In vitro screening of fungicides against Colletotrichum truncatum causing anthracnose of dragon fruit (Hylocereus spp.)

Introduction

Dragon fruit (Hylocereus spp.) is an important tropical and subtropical fruit crop that has gained considerable importance because of its attractive appearance, nutritional value and increasing consumer demand. However, the productivity and quality of dragon fruit are affected by several fungal diseases, among which anthracnose is an important disease causing lesions, necrosis and deterioration of infected plant tissues. Different species of Colletotrichum have been associated with anthracnose of dragon fruit in different production regions, indicating the importance of effective disease management strategies (Masyahit et al., 2009). Colletotrichum truncatum has been reported as an important causal pathogen of anthracnose of dragon fruit. Earlier studies from China documented C. truncatum associated with anthracnose of Hylocereus undatus, while its occurrence on dragon fruit has also been reported from India. The identification of C. truncatum as a pathogen of dragon fruit highlights the need for suitable management measures to reduce pathogen growth and disease development (Guo et al., 2014). Chemical fungicides remain an important component of fungal disease management because they can provide rapid suppression of pathogenic fungi and are particularly useful when disease pressure is high. Fungicides belonging to different chemical groups and combinations of active ingredients may differ considerably in their ability to inhibit mycelial growth of Colletotrichum species. Therefore, screening available fungicides under In vitro conditions provides useful preliminary information for identifying effective molecules or combinations for further disease-management studies. The poisoned food technique is widely used for evaluating the antifungal activity of fungicides by incorporating the test fungicide into a suitable culture medium and measuring the subsequent growth of the pathogen. The reduction in colony growth compared with an untreated control provides an estimate of the inhibitory effect of the tested fungicide. Such laboratory screening helps in comparing the relative efficacy of different fungicides before their further evaluation under greenhouse or field conditions (Nene et al., 1993) [1] .

Considering the importance of anthracnose caused by C. truncatum in dragon fruit, the present investigation was undertaken to screen selected fungicides for their inhibitory effect against the pathogen under In vitro conditions. Seven fungicides representing different active ingredients and combinations were evaluated using the poisoned food technique, and their efficacy was assessed based on colony diameter and percentage inhibition of mycelial growth.

Materials and Methods

Preparation of PDA medium and fungicide solutions

Potato Dextrose Agar (PDA) was used as the basal culture medium for evaluating the efficacy of selected fungicides against Colletotrichum truncatum. The required quantities of each fungicide were incorporated aseptically into molten PDA to obtain the desired concentrations. Streptomycin was added to the medium to avoid bacterial contamination. The fungicides were evaluated at their respective half and recommended dose concentrations.

Poisoned food technique

The efficacy of the fungicides was assessed using the Poisoned Food Technique as described by Nene et al. (1993) [1] . Fungicide-amended PDA medium was dispensed at 20 ml per sterile 90 mm Petri plate and allowed to solidify at room temperature. PDA without fungicide was maintained as the untreated control.

Inoculation of C. truncatum

A 5 mm diameter mycelial disc was cut from the actively growing margin of a seven-day-old culture of C. truncatum using a sterilized cork borer. The disc was placed at the centre of each fungicideamended PDA plate in an inverted position to ensure proper contact with the medium.

Incubation and observation

Three plates were maintained for each treatment along with three untreated control plates. The inoculated Petri plates were incubated at 25 ± 2 °C. Radial growth of the pathogen was observed and colony diameter was recorded after 7 days of incubation.

Assessment of fungicidal efficacy

The colony diameter of C. truncatum was measured in millimetres using a scale. The percentage inhibition of mycelial growth was calculated in comparison with the untreated control using the formula given by Arora et al. (1978) [2] :

Where, C = radial growth of the pathogen in untreated control (mm) T = radial growth of the pathogen in treatment (mm)

Experimental design and statistical analysis

The experiment consisted of eight treatments with three replications and was conducted in a Completely Randomized Design (CRD). The experimental data were statistically analysed, and standard error (S.E.) and critical difference (C.D.) at the 5 per cent level of significance were calculated for comparison of treatments.

Results

In vitro screening of fungicides against Colletotrichum truncatum

The efficacy of seven fungicides against Colletotrichum truncatum causing anthracnose of dragon fruit was evaluated under In vitro conditions using the poisoned food technique. Considerable variation was observed among the fungicides in their ability to suppress the mycelial growth of the pathogen. The colony diameter ranged from

Discussion

The present investigation demonstrated considerable variation in the efficacy of the fungicides evaluated against Colletotrichum truncatum under In vitro conditions. The fungicide treatments differed in their ability to suppress the radial growth of the pathogen, with inhibition ranging from 54.07 to 94.44 per cent. The higher efficacy observed with fungicide combinations indicated their strong inhibitory activity against the pathogen under laboratory conditions. Among the tested fungicides, Metiram 55% + Pyraclostrobin 5% WG recorded the highest inhibition of mycelial growth (94.44%), fol-lowed by Tebuconazole 50% + Trifloxystrobin 25% WG (92.96%) and Carbendazim 12% + Mancozeb 63% WP (90.74%). The higher activity of these combination fungicides may be associated with the presence of active ingredients having different modes of action, resulting in stronger suppression of fungal growth. Similar effectiveness of fungicide combinations against Colletotrichum spp. has been reported by Jagtap et al. (2013) [3] and Mahesh et al. (2020) [4] . Mancozeb 75% WP also showed considerable inhibition of C. truncatum, with 86.66 per cent inhibition of mycelial growth. Similar inhibitory effects of fungicides against Colletotrichum species have been reported by Vani et al. (2018) [5] . Bagade et al. (2020) [6] also reported variation in the efficacy of different fungicides against Colletotrichum causing anthracnose, supporting the observed differences among fungicidal treatments in the present study. The comparatively lower inhibition recorded by Copper Hydroxide 61.41% WG, Captan 70% + Hexaconazole 5% WP and Chlorothalonil 75% WP indicated that their inhibitory activity against C. truncatum was lower than that of the highly effective fungicide combinations. Similar variation in fungicidal efficacy against anthracnose-causing Colletotrichum species has been reported in earlier In vitro studies (Golakiya et al., 2020) [7] . Overall, the present findings indicated that Metiram 55% + Pyraclostrobin 5% WG, Tebuconazole 50% + Trifloxystrobin 25% WG and Carbendazim 12% + Mancozeb 63% WP were highly effective in suppressing the mycelial growth of C. truncatum under In vitro conditions. These findings provide preliminary information for selecting promising fungicides for further evaluation under greenhouse and field conditions.

Conclusion

The present study demonstrated significant variation in the efficacy of fungicides against Colletotrichum truncatum causing anthracnose of dragon fruit under In vitro conditions. Among the tested fungicides, Metiram 55% + Pyraclostrobin 5% WG recorded the highest inhibition of mycelial growth (94.44%), followed by Tebuconazole 50% + Trifloxystrobin 25% WG (92.96%) and Carbendazim 12% + Mancozeb 63% WP (90.74%). These fungicides showed strong inhibitory activity against C. truncatum and can therefore be considered promising candidates for further evaluation under greenhouse and field conditions for the management of dragon fruit anthracnose.

Acknowledgement

AcknowledgmentsThe authors sincerely acknowledge the support of Agriculture Development Trust's Dr. Sharadchandra Pawar College of Agriculture, Baramati and contributions of earlier researchers whose published works provided valuable insights and guidance for the present study.

References

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