Biological Control of Diseases of Bottle Gourd

Abstract

Biological control of plant diseases is an important component of disease management, particularly in the today’s’ world of environmental consciousness and awareness. It is particularly preferred method of disease management under organic production system. Biological control is successful in almost all the crops against a number of diseases but soil borne diseases are most responsive to bio-control methods. The agents of biological control, known as bio-control agents (BCAs) belong to a vast group of micro-organisms, particularly fungi (Trichoderma, Ampelomyces, etc), bacteria (Pseudomonas, Bacillus, etc) and actinomycetes. Bottle gourd is an important vegetable crop belonging to the family Cucurbitaceae. It suffers from a number of diseases like anthracnose, powdery mildew, downy mildew, wilt, etc. The present review shall be an attempt to review the biological control of the major diseases of bottle gourd.

1. Introduction

Fungicides have been used for the management of plant diseases, since the time man became interested in diseases of plants, besides invoking gods for protecting them from the wrath of diseases. However, the dawn of century saw an increasing awareness and consciousness relating to the adverse effects of these chemicals, particularly towards our environment. As a result there is advanced advocacy against the use of fungicides and search for alternative ways of disease management. Resistance to plant diseases is the most preferred way but most of the times not practical due to obvious and inherent reasons of disease resistance. Cultural practices have also been used since times immemorial, often times without the active consciousness of the producer. However, relying on cultural practices alone is not feasible. More so in case of diseases with compound interest type of growth. Biological control has immense potential to be used as viable and alternative disease management strategy and it can be blended well with the integrated disease management capsules [1, 2, 3, 4, 5]. Biological control has proved very effective in disease management, more so of soil borne diseases [6, 7, 8, 9], diseases of fruits [10, 11, 12], foliar diseases [13, 14, 15, 16, 17, 18] as well as nematode diseases [14, 19, 20], besides innumerable other crop diseases and the list is growing day by day.

Bottle gourd is an important crop grown all over the world for its culinary and medicinal properties [21]. However, it is affected by a number of diseases like anthracnose, downy mildew, powdery mildew, bacterial leaf spot, mosaic, etc., all of which lead to severe constraints in yield and reduction in the realization of full genetic potential. Management has been attempted through use of chemicals [22, 23, 24, 25], plant extracts [26, 27], resistance [28, 29, 30] or integrated disease management [31, 32, 33, 34]. However, in the recent past focus has shifted on the biological control of plant diseases and bottle gourd is no exception. In some cases, remarkable success has been obtained through the use of biocontrol agents, while in some cases triumphs have been restricted to lab studies. Following is an attempt to review biological control of diseases of bottle gourd in the last few decades.

2. Fungi as biocontrol agents of diseases of bottle gourd

A number of fungi have been used efficiently against different diseases of bottle gourd. Most commonly used fungi are different species of Trichoderma. These fungi have versatile nature, are easy to isolate from native soils and can easily be cultured, as a result of which many studies have focused on use of Trichoderma as bio-control agents. Also, most of the Trichoderma spp. employ diverse mechanisms for disease control like mycoparasitism, competition antibiosis, as well as induction of systemic resistance [35, 36].

T. harzianum reduced the mycelial growth of Fusarium moniliforme, the causal agent of bottle gourd wilt [37]. Although most of the microbial antagonists viz., Trichoderma harzianum, T. viride, Gliocladium virens, Bacillus subtilis and Stachybotrys atra significantly reduced seedling mortality and root rot infection of F. oxysporum in bottle gourd and cucumber, T. harzianum was found most effective [38]. In Gujarat, India, the isolate of T. viride (Sardarkrushinagar) was most effective against F. oxysporum followed by y T. harzianum (Junagadh), viride (Junagadh), T. viride (Navsari), Bacillus subtilis (Sardarkrushinagar) and Pseudomonas fluorescens (Sardarkrushinagar) [39]. Besides being healthy and free from wilt symptoms, the pathogen quantum inside the host and soil was reduced in bottle gourd seedlings raised from antagonist coated seeds [40]. Besides Trichoderma, Penicillium citrinum and Aspergillus flavus appear to effectively reduce mycelial growth of F. solani [41].

The causal organism of anthracnose of bottle gourd, viz. Colletotrichum lagenarium was inhibited by T. viride [42].

