Controlling Colletotrichum sp. Causing Anthracnose on Yellow Horn Pepper Using Chlorine and Oligochitosan: Efficacy under Net House and Storage Conditions

Introduction

Chili pepper (Capsicum spp.) is a popular and long-cultivated spice vegetable in Vietnam. According to Vo Van Chi (2008), chili pepper is an ingredient in traditional medicine that can treat certain ailments such as indigestion, dysentery, rheumatism, and stimulate the stomach . However, pepper cultivation faces difficulties and reduced yields due to diseases such as: fungal anthracnose, viral mosaic disease, bacterial wilt, fungal damping-off… Among these, anthracnose caused by the fungus Colletotrichum sp. is a disease that seriously harms pepper yield globally [1]. Furthermore, this disease not only causes damage in the field but also affects the fruit after harvest [2].

Some studies apply environmentally friendly, rapidly degrading chemicals or biological pesticides in controlling crop diseases. For example: Controlling post-harvest diseases on NÄ?m Roi pomelo, Sành orange, and Đường mandarin using Chlorine , applying chitosan to inhibit Colletotrichum gloeosporioides causing anthracnose disease on Hoà Lá»?c mango [3,4]. Chili pepper is a fresh spice vegetable, and the safety of post-harvest pesticides is crucial. Chlorine and oligochitosan are substances with very low toxicity, used in this study to investigate their potential to limit anthracnose disease on yellow horn pepper under net house conditions and post-harvest storage.

Research Results and Discussion

Efficacy of Chlorine and Oligochitosan in Controlling Anthracnose Disease on Yellow Horn Pepper Fruit Under Net House Conditions

Table 3 results show that the treatments chlorine 0.2%-PT, chlorine 0.2%-PS, chlorine 0.2%-PTS, oligochitosan 3%-PTS, Ridomil Gold 68WG-PS, Ridomil Gold 68WG-PTS all showed a gradual decrease in disease reduction efficacy over the surveyed time points (5, 7, 9, 11 DAI). The remaining treatments, oligochitosan 3%-PT, oligochitosan 3%-PS, Ridomil Gold 68WG-PT, showed an increase in disease reduction efficacy up to 7 DAI, but then the efficacy gradually decreased.

Table 3: Disease Reduction Efficacy (%) of Chlorine and Oligochitosan on Yellow Horn Pepper Fruit Over Dais After Inoculation

The disease reduction efficacy of the Chlorine 0.2% treatment when sprayed twice (before and after inoculation) was significantly higher than the Ridomil Gold 68WG treatment and other treatments at the recorded time points. At 5 DAI, the disease reduction efficacy in the chlorine 0.2% treatment sprayed twice, before and after, was over 50% (52.74%); subsequently, the efficacy gradually decreased until 11 DAI, where chlorine 0.2%-PTS maintained an efficacy of 31.20%. According to the study by Buck (2006), Chlorine is a strong oxidant, capable of penetrating cells and disrupting the metabolic processes of microorganisms, such as protein synthesis, DNA [3]. Additionally, chlorine can combine with amine molecules in microbial cells to form new substances, which destroy the cell's vital functions, leading to the death of the microorganism [5]. When testing chlorine 0.2% on horn pepper plants under net house conditions, it showed efficacy in inhibiting the anthracnose pathogen caused by the fungal strain Col-8-DT on the fruit (Figure 1). In particular, chlorine applied twice, before and after spraying (chlorine 0.2%-PTS), yielded the highest disease reduction efficacy, reaching 31.20% at 11 DAI.

Figure 1: Disease reduction efficacy of chlorine, oligochitosan, and Ridomil Gold 68WG against the fungus Col-8-DT at 11 DAY under net house conditions

A: NT chlorine 0,2%-PT                                                    B: NT chlorine 0,2%-PS

C: NT chlorine 0,2%-PTS                                                  D: NT oligochitosan 3%-PT

E: NT oligochitosan 3%-PS                                               F: NT oligochitosan 3%-PTS

G: NT Ridomil Gold 68WG-PT                                        H: NT Ridomil Gold 68WG-PS

I: NT Ridomil Gold 68WG-PTS                                  J: NT Distilled water control treatment

Determining the Application Time of Chlorine and Oligochitosan for Inhibitory Efficacy Against the Fungus Col- 8-Dt Under Laboratory Conditions

In Table 4, at 4 DAI, the treatments chlorine 0.2%-5p, chlorine 0.2%-7p, and oligochitosan 3%-3p showed the highest disease inhibition efficacy, reaching 100%, and oligochitosan 3%-7p also showed a comparable inhibition efficacy (93.75%), with no significant statistical difference at the 5% level.

Table 4: Inhibition Efficacy (%) of Chlorine and Oligochitosan Against Anthracnose Disease Caused by the Fungus Col-8-DT at 4, 6, and 8 DAI

At 6 and 8 DAI, the treatment chlorine 0.2%-5p showed the highest disease inhibition efficacy and was statistically different (P < 0.05$) from all other treatments, with inhibition efficacy of 93% (6 DAI) and 15.56% (8 DAI). The chlorine 0.2%-7p treatment showed higher disease inhibition efficacy than the chlorine 0.2%-3p treatment (Figure 2). Overall, the inhibition efficacy of treatments with chlorine and oligochitosan gradually decreased over the time points 4, 6, and 8 DAI.

