An Ultimate Guide To Watermelon Diseases: Causes, Symptoms And Control Measures

Watermelon is an important fruit crop in India widely grown in summer season and has the potential to provide significant economic benefits to farmers. They are easy to grow, require relatively little maintenance and have short growing season. However, the crop is vulnerable to a number of fungal, bacterial and viral diseases that can cause significant yield losses and reduce the quality of the fruit. It can be difficult to control watermelon diseases once they are established in the field. Early detection and prompt management using appropriate fungicides or bactericides can help to limit the spread of disease and minimize crop losses.  

Common types of Watermelon Diseases 

Fungal Diseases in Watermelon

1. Downy Mildew in Watermelon

Downy mildew in watermelon is caused by the fungal pathogen Pseudoperonospora cubensis. 

Causes:  

Presence of infected plant debris and weed hosts cause the primary spread of the disease. Wind and rain splash causes the spread of spores to the healthy plants. High soil moisture, cool, wet and humid conditions and temperature (15 – 23°C) favours disease incidence. 

Symptoms of Downy Mildew in Watermelon:  

• Yellowish angular spots appear on the upper surface of the leaves. The yellowing often starts at the edges of the leaves and then spreads towards the center. 
• As the disease progresses, a white or grayish powdery fungal growth appears on the underside of the leaves when the leaves are wet.  
• These spots later turn brown to black (necrosis). Later, the leaves may wilt and die. 
• It can cause stunted growth of the plants. Affected plants may appear smaller than usual, with fewer fruits. 
• In case of severe infection, the fruits produced may be smaller, misshapen or have bitter taste. 

Management of Downy Mildew in Watermelon:  

2. Powdery Mildew in Watermelon

Powdery mildew is a common and destructive disease caused by the fungal pathogen Erysiphe cichoracearum / Sphaerotheca fuliginea. 

Causes: 

Fungal spores overwintering in dormant buds, plant debris or weeds causes disease development. Wind currents spread the disease. Rainfall, morning dew, dry weather conditions favours disease incidence.   

Symptoms of Powdery Mildew in Watermelon: 

• White, powdery spots or patches on the leaves, stems and young growing parts of the plant. Later, it may spread rapidly and cover the entire leaf surface.  
• The white powdery spots may gradually turn yellow or brown and become necrotic. 
• The affected leaves may curl or become distorted and the fruit may stop growing or become deformed. 
• Leads to premature defoliation and death of affected areas. 

Management of Powdery Mildew in Watermelon: 

3. Anthracnose in Watermelon

Anthracnose is a fungal disease caused by the pathogen Colletotrichum orbiculare / Colletotrichum lagenarium affects all parts of the plant and reduces fruit quality. 

Causes: 

Fungal spores over wintering in the soil causes disease development. Temperature (24 – 30°C), high humidity, leaf wetness favours anthracnose incidence in watermelon plants.  

Symptoms of Anthracnose in Watermelon:

• Small, circular or irregularly shaped spots appear on leaves, stems and fruit that are dark brown or black in color. 
• The spots may have a water-soaked appearance and may be surrounded by a yellow halo. 
• In severe cases, the affected leaves drop prematurely. 
• Lesions on the stem girdle the vascular tissues and causes wilting of wines. 
• Infected fruit may show sunken lesions, cracks and rot. 

Management of Anthracnose in Watermelon: 

4. Alternaria Leaf Spot / Alternaria Blight in Watermelon: 

Alternaria leaf spot is a fungal disease caused by the pathogen Alternaria cucumerina. 

Causes: 

Fungus overwintering in the soil debris causes primary spread of the disease. Weakened plants due to lack of proper fertilization, warm weather, continuous wet conditions favours disease development.  

Symptoms of Alternaria Leaf Spot in Watermelon: 

• Small, circular or irregularly shaped spots appear on leaves that are initially water-soaked, then turn brown or black as they dry out. 
• The spots may have a yellow halo and may merge to form larger lesions. 
• The infected leaves may become distorted, wilt, and eventually die. 
• Affected fruits may have lesions on the surface that are sunken, brown, and dry. 
• In severe cases, the disease can cause complete defoliation of vines and reduce fruit quality and yield.  

Management of Alternaria Leaf Spot in Watermelon: 

5. Fusarium Wilt in Watermelon

Fusarium Wilt is a fungal disease caused by the pathogen Fusarium oxysporum that can cause severe yield loss, if not managed at right time. 

Causes: 

Infected soil, wounds in roots and infected seeds causes fungal spread. Secondary spread occurs through wind, tools or equipments. High soil temperature and soil moisture favours disease infection.  

Symptoms of Fusarium Wilt in Watermelon: 

• Stunted growth of the plant and yellowing of leaves, especially older leaves. 
• Leaves may wilt and become brittle, eventually die. 
• Vascular tissue (xylem) of the stem and roots may show brown discoloration and the stems may develop cracks. 
• In severe cases, the plant may die. 
• Wilting symptoms during maturity stage can cause premature fruit drops, reduced yield and quality. 

Management of Fusarium Wilt in Watermelon:  

6. Gummy Stem Blight in Watermelon

Gummy stem blight is a serious fungal disease caused by the pathogen Didymella bryoniae. 

