*Article* **Exploring the Potentiality of Native Actinobacteria to Combat the Chilli Fruit Rot Pathogens under Post-Harvest Pathosystem**

**Rajamuthu Renuka 1,2,\*, Kupusamy Prabakar 2, Rangasamy Anandham 3, Lakshmanan Pugalendhi 4, Lingam Rajendran 2, Thiruvengadam Raguchander <sup>2</sup> and Gandhi Karthikeyan <sup>2</sup>**


**Abstract:** Chilli is an universal spice cum solanaceous vegetable crop rich in vitamin A, vitamin C, capsaicin and capsanthin. Its cultivation is highly threatened by fruit rot disease which cause yield loss as high as 80–100% under congenial environment conditions. Currently actinobacteria are considered as eco-friendly alternatives to synthetic fungicides at pre and post-harvest pathosystems. Hence, this research work focuses on the exploitation of rhizospheric, phyllospheric and endophytic actinobacteria associated with chilli plants for their antagonistic activity against fruit rot pathogens *viz*., *Colletotrichum scovillei*, *Colletotrichum truncatum* and *Fusarium oxysporum*. In vitro bioassays revealed that the actinobacterial isolate AR26 was found to be the most potent antagonist with multifarious biocontrol mechanisms such as production of volatile, non-volatile, thermostable compounds, siderophores, extracellular lytic enzymes. 16S rRNA gene sequence confirmed that the isolate AR26 belongs to *Streptomyces tuirus*. The results of detached fruit assay revealed that application of liquid bio-formulation of *Stretomyces tuirus* @ 10 mL/L concentration completely inhibited the development of fruit rot symptoms in pepper fruits compared to methanol extracts. Hence, the present research work have a great scope for evaluating the biocontrol potential of native *S. tuirus* AR26 against chilli fruit rot disease under field condition as well against a broad spectrum of post-harvest plant pathogens.

**Keywords:** actinobacteria; *Streptomyces tuirus*; chilli fruit rot; *Colletotrichum scovillei*; *Colletotrichum truncatum*; *Fusarium oxysporum*; liquid bio-formulation

**1. Introduction**

Chilli (*Capsicum annuum* L.) is one of the most economically important spices cum solanaceous vegetable crops and is grown throughout the world for its green and red ripe fruits. It is a universal spice crop of India and occupies a major share in the Indian economy. In addition to adding pungency, taste, aroma and colour to cuisines, chilli have been used for centuries as medicine with countless health benefits, with antioxidant, anti-mutagenic, anti-carcinogenic, anti-arthritic and anti-inflammatory properties. Chilli fruits are rich in capsaicin, an appetite stimulant, and capsanthin, a pigment that gives its distinctive flavour and colour. Green chilli fruits contain more vitamin C than citrus fruits, whereas red chilli fruits contain more vitamin A than carrots [1,2]. Despite its rich nutritional and economic value, its commercial production is greatly threatened by fruit rot disease caused by complex pathogens including different species of *Colletotrichum*, *Fusarium* and *Alternaria* [3–5]. These pathogens extensively damage the fruits and significantly

**Citation:** Renuka, R.; Prabakar, K.; Anandham, R.; Pugalendhi, L.; Rajendran, L.; Raguchander, T.; Karthikeyan, G. Exploring the Potentiality of Native Actinobacteria to Combat the Chilli Fruit Rot Pathogens under Post-Harvest Pathosystem. *Life* **2023**, *13*, 426. https://doi.org/10.3390/ life13020426

Academic Editors: Hakim Manghwar and Wajid Zaman

Received: 28 November 2022 Revised: 13 January 2023 Accepted: 18 January 2023 Published: 2 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

reduce the quality, yield, appearance and marketability of the fruits [6,7]. It is a highly destructive pre- and post-harvest disease which causes yield losses up to 100% under congenial environmental conditions [8–10].

Though this disease can be managed with the repeated application of fungicides, pre- and post-harvest application of synthetic fungicides has been curtailed due to the persistence of fungicides on the fruits, which pose a direct risk to consumers and the environment through food chain contamination [11–13]. The use of naturally occurring bioactive compounds, especially those derived from antagonistic microorganisms, have been explored as prospective alternatives to synthetic fungicides due to their reduced toxicity and impact on humans and the environment [14–16]. Biological control of chilli anthracnose using antagonistic microorganisms or their metabolites is not a new concept, but a sustainable and ecologically acceptable approach in the context of leaving no toxic residues on the produce, safer application methods and ease of delivery, with minimal reliance on chemicals [17,18].

Although different groups of microorganisms have been employed for disease management, several research findings over the past few decades have highlighted the biocontrol potential of actinobacteria against a wide range of plant pathogens [19–21] through various mechanisms, including fungal cell-wall lysis, antibiosis, competition for nutrients, induction of host systemic resistance, phytotoxin degradation, plant growth stimulation, nutrient assimilation, rhizosphere competence and mineral availability [22–28].

Several species of actinobacteria were reported to have strong antagonistic activity against various species of *Colletotrichum* infecting a variety of crops. Taechowisan et al. [29] reported that *Streptomyces* spp. SRM1 exhibited antagonistic activity against *Colletotrichum musae* causing anthracnose in banana. *Streptomyces violaceoruber* reduced the incidence of chilli anthracnose by inhibiting the spore germination and mycelial growth of *Colletotrichum capsici* [30]. *Streptomyces ambofacines* S2 extract completely inhibited the expression of anthracnose symptoms of *Colletotrichum gloeosporioides* in red pepper fruits [18]. Actinobacteria not only prevent post-harvest pathogenic infection but also prolong the shelf life of a variety of crops without upsetting the natural balance.

A diverse group of actinobacteria inhabit the rhizosphere, phyllosphere and endosphere region of the plant and a subset of these provide a wide range of services and benefits to the plant in terms of suppressing plant diseases, promoting plant growth, increasing crop yield and enhancing soil fertility [31–33]. Particularly, when employed to curtail the plant infections, native actinobacterial isolates are more adaptable to their regular niche, have a higher success rate and are more resilient to local environmental challenges than the introduced microbes [34,35]. Furthermore, the introduced microbes must be able to co-habit with the native microbiome in order to provide more benefits to the plants. Hence, precolonization of the host by well adapted native biocontrol agents may prevent the growth and survival of plant pathogens.

The present study was, therefore, undertaken with the following objectives: (1) to isolate native actinobacterial isolates associated with rhizosphere, phyllosphere and surface sterilized tissues of chilli plants; (2) to identify the efficient actinobacterial isolate having antifungal potential against fruit rot pathogens *Colletotrichum* spp., and *Fusarium* sp.; (3) to unravel the antifungal mechanisms of potential actinobacterial isolates against fruit rot pathogens under in vitro conditions; (4) to assess the in vivo antifungal efficacy of liquid formulation and soluble metabolites of potential actinobacterial isolates on chilli fruits.
