (Figure 2a) [37]. Isolates of K. clavus, sourced from these diseased trees within these forests, had the ability to infect macadamia trees [37], suggesting that they could be a significant source of infection for macadamia. Other recognized sources of infection include sporangia and zoospores of P. tropicalis, basidiospores of P. gilvus, stromata of K. clavus, ascospores, marconidia and microconidia of N. rugulosa and X. arbuscula, as well as conidia of A. recifei [5,38,39]. This suggests that the fruiting bodies from diseased macadamia may be the primary pathosystem for the decline disease [40]. When macadamia orchards become infected with these pathogens, diseased tissues and fruiting bodies generate propagules on exposed roots [8]. Moreover, certain pathogens are more likely to attack tree trunks from the base upwards, subsequently spreading to the upper trunk and branches [25,32]. Consequently, the macadamia decline pathogens can be extracted from the roots, rhizosphere, raceme lesions, leaf, and stem [10]. The presence of fruiting bodies on living trees indicates that these fungi may reside inside the trunk for an extended period [8]. When X. arbuscula and P. gilvus fruiting bodies were excised from diseased trees, over 90% of the cross-sectional surface was decayed [25,32]. Fruiting bodies of diseased tissues can be washed away by rainwater and spread over long distances [41]. The pathogens can persist in soils for more than 10 years and have the potential to infect the roots of neighboring healthy trees when macadamia is planted [42].

2.2.2. Internal Damage

Macadamia trees have substantial resilience and can sustain growth to some degree in the absence of conspicuous aboveground symptoms. This resilience is primarily due to several factors. First, the high crystallinity of cellulose in the plant can provide a certain physical barrier to prevent the rapid invasion of pathogens. Second, the high C/N ratios of tree biomass may inhibit the proliferation of pathogens in the heartwood of trees [40]. Despite these natural defenses, the pathogens can still infect macadamia trees, since their roots are short and most proteiod roots are close to the soil surface [43]. This kind of root system is susceptible to infection by pathogens such as X. arbuscula, resulting in approximately 10% of roots becoming rotted after a period of five years [5]. The root system is pivotal in coordinating the tree’s response to various stresses, including preventing pathogen attacks [44]. Therefore, damage to the root system by pathogens would facilitate pathogen proliferation within the tree and potentially impair the functions of xylem and cambium tissues. Xylem, an organ that is responsible for nutrient and water transportation, would be adversely affected when macadamia decline occurs [45]. Consequently, the damaged roots would result in diminished water and mineral nutrient transportation from the soil to the leaves [29], facilitating pathogen spread to the tree trunk and leaves (Figure 2b). When pathogens inflict severe damage on the vascular system, trees may die in a relatively short period due to a limited water supply for their growth and functionality [8]. During the terminal stages of decline, pathogens would infect 90–100% of the bark and 58–97% of the wood [32,35].

2.2.3. External Factors

Environmental factors can significantly influence the emergence and spread of macadamia decline. Temperature is a crucial factor, as it affects the growth and sporulation of pathogens. High temperatures, particularly those exceeding 30 °C, can cause a rapid surge in the number of sporangia of pathogens like P. cinnamomi [46,47] (Figure 2c). The isolates of Phytophthora have an optimal growth temperature of 34 °C, which is higher than the mean annual temperature in tropical regions [48]. Rainfall plays a significant role in the spread of decline disease (Figure 2c). Fruiting bodies, often transported by rainwater from infected trees to the soils around other trees, remain in the rhizosphere until conditions are favorable for their subsequent outbreak [49]. Certain insects can also contribute to the propagation of this disease by carrying and transmitting pathogens [50]. Additionally, long-term, continuous monocropping negatively impacts both the soil environment and the health of macadamia trees [51], thereby increasing their susceptibility to pathogen infection [52]. For the purpose of maintaining or improving soil quality and disease resistance, intercropping systems should be recommended.

