Microbial Antagonists for the Control of Plant Diseases in Solanaceae Crops: Current Status, Challenges, and Global Perspectives

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Please find the attachment with minor editing corrections.
The manuscript "Microbial antagonists for control of plant diseases in Solanaceae crops: Current status, challenges, and global perspectives" has an interesting and promising title, and...  misleading. It is a general manuscript on mostly bacterial genera used for the biocontrol of various crop diseases, not only Solanaceae. Including a literature search and a list of the most common bacterial and fungal biocontrol agents used in Solanaceae crops is not enough. The main text lacks information about the primary Solanaceae diseases. It is also difficult to tell whether authors write about Solanaceae biocontrol in South Africa because of a literature search or if they mean global biocontrol and food security of crops because of the "Mode of action of antagonistic microbes for the management of plant diseases" chapter. For example, the chapter "Biodiversity of antagonistic microbes" describes a few genera of BCAs that are mostly not in the context of Solanaceae diseases but general crop diseases. And to be true, a whole manuscript is like that.
The authors should focus strictly on the biocontrol of Solanaceae diseases in South Africa or globally. However, it would be more interesting for readers to focus on this topic in South Africa and only comment on the broad perspective. The information in the manuscript (main text, not figures or tables) should encompass Solanaceae diseases, how BCAs can help to fight them in the biological context, how BCAs are applied, and so on, but in Solanaceae plants. Chapters "Difficulties encountered during commercialization Biocontrol agents" and "Future Directions and Challenges" should not be general but focus on Solanaceae plants.

Comments for author File: Comments.pdf

Author Response

1. The manuscript "Microbial antagonists for control of plant diseases in Solanaceaecrops: Current status, challenges, and global perspectives" has an interesting and promising title, and...  misleading.

 The contents of the manuscript fit the title description. Other crops which do not belong to the family Solanaceae were removed.

2. It is a general manuscript on mostly bacterial genera used for the biocontrol of various crop diseases, not only Solanaceae.

Any crop that does not belong to the Solanaceae were removed from the manuscript

Table 1. Only biocontrol agents used in Solanaceae were listed on the table.

 

3. Including a literature search and a list of the most common bacterial and fungal biocontrol agents used in Solanaceaecrops is not enough.

 Azospirillum, Azotobacter, Klebsiella, and Enterobacter were added.

Enterobacter have been well documented as an effective BCA against Ralstonia solanacearum (Smith) [64], the bacterium that causes bacterial wilt in potato and eggplant. Enterobacter cloacae PS14 reduced the potato wilt disease severity by 25%, while increasing the yield by 40% through the induction of plant systemic resistance and by supressing the pathogen modes of action [65]. The induction of systemic resistance can be confirmed by observing an increase in the total phenol and salicylic acid contents [66]. In tomato, E. cloacae TR1 suppresses pathogen growth by producing antifungal volatile organic compounds with 3-Methylbutan-1 against B. cinerea [67].

 

4. The main text lacks information about the primary Solanaceaediseases. It is also difficult to tell whether authors write about Solanaceae biocontrol in South Africa because of a literature search or if they mean global biocontrol and food security of crops because of the "Mode of action of antagonistic microbes for the management of plant diseases" chapter. For example, the chapter "Biodiversity of antagonistic microbes" describes a few genera of BCAs that are mostly not in the context of Solanaceae diseases but general crop diseases. And to be true, a whole manuscript is like that.
   The authors should focus strictly on the biocontrol of Solanaceae diseases in South Africa or globally. However, it would be more interesting for readers to focus on this topic in South Africa and only comment on the broad perspective. The information in the manuscript (main text, not figures or tables) should encompass Solanaceae diseases, how BCAs can help to fight them in the biological context, how BCAs are applied, and so on, but in Solanaceae plants.

The manuscript mainly focuses on BCA globally because most of the commercially available BCAs are outside South Africa. South Africa was discussed because most of the biocontrol studies in Africa occurred in SA, this was supported by bibliometric analysis

 

Lines 221-272; Some of diseases that affect solanaceae were summarized below

 

3. Solanaceaediseases

Bacterial soft rot is a postharvest disease which poses a serious threat to potato production worldwide [85]. Infected tubers become soft, watery, followed by foul odour smell, which is caused by two genera, Pectobacterium and Dickeya. Pectobacterium was previously known as Erwinia [86]. Contamination of potato tubers occurs during harvest, handling and washing. Pathogen remains dormant within the plant until favourable environmental conditions like humidity, causes a shift from latency to disease development. In South Africa, the disease has contributed to yield losses and was caused by climate change and P. brasiliense [87]. Pectobacterium brasiliense prevail in temperatures between 20 °C and 38 °C, during the potato growing seasons in South Africa. The disease can be managed by spaying chemicals with copper on the wounded area to reduce the spread of bacteria to healthy plants. BCAs which have an induced systemic resistance mechanism as their during mode of action can induce plant systemic resistance against soft rot by activating the salicylic acid-dependent pathway [88].