T. hamatum was found more promising than T. harzianum, although both inhibited the growth and sporulation of Alternaria alternata, the causal organism of black rot of bottle gourd [43].

Chatur and Anil (2014) reported that seed treatment and soil application of Trichoderma harzianum and T. viride were effective for the management of gummy stem blight and these treatments also increased fruit yields [44]. Patel et al. (2017) reported similar results against the same disease using same bio-agents and reported that next in order were Bacillus subtilis and Pseudomonas fluorescence [45]. Soil application of spent mushroom substrate enriched with T. harzianum significantly increased yield and simultaneously decreased disease incidence of gummosis [46]. Gliocladium virens was found most effective in reduction of seed and root infection caused by Lasiodiplodia theobromae, whereas, B. subtilis helped in the reduction of seed and seedling infection of bottle gourd under in vitro conditions [47].

Seed priming by aqueous solutions of culture filtrates of Trichoderma can be used for vegetable seed treatment, including bottle gourd, for controlling seed-borne fungal infection [48]. Seed treatment with T. harzianum resulted in the induction of systemic resistance [49]. Besides, Trichoderma increases the germination percentage of treated seeds [50]. As a result of these studies, it has been recommended that seed treatment with Trichoderma, may be included in the IPM schedule to increase the net return of farmers [51]. Trichoderma enriched bio-fertilizers promote crop cultivation of bottle gourd with subsequent reduction in the usage of nitrogen fertilizers [52].

The fungus Paecilomyces lilacinus had better rate of success against second stage juveniles of root knot nematode infesting bottle gourd than either Trichoderma or Pseudomonas [53]. Tricho-compost was effective in reducing the root knot severity and increasing plant growth and yield of bottle gourd [54].

3. Bacteria as biocontrol agents of diseases of bottle gourd

In Japan, the root systems of an associate crop of welsh onion or Chinese chive were dipped in broth cultures of Pseudomonas gladioli and then bottle gourd was mixed cropped with the associate crop. It was observed that by this treatment, the occurrence of Fusarium wilt was suppressed to a large extent [55].

Pseudomonas fluorescens as foliar spray has been used for the management of Alternaria leaf blight of bottle gourd [24]. Along with some other fungal biocontrol agents, particularly Trichoderma, bacterial biocontrol agents like Bacillus subtilis controlled the pre- and post-emergence infection of Lasiodiplodia theobromae in seedlings of bottle gourd under both in vitro and in vivo conditions [47]. In another experiment on the efficacy of biocontrol agents against gummy stem blight of bottle gourd, it was found that although the bacterial biocontrol agents were not much effective against the disease, but treatment with Bacillus subtilis and Pseudomonas fluorescens did increase the fruit yield over control [45]. Combined treatment of bottle gourd with carbendazim (seed treatment), mancozeb (foliar spray) and Pseudomonas fluorescens (foliar spray) resulted in minimum disease incidence and minimum disease severity of Alternaria blight with maximum disease control [24].

A study by Rani et al. (2022) revealed that bacteria like Bacillus amyloquefaciens, B. megaterium, P. fluorescens, and P. putida, could be used for the management of root knot nematode both under in vivo and in vitro conditions [56]. Seed treatment with Bacillus subtilis reslted in the reduction of seed borne fungi of vegetable seeds including bottle gourd [57]. Pseudomonas fluorescens was the second best bioagent after Paecilomyces lilacinus for causing the most mortality of second stage juveniles of root knot nematodes [53].

4. Mechanism of biocontrol

The biocontrol agents utilize a variety of mechanisms for reducing the pathogen populations and promote plant growth. The general mechanisms of biocontrol agents have been reviewed by a number of scientists [58, 59]. Most of the mechanisms can be categorized as (a) antibiosis, (b) competition for food and space, (c) hyperparasitism or mycoparasitism, (d) cell wall degrading enzymes and (e) induction of systemic resistance. The mechanisms of action of BCAs have been demonstrated using microbiological, microscopic and biochemical techniques. In the recent past, development and use of molecular techniques have yielded significant results [60].