3. Hotspots and Frontiers of the Research of Macadamia Decline

A bibliometric analysis was performed using the literature from January 1977 to May 2023 in the Web of Science Core Collection database (SCI-EXPANDED). The analysis focused on the topic “macadamia decline”, using the query “macadamia” AND “decline OR die OR died OR death OR dieback”. A total of 59 papers were recorded, comprising 49 original articles and 5 review articles (Figure 3a). The USA and South Africa contributed 28.13% and 10.94% to the total publications, respectively (Figure 3b). Five of the top ten contributing institutions were from the USA, while the others were located in Australia. The authors with the highest number of contributions from these institutions were Olufemi A. Akinsanmi, Wen-Hsiung Ko, and Olumide S. Jeff-Ego (Figure 3c). The articles were primarily published in Plant Pathology, Plant Disease, Australasian Plant Pathology, and Phytopathology. To enable subsequent statistical analysis, we grouped similar keywords together. For example, Macadamia integrifolia was treated as macadamia, oomycete as oomycetes, and quick decline and dieback as decline. Consequently, macadamia was the most frequent keyword in the keyword analysis, followed by decline and oomycetes (Figure 3d). The disease-related terms included “Kretzscmaria-clavus”, “Acremonium recifei”, “Nectria rugulosa”, “Xylaria arbuscula”, “disease”, “phytoplasma”, and “soil-borne pathogen”. A word cloud was generated from the keywords. It should be noted that in these articles, “Oomycetes” mainly referred to Phytophthora species known to cause diseases in more than 5000 different plant species being infected [53]. The keyword analysis highlighted that macadamia decline is affected by various fungal pathogens, including Phytophthora species, K. clavus, A. recifei, N. rugulosa, and X. arbuscula.

4. Control Strategies of Macadamia Decline

The control strategies of macadamia decline can be categorized into chemical intervention, the use of resistant cultivars, agronomic measures, and biological control measures (Figure 4). Nevertheless, integrated control strategies that utilize two or more of these measures are becoming more and more popular.

4.1. Chemical Strategies

Fungicides are extensively utilized to control the pathogens of macadamia decline [54], with a particular focus on Phytophthora species (Figure 4a). Different fungicides have been registered and used worldwide, such as carboxylic acid amide fungicides (dimethomorph, flumorph, pyrimorph, and mandipropamid) and benzamide fungicides (fluopicolide and propamocarb) [55]. However, concerns have been raised regarding fungicide resistance; i.e., certain strains of Phytophthora have been found to be resistant to commonly used fungicides, e.g., metalaxyl [56]. To avoid fungicide resistance, the simultaneous application of a combination of fungicides has been proven to be effective. For example, the mixture of Melody Duo (iprovalicarb 55 g kg−1, propineb 613 g kg−1), Nordox 75 WP (copper oxide 86% w/w, 75% metallic copper 14% w/w), and Victory 72 WP (metalaxyl 80 g kg−1, Mancozeb 640 g kg−1) is effective in controlling Phytophthora diseases [57]. The fungicide phosphite (comprising 53% monopotassium phosphate and dipotassium phosphate) can efficiently suppress quick decline by decreasing lesion size by 70%, extending the lifespan of infected trees by 700 days [11]. However, relying solely on chemical strategies may only mitigate, rather than eradicate, macadamia decline [58]. Furthermore, an excessive or prolonged application of fungicides can be detrimental to plants [9], resulting in phytotoxicity and other adverse effects [59], or leading to drug resistance in pathogens, soil degradation, and environmental pollution.

4.2. Resistant Cultivars

Plant breeding strategies, aiming to enhance belowground traits that positively influence the rhizosphere microbiome, present a promising avenue for sustainable crop production [60]. The severity of macadamia decline may be partly attributed to genetic factors [27]. Although identifying and utilizing disease-resistant cultivars are challenging, it is rewarding [61]. In addition to breeding selection focusing on yield enhancement, breeding more tolerant macadamia cultivars could reduce the incidence and severity of decline disease (Figure 4b) [11]. Wild germplasms such as Macadamia integrifolia and M. tetraphylla are commonly used as resistance sources in macadamia breeding programs, demonstrating resistance to both P. cinnamomi and P. multivora in an in vivo assay [62]. Research has also indicated that commercial cultivars of M. integrifolia exhibit resistance to P. cinnamomi [6]. Among five macadamia cultivars (namely ”HAES 816”, “A16”, “HAES 246”, “HAES 344”, and “HAES 741”), “HAES 344” was found to have the highest resistance to P. cinnamomi [62,63,64]. Despite the advantages, few resistant cultivars have been applied, since the process of breeding new cultivars typically requires 8–10 years using conventional breeding approaches [65]. Disease-resistant rootstocks have also been extensively employed in commercial orchards as a disease management strategy (Figure 4b). The preferred rootstocks include M. integrifolia cultivar H2 (Hinde), M. integrifolia, and M. tetraphylla hybrid cultivar D4 (Renown) [1]. However, the selection of rootstocks often prioritizes rapid germination, a high grafting success rate, and a robust seedling vigor over stress resistance [66,67].