 

 

Rhizoctonia disease also known as a black scurf is a fungal disease caused by Rhizoctonia solani which affects potato tubers. Although is not destructive, it can reduce the value of potatoes because it makes potatoes to be appear hard and have black patches on the surface [89]. The disease can be managed using fungicide, resistant cultivars, crop rotation and avoid planting deeply [90]. Infection can occur at any time during the growing season throughout the world including South Africa. Rhizoctonia disease was first recorded in South Africa in 1918 and currently all 10 anastomosis groupings (AG) of Rhizoctonia solani associated with black scurf have been isolated in South Africa [91]. It was only recently in 2023 that a destructive black scurf disease was recorded in Lesotho, the neighbouring country of South Africa [92]. Although most of the Trichoderma species have not been approved as BCA, in vitro studies have shown that Trichoderma species (T. viride, T. herzianum, T. helicum, T. asperellum, and T. hamatum) suppress the growth of Rhizoctonia solani by 70% [93] through different modes of action including competition for the substrate, antibiosis and mycoparasitism [94]. Trichoderma harzianum and T. viride are registered as active BCAs.

 

 

Ralstonia bacterial wilt caused by Ralstonia solanacearum and Ralstonia pseudosolanacearum, is a devastating disease affecting tomato production worldwide [95]. Ralstonia solanacearum is a highly heterogeneous bacterial which can metabolize sugar alcohols and disaccharides [96]. The bacterium enters the roots through wounds made by insects, transplanting, and cultivation. Endophytic Bacillus species have been previously used in in vitro studies to control tomato bacterial wilt by foliar spray application. Less pathogens were detected on plants treated with BCA, implying that the induction of jasmonic acid, salicylic acid and ethylene-dependent defenses was involved in the protective effects as observed using real-time Polymerase chain reaction (PCR) method [97]. Pseudomonas fluorescens and P. aeruginosa can control bacterial wilt in tomato by reducing disease severity by more than 50% [98]. Evidence showed that Bacillus and Pseudomonas species as BCA is promising in eradicating for bacterial wilt.

 

Botrytis also known as gray mold diseases, is caused by Ascomycota Botrytis cinerea, necrotrophic pathogenic fungus which causes substantial losses in chili peppers, tomatoes and potatoes worldwide [99]. The disease shows various symptoms ranging from soft rotting of plant parts to brown lesions and not forgetting the production fuzzy gray-brown mold [100]. Botrytis cinerea can be controlled by colonizing the pathogen with Beauveria bassiana which is an antagonism that have a high antifungal effect [101].

 

5. Chapters "Difficulties encountered during commercialization Biocontrol agents" and "Future Directions and Challenges" should not be general but focus on Solanaceaeplants.

Line 391- Challenges which include BCAs might not be effective as the well-established pesticides, environmental factors such as temperature can be a factor because some of the bacterial strains function better at optimal temperatures, and the cost of manufacturing might be higher than profitability, making them not accessible to most of the farmers.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The review discusses the emerging trends and investigates the microbial antagonists as biocontrol agents for plant diseases affecting the Solanaceae family, the mechanism of action, and future applications in plant disease management.

In the present era, bio-based agents are gaining recognition for pest/pathogen control compared to agrochemicals to minimize adverse impact on the environment, consumer health, and to prevent the spread of pesticide resistance.  Although microbial biocontrol agents (BCA) demonstrate prospects to control plant pathogens, challenges exist with the optimization of microbial efficacies and safety assessment before large-scale field applications.

For the improvement of the manuscript, some suggestions are provided.

Considering the promising scope of the research on BCAs, how does the present article improve our knowledge in this direction? What are the future implications of the literature review? Discuss.

Keywords: line 30: Postharvest diseases 1, microbial antagonist 2, sustainable, management 3. What do the numbers 1, 2, and 3 indicate?