In some cases, biocontrol efficacy can be increased by the use of more than one biocontrol agent with more than one mode of action. An example of this situation is the use of a yeast (Pichia guilermondii) which caused inhibition of conidial germination of Botrytis cinerea, and the bacteria (Bacillus mycoides) that caused breakdown and destruction of conidia [61]. However, we have to tread this path with caution because it has been found that in combined use of BCAs, antagonistic interactions among BCAs are more likely to occur than synergistic interactions [62]. For example, it has been found that DAPG from Pseudomonas fluorescens strains enhanced nag1 N-acetyl-β-d-glucosaminidase, but not ech42 endochitinase expression, whereas an unknown substance from P. fluorescens CHA0 repressed expression of both Trichoderma chitinases [63].

The production of plant growth promoting metabolites as well as antagonistic potential of different BCAs varies with respect to disease control of bottle gourd is expected. Kotasthane et al. (2015) found that the production of metabolites in 20 different isolates of Trichoderma did not corelate with enhanced growth on cucumber, bottle gourd and bitter gourd [64]. The isolate viz. T. viride isolate (T14) was identified as highest producer of inorganic phosphate, IAA and siderophore and exhibited high antagonistic and plant growth promoting ability. T. harzianum strain T-A66 promoted growth of bitter gourd and induced disease resistance to Fusarium oxysporum by inducing quick H₂O₂ burst and callose deposition, as well as increasing antioxidant enzyme activities and phenolic compounds content [65]. Munir et al. (2019) demonstrated the inhibitory role of chitinolytic enzyme extracts of Trichoderma against fungal pathogens of bottle gourd, whereas, Shah et al., showed the inhibitory effect of various Trichoderma isolates on the mycelia of pathogens of bottle gourd [66, 67].

In another study on powdery mildew of Cucurbitaceae, it was found that the antagonistic strain of Bacillus subtilis confers protection against cucurbit powdery mildew by the production of reactive oxygen species and cell wall reinforcement and by activation of jasmonate and salicylic dependent responses [68]. The scanning electron micrographs revealed that the antagonistic bacteria colonized the leaves by forming orderly microcolonies following epidermal cell junctions and were closely attached to Podosphaera fusca conidia and hyphae resulting in the collapse of latter [69]. The lipopeptides produced by B. subtilis are also able to reduce the disease by arresting conidial germination, probably due to induction of cytological alterations [70]. Similarly, the epiphytic yeast Pseudozyma aphidis proliferated on the infected tissue and its long hyphae parasitized the powdery mildew hyphae and spores as an ectoparasite, besides producing antibiotics [71].

Ongena et al. (1999) suggested that antifungal compounds induced by inoculation of cucumber roots with fluorescent Pseudomonas sytrains protected the cucumber plants against Pythium aphanidermatum and siderophores or antibiosis had minimal role in protection against disease [72].

In case of bacterial diseases compounds like iturin like lipopeptides have been found to be of considerable importance. The antibacterial activity is absent in iturin deficient mutants. Fluoresence and transmission electron microscopic studies have revealed that these compounds are cytotoxic to the bacterial plasma membrane [73]. Iturins have also been implicated in the antagonism of B. subtilis towards P. fusca [74].

5. Conclusion

Biological control has minimized the safety concerns regrading chemical fungicides and pesticides. Bio-control agents do not leave harmful residues in soil, are long lasting, environment friendly and self-sustaining in the long run. The problem of disease resistance is minimized and most of the biocontrol agents have plant growth promoting properties as well. However, they can be less effective than chemicals, particularly when time has to be taken into consideration. Although biological control proves to be cheaper in the long run, the initial investment and startup production costs limits its Commercialization and formulation of successful biocontrol agents pose another problem. There is the problem of culturing biocontrol agents in large quantities. They cannot be stored for long periods of time and hence have low shelf life. Moreover, there is very little evidence that biological control in itself is sufficient for practical management of plant diseases. In most of the cases, biological control needs to be integrated with other methods of disease control for effective management of diseases. It also has to be planned well in advance and is not efficient strategy for mitigation of emergencies like outbreaks of blights where the disease progresses at an exponential rate.

Biological control is the latest and most interesting strategy that is being used in the management of diseases of plants including bottle gourd. However, at present very little literature is available on the biological control of different diseases of bottle gourd and the work on plant growth promoting microbes is scantier. The need of the hour is to accelerate work on this aspect and search for BCAs with excellent biocontrol potential against maximum diseases of bottle gourd with bio-stimulatory action as well. These microbes need to be formulated and commercialized for the benefit of mankind in general and farmers in particular.