4.3. Agronomic Measures

Good orchard hygiene helps reduce the spread of decline disease by minimizing pathogen infection (Figure 4c). Several key practices are recommended for pathogen suppression, including the removal of dead or dying limbs from the crown, the modification of canopy coverage in accordance with the severity of macadamia decline symptoms, and the installation of shade nets [68]. Furthermore, temperature and humidity managements within the orchard are vital as they significantly affect the propagation of pathogens. P. cinnamomi infection is more likely in soils with poor drainage, areas with lower elevations, or during periods of heavy rainfall [28]. Hence, selecting suitable planting locations or adjusting the microenvironment can mitigate macadamia decline. Organic fertilization emerges as a promising control strategy for controlling soil-borne diseases [69]. The application of composts or animal manure can enhance soil fertility and inhibit the proliferation of pathogens such as P. cinnamomi [70]. Compared to chemical fertilizers, the long-term application of organic fertilizers like cow manure and green manure can elevate microbial abundance and enhance soil enzyme activity [71,72]. The role of soil microflora is pivotal in maintaining soil health and suppressing disease [73]. The rhizosphere microbiome, in conjunction with its interaction with plant roots, exerts a significant impact on overall health [74].

4.4. Biological Control Measures

Biological control strategies, utilizing microbial antagonists (bacteria and fungi) [17] or beneficial insects [75], has received tremendous attention as a safe and potentially efficacious approach against soil-borne pathogens (Figure 4d). Certain beneficial microbes, e.g., Trichoderma spp., have been shown to enhance the resistance of macadamia to decline diseases. For example, T. hamatum was employed as a biocontrol agent to shield macadamia from infection by Lasiodiplodia theobromae, a pathogen responsible for kernel rot, branch dieback, and macadamia decline [7]. The application of a T. hamatum conidial suspension significantly reduced the size of lesions caused by L. theobromae on macadamia leaves [76]. In another study, Trichoderma spp. isolated from the macadamia orchard effectively mitigated infection by Rosellinia spp., a pathogen associated with macadamia decline [33]. Native isolates of Trichoderma spp. exhibited potential as biological control agents against Rosellinia spp. [33]. Xyleborus beetles, particularly X. ferrugineus, X. affinis, and X. perforans, may exacerbate macadamia decline by being attracted by ethanol produced by stressed trees [50,77]. Phymastichus LaSalle species, including P. xylebori LaSalle, P. coffea LaSalle, and P. holohol, were identified as biological control agents against Xyleborus beetles [75,78].
Research on the biological control of macadamia decline is currently very limited. Strains of Trichoderma, Pseudomonas, Bacillus, and Actinomycetes have been identified as effective biological control agents against diseases caused by Phytophthora [79], which is the primary pathogen contributing to macadamia decline. Among them, Trichoderma strains have been extensively studied. For instance, T. harzianum strains effectively reduced pear collar rot by 97% [80]. Serratia plymuthica and its siderophore molecule (serratiochelin C) showed inhibitory effects on P. cinnamomi [81]. Furthermore, Bacillus amyloliquefaciens, Burkholderia metallica, Burkholderia cepacian, and Pseudomonas aeruginosa were found to inhibit P. capsici sporangium formation and zoosporogenesis, thereby enhancing seed germination and plant growth [82,83]. Since few studies have been reported regarding the applications of antagonistic microbes in combating the pathogens of macadamia decline, research is urgently needed.