What is the rationale for focusing only on the plants of the family Solanaceae? Are there any microbial formulations commercialized for applications? Please mention in the conclusion section.

Discussing the microbial preparations as BCAs, it is important to discuss the current biotechnological advances in this direction. A summarized table on commercial products and their microbial component would add value to the manuscript.

Diagrammatic representations are well-organized and presented, providing key information on the bibliometric aspect, literature survey, and action mechanisms of the microbes. Just one point, the alignment of the figures should be in line with the text. Arrange accordingly.

 

Figure 2. Key mechanisms of action involved in the biological control of potato late blight disease by microbial antagonists. In the figure, instead of the compound ID, it would be better to mention the names of the compounds 1,2,3,4,5,6 (either in the figure or in the caption).

Moreover, the tables, figures, and text need to be realigned as per the MDPI format for clarity and consistency.

Abstract: The plant names should be in scientific form at the first mention, and then abbreviated names can be used. Likewise, common names are in the brackets.

Please be consistent in using the abbreviations. The complete name should be mentioned at first, followed by the abbreviation, e.g., line 26, biocontrol agents should be BCA, and likewise in the manuscript.

Another point is to maintain consistency in mentioning biological control agents or biocontrol agents in all sections.

Substantial English revisions are necessary to improve the readability and clarity of the article.

References should be revised as per MDPI format.

Comments on the Quality of English Language

Substantial English revisions are required in the manuscript.

Author Response

1. Considering the promising scope of the research on BCAs, how does the present article improve our knowledge in this direction? What are the future implications of the literature review? Discuss.

The future implications of BCA were discussed in Section 6 highlights several challenges that need to be addressed to fully utilize most of BCAs to be produced. Large scale production is still a big factor and the compatibility of BCAs with the environment because their mode of action is not always known.

Line 391-398 Challenges which include BCAs might not be effective as the well-established pesticides, environmental factors such as temperature can be a factor because some of the bacterial strains function better at optimal temperatures, and the cost of manufacturing might be higher than profitability, making them not accessible to most of the farmers. Most developing countries continue to produce and use BCAs without registration, to avoid the high registration costs. While some companies register BCAs as biofertilizer not as BCAs, for general use in all agricultural crops.

 

However, the current review gives hope that by addressing most of these challenges, the potential BCAs might contribute to the sustainability and effectiveness of disease management, while promoting a more environmentally friendly agricultural system

Section 6. Future Directions and Challenges

Utilizing microbial antagonists to control postharvest diseases is a viable strategy for reducing the need for agrochemicals and mitigating the detrimental effects of phytopathogens on crop productivity. However, there are still several challenges that need to be addressed to fully utilize these BCAs effectively and sustainably.

 

Since BCAs impacts the natural environment, one must preferably predict any possible risks that could result from its implementation because their mechanism of action on how they suppress the pathogens is not always known. Therefore, a comprehensive analysis of the advantages and hazards should be conducted to give stakeholders the knowledge they need for effective, secure, and long-term pest control and production and to eliminate the fear of the unknown. Knowing the molecular mechanisms behind these interactions is a crucial part of understanding how the plant host reacts to this interaction as well as how phytopathogens react to BCA or vice versa. Furthermore, the discernment by which plants distinguish between beneficial and harmful bacteria is still unknown. However, it has been proposed that plants can adjust their microbiome by releasing certain nutrients to increase competitiveness. Nevertheless, with all the proper isolation, screening, and field trials to scale up the potential microbes from the lab to the agriculture field, there is always a delay in the regulatory approval, registration process and marketing of products. Although, diverse effective BCAs are available commercially.

Line 410- By addressing most of these challenges, the potential BCAs might contribute to the sustainability and effectiveness of disease management, while promoting a more environmentally friendly agricultural system

Line 425- Solanaceae crops such as Solanum tuberosum, Solanum lycopersicum, Capsicum annuum and Solanum melongena are globally important and are known to be susceptible to a wide range of bacterial and fungal pathogens, resulting in substantial economic losses. This susceptibility makes them an ideal target for researcher to develop BCA to reduce reliance of fungicides and pesticides to fight pathogens to ensure food security and economic stability. Although microbial formulations for commercialized BCAs exists such as liquid and solid formulations, a major bridge between fermentation and field application still exists because of factors such as production in large scale, stabilizing the BCA during shelf life, handling and use of the product and ensuring the viability efficacy at the target site.

