Dr. Md. Mahfuz Alam

Watermelon is a highly popular fruit in worldwide and renowned for its juiciness, refreshing taste and exceptional nutritional value. Packed with vitamins, minerals and antioxidants, it is a powerhouse of health benefits. With 92% water content, watermelon serves as an essential source of hydration, especially in arid regions and during dry seasons. China leads global watermelon production, contributing approximately 70% of the total output.

Other significant producers include Turkey, Iran, Brazil, Uzbekistan, Algeria, the USA, Russia, Egypt, and Mexico. In Bangladesh, the soil and climate are particularly conducive to watermelon cultivation, making it a widespread crop across the country. Due to its cultivation being similar to that of vegetables and requiring relatively less irrigation, farmers in the southern regions of Bangladesh are increasingly shifting from boro rice to watermelon as a profitable commercial crop during the boro season.

Globally, over 20 diseases have been identified in watermelon, caused by various pathogens, including fungi, bacteria, viruses and nematodes. Among these, Gummy Stem Blight is one of the most most destructive fungal diseases, caused by the pathogen Didymella bryoniae and its anamorph Phoma cucurbitacearum. This disease can also affect other cucurbit crops such as cucumber, pumpkin and melon. Under favorable conditions, Gummy Stem Blight can result in up to a 40% yield loss with high inoculum levels and conducive environmental conditions. Research conducted by the Bangladesh Agricultural Research Council (BARC) and the Bangladesh Agricultural Research Institute (BARI), with funding from the Krishi Gobeshona Foundation (KGF) between 2019 and 2023, highlighted the prevalence of Gummy Stem Blight in major watermelon producing districts of Bangladesh. These districts include Noakhali, Patuakhali, Bhola, Khulna, Cox's Bazar, Chuadanga, Gopalganj, Natore, Panchagarh, and Sylhet, where the disease incidence ranges from 10% to 60%.

Causes of the Disease

The primary cause of Gummy Stem Blight is the fungus Didymella bryoniae. This pathogen can persist in the soil, infected plant debris and seeds, remaining dormant until favorable conditions allow it to spread rapidly within plants. The key factors contributing to the spread of Gummy Stem Blight include:
1. Infected Seeds: Fungal spores often remain hidden within watermelon seeds, enabling the fungus to infect plants during new cultivation cycles.
2. Plant Residues: Residual parts of infected plants left in the field can act as a source of fungal infection for subsequent crops.
3. Contaminated Soil: Spores from infected plant residues can contaminate the soil, allowing the fungus to infect new plants through their root systems.
4. Air and Water: During rainfall, fungal spores can be dispersed by water splashes and carried by air, spreading the infection to different parts of the plants.

Favorable Weather Conditions for Disease Spread

Weather conditions play a critical role in the spread of Gummy Stem Blight in watermelon. Warm and humid environments significantly enhance the infection and proliferation of this disease. Key weather factors that contribute to its spread include:
1. High Humidity: Elevated humidity levels create favorable conditions for the rapid growth and infection of the fungus responsible for Gummy Stem Blight. The fungus thrives when water accumulates on leaves and stems, particularly during rainy weather.
2. Moderate to High Temperatures: Temperatures between 24°C and 30°C activate the fungal spores, providing an optimal environment for infection.
3. Rainfall: Prolonged rainfall results in water accumulation on plant surfaces, facilitating the spread of fungal spores. Rainwater splashes further disperse the spores, increasing the risk of infection.
4. Excessive Irrigation: Overwatering can lead to waterlogging around the plant base and in the field, creating conditions that promote fungal infection. Proper irrigation management is essential to minimize the risk of disease spread.

Nature of Damage

Gummy Stem Blight significantly impacts various parts of the watermelon plant, leading to reduced growth, compromised fruit quality and decreased overall yield. The damage caused by this disease is as follows:
1. Leaf blight: The disease manifests as circular or irregular brown spots on the leaves (Figure 1). Over time, the affected leaves turn black or yellow, eventually dropping off. This leaf loss disrupts the plant's ability to photosynthesize and transport nutrients effectively.
2. Stem blight: The base of the plant's stem is particularly vulnerable, often exuding a sticky substance (Figure 2). Dark lesions appear on the stem and the fungus invades the internal tissues, weakening the plant’s structure and causing it to collapse.
3. Fruit rot: The disease begins with small spots on the fruit, which gradually enlarge (Figure 3). This results in deformation of the fruit, deterioration in taste and reduced nutritional quality. Consequently, the market value of the fruit is significantly affected.

4. Reduction in Productivity: Affected plants experience stunted growth and reduced yield potential. The overall health of the plant declines, leading to lower fruit production and diminished productivity.
5. Economic Impact: Gummy Stem Blight has led to considerable economic losses in Bangladesh's watermelon production sector. The disease reduces crop yield, imposing financial strain on farmers. Additionally, the decline in fruit quality lowers its market value, compounding the economic burden on producers.

Management Practices

Effective measures can be implemented to control Gummy Stem Blight in watermelon, safeguarding plants from infection and preventing the disease's spread. Below are key strategies for disease management:
1. Crop Rotation: Practice crop rotation to reduce fungal activity in the soil. Avoid planting watermelon consecutively in the same field and alternate with non-host crops to limit the spread of Gummy Stem Blight.
2. Selection of Resistant Varieties: Cultivate watermelon varieties resistant to Gummy Stem Blight. Resistant varieties enhance the plants' natural ability to defend against fungal infections, significantly lowering disease risks.
3. Soil Treatment: Maintain soil health by applying organic compost and avoiding the retention of root residues. Keep fields clean and free of plant debris. Soil treatment with fungicides can also help reduce fungal presence and prevent disease outbreaks.
4. Clean Cultivation: Maintain a clean cultivation environment. Remove crop residues, leaves and roots from previous crops to prevent the buildup and spread of the Gummy Stem Blight fungus.
5. Use of Fungicides: Use appropriate fungicides, such as azoxystrobin, difenoconazole, copper based fungicides, or mancozeb, to control the disease. Always follow proper dosage instructions and application guidelines to ensure effectiveness and safety.
6. Biological Control: Employ beneficial microorganisms like Trichoderma spp., Bacillus subtilis, and Pseudomonas fluorescens to suppress fungal activity in the soil. Organic products like Trichost and Bio-darma not only reduce fungal presence but also enhance soil nutrient content.
7. Integrated Management: Adopt integrated management practices for effective control:
• Treat seeds and plant them in sterilized soil within polybags under net houses.
• Perform bio-fumigation using 3-week-old BARI Sarisha11 mustard plants in the field soil.
• Prepare raised beds with proper drainage, apply organic fungicides, and cover the beds with mulch paper. Create planting holes 4-6 cm in diameter.
• Apply organic pesticides 15-20 days after transplanting.
• Ensure adequate irrigation and apply fungicides promptly at the first sign of disease symptoms.

Conclusion

Gummy Stem Blight is a major disease affecting watermelon cultivation, significantly reducing both yield and fruit quality. However, its impact can be effectively mitigated through proper care and management practices. Raising farmers' awareness, adopting advanced agricultural techniques, and integrating biological and chemical control methods can substantially minimize the effects of Gummy Stem Blight. These efforts will not only improve watermelon productivity but also contribute to nutrition security and strengthen the agricultural sector of the country.

The writer is a, PSO, BARC, Dhaka Khondokar Mohammad Alam, SO, BARI.