4.5. Control with Multiple Measures

Besides the implementation of individual control measures, there is an escalating focus on the simultaneous use of multiple strategies for mitigating macadamia decline. In China, a combination of Trichoderma harzianum, humic acid, and urea was found to be effective in preventing and controlling macadamia decline [68]. The management procedure includes the removal of dry branches and leaves based on the severity of decline symptoms, the installation of shade nets, and the irrigation of roots with a recovery solution. For long-term control, late topdressing was implemented by applying 5 kg of organic fertilizer per plant in ring channels 60–80 cm around the stem. Meanwhile, foliar fertilization was conducted to foster the recovery of macadamia trees and accelerate the growth of new leaves. This integrated approach resulted in the recovery of over 96% of the infected macadamia trees [68]. This highlights the potential effectiveness of integrated management strategies in combating macadamia decline and ensuring the sustainable health of macadamia orchards.

5. Macadamia Decline Prevention: SynComs Application via Multiple Approaches

5.1. Identification of Antagonistic Microbes and Construction of SynComs

The primary objective is to identify the beneficial microbes with antagonistic effects on decline pathogens (Figure 5). The disease-suppressive soils present an optimal resource for screening and isolating potential biological control agents [80]. Core microbiomes, composed of antagonistic microbes, are instrumental in inhibiting disease incidence and fostering plant growth [84]. Nevertheless, the inoculation with individual beneficial microbes for controlling soil-borne pathogens suffers from ineffective colonization in the rhizosphere and inconsistent field efficiency [85]. Given that decline diseases in a macadamia orchard are usually attributed to multiple pathogens, the application of a combination of beneficial microbes, i.e., SynComs, should be recommended, by mixing several strains with different functions, including growth promotion, disease suppression, and high temperature and humidity tolerance (Figure 5). High-throughput and genomic sequencing techniques have been widely used to identify the isolated strains and ascertain their taxonomic status, functional characteristics, and abundance.

5.2. Macadamia Decline Prevention Strategies Using SynComs

Several studies have demonstrated that the effectiveness of biological control can be influenced by different inoculation methods [86,87]. To improve the survival rate of SynComs and enhance the management of macadamia decline, a combination of SynComs with various strategies is recommended, including seed coating, root dipping, seedling substrates, soil drenching, foliar spraying, and bio-organic fertilizer application (Figure 5).

5.2.1. Seed Coating and Root Dipping with SynComs

The seed coating delivery of biocontrol inocula is recognized as a cost-effective and efficient technology for safeguarding crops against both seed-borne and soil-borne phytopathogens [88,89]. Recent attempts have integrated biocontrol agents such as Pseudomonas fluorescens [90], Bacillus subtilis [91,92], Yersinia spp. [93], Serratia entomophila [94], Paenibacillus alvei, nonpathogenic Fusarium oxysporum [95], and Rhizobium radiobacter [96] into seed coating. These studies have demonstrated that seed coating is efficient in protecting plants against soil-borne fungal pathogens, thereby fostering plant growth. It is imperative to sterilize and coat macadamia seeds with SynComs to mitigate biological disturbances or invasion, given that the transmission of pathogens by seeds is the first step of disease occurrence [97]. Additionally, the practice of dipping seedling roots in a suspension with beneficial microbial strains is an effective strategy for improving plant resistance to pathogens [86]. For example, immersing angelica seedling roots in a suspension of Enterobacter cloacae and Serratia ficaria effectively inhibited the growth of Phytophthora cactorum, significantly suppressing the incidence of Phytophthora root rot in a pot experiment or when the treated seedlings were planted in naturally infested soil [98]. Therefore, it is worth investigating the effects of seed coating and root dipping with SynComs on macadamia decline prevention or control.

5.2.2. Seedling Substrate and Soil Drenching

The quality of germination substrates can significantly influence the growth and survival of plant seedlings [99]. A substrate mixed with beneficial microbes benefits from the successful transplantation of seedlings into the field, and helps establish a robust biological barrier during their initial stages [100]. For example, substrates inoculated with lysine, sucrose, and anaerobic digestion slurry, improved the ability of tomato to resist bacterial wilt [101]. Similarly, incorporating suitable SynComs into the substrate offers a novel approach to promote the growth and disease resistance of macadamia seedlings. Moreover, soil drench methods, which involve the application of suspensions around the root zone of plants, have been proven effective in improving plant growth [100,102]. A recent study revealed that the addition of T. atroviride conidial suspensions suppressed Fusarium wilt disease of tomato seedlings [103]. However, the colonization rate of single strains using this method is relatively low [86]. Hence, SynComs are essential to enhance the ability of colonization for nutrient competition, niche occupation, and the induction of systemic resistance.