 

2. Keywords: line 30: Postharvest diseases 1, microbial antagonist 2, sustainable, management 3. What do the numbers 1, 2, and 3 indicate?

Line 30: Numbers were removed

3. What is the rationale for focusing only on the plants of the family Solanaceae? Are there any microbial formulations commercialized for applications? Please mention in the conclusion section.

428-437-Solanaceae family is the third largest family which comprises of crops such as Solanum tuberosum, Solanum lycopersicum, Capsicum annuum and Solanum melongena that are economically important and are known to be susceptible to a wide range of bacterial and fungal pathogens, resulting in substantial economic losses. This susceptibility makes them an ideal target for researcher to develop BCA to reduce reliance of fungicides and pesticides to fight pathogens to ensure food security and economic stability. Although microbial formulations for commercialized BCAs exists such as liquid and solid formulations, a major bridge between fermentation and field application still exists because of factors such as production in large scale, stabilizing the BCA during shelf life, handling and use of the product and ensuring the viability efficacy at the target site.

4. Discussing the microbial preparations as BCAs, it is important to discuss the current biotechnological advances in this direction. A summarized table on commercial products and their microbial component would add value to the manuscript.

 

Table 2. List of approved Biological Control Agents and their target pathogens.

BCA	Commercial Name	Target Pathogens/ disease
Bacillus amyloliquefaciens strain MBI 600	SERIFEL®	Botrytis cinerea, Sclerotinia spp
Bacillus amyloliquefaciens strain FZB24	TAEAGRO®	Powdery mildews diseases, Botrytis sp.
Bacillus amyloliquefaciens subsp. plantarum strain D747	AMYLO-X®	Botrytis cinerea, Monilinia spp., Sclerotinia spp.
Bacillus pumilus strain QST 2808	SONATA®	Powdery mildews diseases
Pseudomonas chlororaphis strain MA342	PRORADIX®	Rhizoctonia spp., Helmintosporium solani, Fusarium spp.
Streptomyces lydicus strain WYEC 108	ACTINOVATE® AG	Rhizoctonia spp., Verticillium spp, Phytophthora spp., Fusarium spp., Alternaria spp., Botrytis spp
Pseudomonas chlororaphis strain 63-28	AtEze®	Pythium spp., Rhizoctonia solani, and Fusarium oxysporum

 

5. Diagrammatic representations are well-organized and presented, providing key information on the bibliometric aspect, literature survey, and action mechanisms of the microbes. Just one point, the alignment of the figures should be in line with the text. Arrange accordingly.

Figures were properly arranged in the text 

6. Figure 2. Key mechanisms of action involved in the biological control of potato late blight disease by microbial antagonists. In the figure, instead of the compound ID, it would be better to mention the names of the compounds 1,2,3,4,5,6 (either in the figure or in the caption).

Compound ID represents toxic metabolites; 1-atropine, 2-hyoscyamine, 3-scopolamine, 4-pyrrolic alkaloids, and 5-pyrrodine.

7. Moreover, the tables, figures, and text need to be realigned as per the MDPI format for clarity and consistency.

Figures and tables were realigned as per MDPI format

8. Abstract: The plant names should be in scientific form at the first mention, and then abbreviated names can be used. Likewise, common names are in the brackets.

Line 9-10. Plant names were added; potato (Solanum tuberosum), tomato (Solanum lycopersicum), bell peppers (Capsicum annuum).

Line 2- Solanaceae was italized throughout the manuscript including the title.

9. Please be consistent in using the abbreviations. The complete name should be mentioned at first, followed by the abbreviation, e.g., line 26, biocontrol agents should be BCA, and likewise in the manuscript.

BCA abbreviation was used and for plural BCAs was used.

10. Another point is to maintain consistency in mentioning biological control agents or biocontrol agents in all sections.

Biocontrol agents was retained throughout the

11. Substantial English revisions are necessary to improve the readability and clarity of the article.

The Manuscript will be submitted for language editing at MDPI.

12. References should be revised as per MDPI format.

References were revised as per MDPI format.

13. Comments on the Quality of English Language. Substantial English revisions are required in the manuscript.

The Manuscript will be submitted for language editing at MDPI.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript has been corrected according to the reviewer`s suggestions and comments.

Reviewer 2 Report

Comments and Suggestions for Authors

I recommend the publication of the manuscript in the current form.