Materials and Research Methods

• Fungal Source: Colletotrichum sp. (Col-8-DT) causing anthracnose disease on pepper (provided by the Faculty of Plant Protection).

• Pepper Variety: Yellow horn pepper from Trung Nong Plant Seed Co., Ltd.

• Pesticide: Ridomil Gold 68WG (a fungicide selected as the positive control) is effective in controlling the fungus Colletotrichum sp.

• PDA medium (Shurleff and Averre III, 1997):

Potato 200 g

Dextrose 20 g

Agar 20 g

Distilled water 1000 ml

pH 6.8 - 7.0

Experiment 1: Evaluating the efficacy of chlorine and oligochitosan in controlling anthracnose disease on yellow horn pepper fruit under net house conditions

• Experimental Objective: Investigate the ability of chlorine and oligochitosan to control anthracnose disease on yellow horn pepper.

• Experimental Method: The experiment was arranged in a completely randomized design, with 4 replications, including 10 treatments (Table 1); each replication was 1 pepper plant with uniform fruits (first flush fruit) artificially inoculated on 4 fruits/plant.

Table 1: Treatments Arranged in the Net House

LBNT: Artificial Inoculation

• Experimental Procedure:

 Yellow horn pepper plants were grown in pots until they produced uniform fruits, then artificial inoculation was performed.

Fungal Source Preparation: The fungus Col-8-DT was cultured on PDA medium in Petri dishes.

Artificial Inoculation: Each pepper fruit was injured in the middle with a sterile needle bundle (5 needles/bundle), and then a 5 mm diameter paper disc, soaked with a spore suspension of the fungus Col-8-DT at a concentration of 10^7 spores/ml, was placed on the wound.

Monitoring Indicators: Record the length and width of the lesion at 5, 7, 9, 11 DAIs after Inoculation (DAI). The efficacy of the substances (HQT) was calculated according to the formula of Abbott (1925):

In there: - C: average lesion length in the control treatment.

• T: average lesion length in the treated treatment.

Experiment 2: Determining the application time of chlorine and oligochitosan for inhibitory efficacy against the fungus Col- 8-DT causing anthracnose disease on horn pepper fruit under laboratory conditions.

Experimental objective: Investigate the application time of chlorine and oligochitosan on post-harvest horn pepper fruit to limit anthracnose inoculum [9].

• Experimental Method:

The experiment was arranged in a completely randomized design with 7 treatments (Table 2): 6 treatments were soaked in chlorine 0.2% and oligochitosan 3% at time points 3, 5, 7 minutes, and 1 control treatment. Each treatment had 4 replications, and each replication consisted of 15 pepper fruits.

Table 2: Treatments Arranged in the Laboratory

Fungal Source Preparation: the fungus was cultured on PDA medium.

Procedure for Treating Pepper Fruit with Chlorine and Oligochitosan: Select pepper fruits that have reached physiological ripeness, are uniform in size and color, wash thoroughly with water, allow to air-dry naturally, and wipe the fruit with 70% alcohol. Then, place the pepper fruit in a container with 500 ml of chlorine 0.2% or oligochitosan 3% solution, and soak the fruits completely for 3, 5, or 7 minutes.

Inoculation Method: Remove the fruits and place them on a plate, then spray 3 ml of fungal spore suspension at a concentration of 10^7 spores/ml. Each plate of pepper, after spraying the fungal spores, was placed in a nylon bag and incubated at a temperature of 25^\circC for 48 hours, then the treatments were moved to laboratory conditions (28 - 30^\ circC).

Monitoring Indicators: Record the percentage of fruits showing disease symptoms (%), once every 2 DAIs, until the control treatment reaches 100% diseased fruits.

Disease inhibition efficacy was calculated according to the formula of Yu et al. (2012):

In there: HQUC: Disease inhibition efficacy of the applied chemicals.

TLB_{dc}: Disease rate (number of diseased fruits on the total number of fruits in the control treatment) in the control treatment.

TLB_{i}: Disease rate (number of diseased fruits on the total number of fruits in the treated treatment) in the treated treatment.

Conclusion and Recommendations

Under net house conditions, chlorine 0.2% and oligochitosan 3% both effectively control anthracnose disease on pepper, and the efficacy is higher when sprayed twice—once before inoculation and once after inoculation. The disease reduction efficacy of the Chlorine 0.2%-PTS treatment (31.2%) was higher than that of Oligochitosan 3%-PTS (22.55%) and Ridomil Gold 68WG-PTS (22.17%) at 11 DAI. Under laboratory conditions, chlorine 0.2% and oligochitosan 3% both showed high efficacy in controlling anthracnose disease on pepper fruit after harvest. The disease reduction efficacy when soaking the pepper fruit for 5 minutes was higher than for 3 minutes and 7 minutes. For chlorine 0.2%, the efficacies were 93%, 69.75%, and 86%, respectively; for oligochitosan 3%, the efficacies were 79%, 74.5%, and 76.49%, respectively, at 6 DAI. Further studies on the potential of chlorine and oligochitosan for controlling anthracnose disease on pepper under field conditions are needed. The author wishes to express deep gratitude for the significant assistance from the AI assistant Gemini, including but not limited to the aspects of: suggesting ideas for student survey questions, checking for spelling errors and scientific terminology, ensuring accurate academic translation into English, and consulting on the design of data collection tables.

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