5.2.3. Foliar Spraying and Bio-Organic Fertilizer Application

Biological control agents are capable of suppressing pathogen infection during seedling germination and early growth stages prior to transplantation into the field. However, the pathogens can persist in the rhizosphere soil for long periods. To improve plant resistance for growth, it is imperative to implement biological control strategies at different growth stages [100]. Foliar spraying with an antagonistic microbial suspension is a widely used alternative method in the field to inhibit plant diseases and promote growth [104]. The application of bio-organic fertilizer may be a crucial strategy for SynComs utilization. Compared to seed coating, root dipping, and soil drenching, foliar spraying and bio-organic fertilizers may be superior in disease control [100].

5.3. Mechanisms Underlying the Inhibitory Effects of SynComs on Macadamia Decline

The assembly of plant-associated rhizosphere microbiomes is highly complex due to their inherent heterogeneity [105]. Advanced multi-omics technologies, including metagenomics, transcriptomics, proteomics, and metabolomics, have been utilized to elucidate the function of the microbiome in the rhizosphere [106,107,108], and to explore plant–microbe and microbe–microbe interactions under SynComs inoculation. A comprehensive understanding of the synthetic microbiome’s genome characteristics through metagenomics can pinpoint the antagonistic genes against specific pathogens [109]. Transcriptomics serves as the most effective method for unveiling alterations in gene expression when plants interact with SynComs [110], thereby identifying the genes of plants responding to SynComs and inferring the metabolic pathways and biological processes involved. A proteomics approach allows for the identification of proteins associated with the biocontrol processes and differential expression [110]. Metabolomic analyses have the potential to reveal perturbations in signaling or output pathways that significantly influence the outcome of a plant–microbe interaction [111]. The combination of multiple omics analysis methods is helpful to elucidate information pertaining to the pathogenicity of plant pathogens, enhancing the efficacy of plant disease diagnosis and management through the inoculation of SynComs. The integration of metabolomics and transcriptomics has unveiled that the assembly of rhizosphere microbiomes is responsible for the systemic induction of root exudation of metabolites at both molecular and chemical levels [112]. Root exudates are pivotal in plant–soil–microbe interactions [113], offering significant insights into the mechanism by which SynComs regulate rhizosphere microbiota to control soil-borne diseases. Therefore, multi-omics technologies may provide new insights into the mechanisms underlying the inhibitory effects of SynComs on macadamia decline.

6. Conclusions

This review provides a comprehensive overview of macadamia decline and its associated control measures. Given that limited information is available regarding the biological control of macadamia decline, we largely explored the potential of biological control in managing macadamia decline. Our proposed approach involves the use of SynComs, a promising biological control method, in conjunction with various measures such as seed coating, root dipping, seedling substrate, soil drenching, foliar spraying, and bio-organic fertilizer application to effectively manage macadamia decline.

