Dr. Md. Mahfuz Alam

Watermelon is a very juicy and nutritious popular fruit. The soil and climate of Bangladesh are very suitable for watermelon cultivation. Farmers are commercially cultivating watermelon instead of paddy during the Boro season as the cultivation method is similar to vegetables and requires less irrigation. More than 20 watermelon diseases have been identified in different countries of the world so far. Diseases are caused by various organisms such as fungi, bacteria, viruses and nematodes. Among these diseases, fungal downy mildew causes the most damage to watermelon. Watermelon wilt (Fusarium Wilt) is a soil-borne fungal disease caused by Fusarium oxysporum f. sp. It is caused by a fungus called niveum. It mainly attacks the roots and branches of the watermelon plant and blocks the flow of water and nutrients inside the plant. As a result, trees wilt quickly, wither, and crop productivity and quality decline. Downy mildew spreads from the watermelon roots to the whole plant and quickly damages the crop, becoming a major problem in watermelon cultivation. This wilting disease of watermelon plants is especially prevalent in hot and humid climates. Downy mildew can cause up to 100% crop loss, respectively, under favorable conditions. In 2018, about 30-70% of the watermelon fields were affected by this disease in Subarnachar, Noakhali. Also, in the last 2019-2023 KGF-funded study conducted by BARC and Bari, the prevalence of the disease was observed at 20-63% in major watermelon producing districts across Bangladesh such as Noakhali, Patuakhali, Bhola, Khulna, Cox's Bazar, Chuadanga, Gopalganj, Natore, Panchagarh and Sylhet. . It is possible to increase the production of watermelon by suppressing this disease through proper identification and integrated disease control management.

cause of disease

Fusarium oxysporum f. sp. niveum, a fungus that exists readily in the soil and once infected in the soil can persist for a long time. This fungus lives deep in the soil and enters the plant through the plant's roots. By destroying the xylem in the veins of the plant, it obstructs the flow of water and nutrients and the plant gradually weakens and droops. The causes of infection are:

1. Infected soil: The Fusarium fungus is present in soil and can survive for years. It enters the plant roots through the soil and cuts off the flow of water and nutrients from there, causing the plant to droop.
2. Infected seeds: This fungus can spread even if infected seeds or seedlings are used.
3. Infected plant residues: If residues from infected plants remain in the field, they become a source of disease for new crops.
4. Humans and Farm Equipment: The fungus can spread to other fields through contact with infected soil by farm equipment and farmers.
favorable weather
Certain climatic factors play an important role in the spread of watermelon wilt disease. A warm and humid environment is most conducive for the infection and spread of this disease. Below are the climatic factors that favor the spread of this disease:
1. Warm weather: Temperatures of 25-30 degrees Celsius are ideal for this fungus. Fusarium spores are active and able to multiply rapidly in warm weather.
2. High moisture: Soil moisture is conducive to the spread and growth of Fusarium fungi. Outbreaks of this disease occur when soil moisture is high, especially during rainwater or excessive irrigation.
3. Water status: Waterlogging in the field and lack of drainage creates an ideal infection environment for the Fusarium fungus.
4. Soil pH: Fusarium fungi thrives in slightly acidic or neutral pH. Fungal activity is more when soil pH is 6.0 to 6.5.
Nature of loss
The damage caused to the plant due to the wilt disease of watermelon is:
1. Root damage: This fungus first penetrates the plant roots and destroys the water and nutrient transport capacity of the roots, resulting in nutrient deficiency at the surface of the plant and the plant drooping (Figure 1).
2. Fungal spread within the stem: Fusarium fungi enter the stem and spread rapidly through the vascular or vein cells, thereby disrupting nutrient circulation throughout the plant (Figure 2).
3. Yellowing of leaves and branches: The leaves turn yellow and wilt as water and nutrients flow from the plant's trunk and roots to stop. The branches of the tree wither and the tree dies (Figure 3).
4. Impact on crop production: Watermelon plants are likely to die completely due to wilting disease. It greatly reduces yield and quality of watermelon.
Suppression Management
Some effective management can be adopted to control the outbreak of downy mildew disease in watermelon. The following steps can help prevent the spread of downy mildew and protect watermelon plants:
1. Crop Rotation
Fusarium wilt activity can be reduced if crop rotation is adopted. Watermelon should be cultivated in cycles with other crops rather than cultivating it more than once on the same land. It prevents the spread of Fusarium fungus and helps to reduce fungal activity in soil.
2. Soil treatment
Soil health tests can be used to adjust soil pH levels and increase alkalinity, as Fusarium fungi are generally more active in acidic soils. In addition, organic fertilizers or compost can be added to the soil, which improves soil health and reduces fungal activity.
3. Soil Solarization
It is an effective method in which the soil is sterilized using direct heat from the sun. In this method, the soil is sterilized by sunlight by covering it with plastic black polythene.
4. Clean cultivation
The land should be kept clean before and after the start of cultivation and try not to leave remnants of the previous crop such as leaves or roots on the land. Maintaining cleanliness in the soil reduces the spread of Fusarium fungus.
5. Disinfection of equipment
Cultivation implements like spades, plows etc. should be sterilized to prevent the spread of Fusarium fungus from infected soil or plants. Disinfection of equipment used during field cultivation can control the spread of disease.
6. Raised beds and drainage systems
After 2-3 days of making the raised bed and drainage system, the bed should be made. Before planting the seeds or saplings, the land should be plowed and the beds should be prepared with a ladder. After preparing the land, beds of 2m width-length and 20-25 cm height should be made. By creating a raised bed, water does not accumulate at the base of the plants due to untimely rains and heavy rains. As a result, the incidence of disease will be reduced. 75-100 cm wide irrigation and drainage channels should be kept between the two beds for quick drainage of rain and irrigation water.
7. Biological Control
Soil application of beneficial bacteria such as Trichoderma spp., Bacillus subtilis and Pseudomonas florescense can help reduce the activity of Fusarium fungi. These beneficial bacteria inhibit the growth of Fusarium fungi and limit the spread of Fusarium fungi in the soil. Organic fertilizers such as Tricost, Biordama can be used to improve soil health and control Fusarium wilt disease. It increases soil nutrients and reduces fungal activity.
8. Fungicide
Fungicides such as carbendazim, mancozeb, ezoxystrobin or copper oxychloride can be used to control downy mildew of watermelon. However, it is very important to follow the rules of use and quantity in the use of fungicides.
9. Integrated Management
Integrated management involves first treating the seeds and sowing the seeds in polybags of treated soil to produce seedlings inside mosquito nets. Bio fumigation should be done on the soil of the land and the beds should be raised and proper drainage system should be maintained. Organic fungicides should be applied to the soil, the bed should be covered with mulch paper and only 4-6 cm along the soil. Diameter holes should be taken. After 15-20 days of transplanting in the field, organic herbicides should be applied, moderate irrigation should be given and fungicides should be applied as soon as disease symptoms appear.
Conclusion
It is very important to take appropriate measures to control the downy mildew of watermelon. Watermelon growers suffer severe losses due to downy mildew and reduced crop productivity. The impact of this disease can be minimized by proper management, crop rotation practices, use of resistant varieties, proper irrigation management, and appropriate fungicide application.

The writer is a, Dr. Md. Mahfuz Alam, Chief Scientific Officer (Cereals), BARC, Dhaka.