Author Contributions

Conceptualization, D.L.; methodology, X.Y.; writing—original draft preparation, X.Y.; writing—review and editing, D.L. and Y.L.; visualization, X.Y.; funding acquisition, D.L., Y.L., and Q.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Key Research and Development Program of China, grant number 2022YFF1300704; Guangxi Bagui Scholarship Program, no grant number; National Natural Science Foundation of China, grant number 42201068; and Agricultural Science and Technology Innovation Program, grant number CAAS-ZDRW202308.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Symptoms and pathogens of macadamia decline disease: (a) Diagram of slow decline and quick decline. (b) Typical brown leaves and roots of quick decline. (c) Infected sites and their characteristics of quick decline. (d) Timeline of the major decline pathogen studies over the past half-century.
Figure 1. Symptoms and pathogens of macadamia decline disease: (a) Diagram of slow decline and quick decline. (b) Typical brown leaves and roots of quick decline. (c) Infected sites and their characteristics of quick decline. (d) Timeline of the major decline pathogen studies over the past half-century.
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Figure 2. Bottom-up infection caused by macadamia decline pathogens. (a) Infection by the original pathogens: pathogens are derived from nearby forest plants or soil-borne pathogen carriers, including diseased branches, leaves, and roots. Some pathogens infiltrate into plants via root system, leading to an outbreak of pathogens in the rhizosphere soil. (b) Internal damage to macadamia by pathogens: pathological tissues carrying some pathogens infect neighboring macadamia trees, causing root damage. Additionally, stems and leaves become infested, resulting in plant disease or death. (c) Environmental factors accelerate decline progression: high temperatures can accelerate the spread of pathogenic spores. Rainwater can carry plant remnants carrying pathogenic spores to new orchards and cause new decline outbreak. Another way by which the decline disease can be spread is though insects such as beetles.
Figure 2. Bottom-up infection caused by macadamia decline pathogens. (a) Infection by the original pathogens: pathogens are derived from nearby forest plants or soil-borne pathogen carriers, including diseased branches, leaves, and roots. Some pathogens infiltrate into plants via root system, leading to an outbreak of pathogens in the rhizosphere soil. (b) Internal damage to macadamia by pathogens: pathological tissues carrying some pathogens infect neighboring macadamia trees, causing root damage. Additionally, stems and leaves become infested, resulting in plant disease or death. (c) Environmental factors accelerate decline progression: high temperatures can accelerate the spread of pathogenic spores. Rainwater can carry plant remnants carrying pathogenic spores to new orchards and cause new decline outbreak. Another way by which the decline disease can be spread is though insects such as beetles.
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Figure 3. Bibliometric analyses of the macadamia decline research based on data from the Web of Science Core Collection database, spanning the period from January 1977 to May 2023: (a) Bibliometric quantity of articles. (b) The top ten countries with the most publications related to macadamia decline with the number of publications presented. (c) Network analysis of citation interaction. (d) Word cloud generated from keywords of articles. The size of the circles represents word frequency.
Figure 3. Bibliometric analyses of the macadamia decline research based on data from the Web of Science Core Collection database, spanning the period from January 1977 to May 2023: (a) Bibliometric quantity of articles. (b) The top ten countries with the most publications related to macadamia decline with the number of publications presented. (c) Network analysis of citation interaction. (d) Word cloud generated from keywords of articles. The size of the circles represents word frequency.
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Figure 4. Control strategies of macadamia decline: (a) chemical strategies, (b) resistant cultivars, (c) agronomic measure, and (d) biocontrol measure.
Figure 4. Control strategies of macadamia decline: (a) chemical strategies, (b) resistant cultivars, (c) agronomic measure, and (d) biocontrol measure.
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Figure 5. Biocontrol application in mitigating macadamia decline: The initial step involves the isolation, screening, and identification of decline pathogens and antagonists. The subsequent step entails the construction of SynComs and their integration with various strategies such as seed coating, root dipping, seedling substrates, soil drenching, foliar spraying, and bio-organic fertilizer application to augment the colonization of SynComs in the rhizosphere soil. Ultimately, SynComs can serve as an alternative to plant protection by inducing systemic resistance and facilitating the release of root secretions including organic acids, defense enzymes, volatile organic compounds, and secondary metabolites. SynComs: Synthetic microbial communities.
Figure 5. Biocontrol application in mitigating macadamia decline: The initial step involves the isolation, screening, and identification of decline pathogens and antagonists. The subsequent step entails the construction of SynComs and their integration with various strategies such as seed coating, root dipping, seedling substrates, soil drenching, foliar spraying, and bio-organic fertilizer application to augment the colonization of SynComs in the rhizosphere soil. Ultimately, SynComs can serve as an alternative to plant protection by inducing systemic resistance and facilitating the release of root secretions including organic acids, defense enzymes, volatile organic compounds, and secondary metabolites. SynComs: Synthetic microbial communities.
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Yao, X.; Liu, Q.; Liu, Y.; Li, D. Managing Macadamia Decline: A Review and Proposed Biological Control Strategies. Agronomy 2024, 14, 308. https://doi.org/10.3390/agronomy14020308

AMA Style

Yao X, Liu Q, Liu Y, Li D. Managing Macadamia Decline: A Review and Proposed Biological Control Strategies. Agronomy. 2024; 14(2):308. https://doi.org/10.3390/agronomy14020308

Chicago/Turabian Style

Yao, Xiaofang, Qiumei Liu, Yongxin Liu, and Dejun Li. 2024. "Managing Macadamia Decline: A Review and Proposed Biological Control Strategies" Agronomy 14, no. 2: 308. https://doi.org/10.3390/agronomy14020308

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