Causes: 

Humidity (>85%), rainfall, longer period of leaf wetness, overhead irrigation and infected seeds/transplant materials may cause disease development. The optimum temperature for the disease incidence is about 24°C. Presence of wounds, feeding activity of cucumber beetle and aphids, along with powdery mildew incidence may induce development of gummy stem blight infection. 

Symptoms of Gummy Stem Blight in Watermelon: 

• Brown to black, circular water-soaked lesions appear on stems which later become dry and wilt 
• Irregular tan to brown blotches appears on the leaves which may lead to wilting and blight of foliage. 
• The development of a gummy, reddish brown substance on the surface of the lesions, which gives the disease its name. 
• Premature senescence and defoliation of the plants. 

Management of Gummy Stem Blight in Watermelon: 

Bacterial Diseases in Watermelon

7. Bacterial wilt in Watermelon

Bacterial wilt is a destructive disease caused by the bacterium Erwinia tracheiphila.  

Vector – Cucumber beetle 

Causes: 

The bacterium responsible for bacterial wilt is transmitted by the striped or spotted cucumber beetle, which feeds on the leaves of the plant and then transfers the bacteria to the stem. Presence of plant debris or alternate host, wounds in root system, high temperature, high humidity and soil moisture and alkaline pH of soil favours disease incidence. Heavy soils that can retain soil moisture for long duration are highly vulnerable to the disease.  

Symptoms of Bacterial Wilt in Watermelon: 

• Sudden wilting of leaves which later turn dull green but remain attached to the stem. 
• Wilting of the entire plant, which progresses from the point of bacterial entry through vascular tissue towards the main stem.  
• Death of the plant within a few days of infection. 
• Sticky or slimy exudates on the surface of the stem near the soil line. 
• Collapse of the stem when the plant is cut at or above the soil line.  

Management of Bacterial Wilt in Watermelon: 

Management of Vector – Cucumber Beetle that transmit Bacterial Wilt:  

8. Bacterial Fruit Blotch of Watermelon

Bacterial fruit blotch is a bacterial disease caused by the pathogen Acidovorax citrulli. 

Causes: 

Seeds from infected fruit, plant debris in the soil, presence of weed hosts causes bacterial fruit blotch infection. Infected seeds are the primary source of this disease spread. While over-head irrigation, mechanical transmission through workers’ hands and tools or equipment acts as secondary source of infection. High temperature (>32°C) and high humidity also favours disease incidence. 

Symptoms of Bacterial Fruit Blotch in Watermelon: 

• Dark reddish-brown spots may develop along the leaf veins. 
• Dark green to brown, water-soaked spots on the fruit, which can appear in a circular pattern or as a long streak. It can cause the development of distinctive cracks on the fruit surface. 
• The flesh underneath the spots may become soft, watery and discolored. 
• Infected fruit may develop a sour smell and ooze a sticky, tan-colored substance. 
• Stunting and reduced yield of the plant. 

Management of Bacterial Fruit Blotch in Watermelon: 

Viral Diseases in Watermelon

9. Bud Necrosis Disease in Watermelon

Bud necrosis is caused by Tomato Spotted Wilt Virus (TOSPO virus) 

Vector – Thrips 

Causes: 

Main source of transmitting this virus in watermelon is thrips. Presence of alternate hosts, dense planting, hot and dry weather favors thrips population which increases the spread of the disease.  

Symptoms of Bud Necrosis Disease in Watermelon: 

• Yellowing or bronzing of leaves, especially younger leaves. 
• Small, dark brown or black spots or rings may develop on the leaves. 
• The new growth or buds may become stunted and show brown or black necrotic spots and hence the disease name ‘Bud necrosis’. It results in decay of floral structures. 
• The fruit surface may also showring spots and may become deformed or discolored. 
• In severe cases, the plant may become stunted and eventually die. 

10. Cucumber Mosaic Virus (CMV) in Watermelon

Vector – Aphids 

Causes:  

The virus is carried and transmitted through vector aphids. Other means of transmission include infected seeds and grafts, weeds, mechanical transmission through tools or equipments and farm workers hands. 

Symptoms of Cucumber Mosaic Virus in Watermelon:  

• Leaves may curl downward and show mottled or streaked discoloration, usually with a yellow-green color. 
• Leaves may also show puckering or distortion and may become brittle or necrotic. 
• Veins of the affected leaves appear bushy due to shortening of internodes. 
• The fruit may show a yellow or green mosaic pattern and may become deformed or small. 
• The plant may become stunted or distorted, and in severe cases, may die. 
• The virus can also reduce the overall yield and quality of the fruit. 

Management of Aphids and Thrips in Watermelon: 

NOTE:  

• Refer to the product’s label before use to find the right time of application.  
• Bio-fungicide and chemical fungicide are incompatible., therefore they should not be mixed together.  
• It is advised to spray Bio-fungicides during early morning or evening hours when the temperature is relatively cool to get best results. 

Conclusion 

Watermelon is susceptible to several diseases caused by fungi, bacteria, and viruses. To manage these diseases, it is important to implement good cultural practices such as crop rotation, proper irrigation, fertilization and spacing of plants. In addition, maintaining proper field sanitation can be effective in preventing or controlling disease outbreaks since most of the diseases are spread through over wintering spores in crop debris. Right time and right dose of application of chemicals to control diseases and disease-causing vectors can help to reduce yield loss. By implementing these measures, you can protect your watermelon crop and achieve maximum yields.  

Discover how to safeguard your watermelon crop from these pests. CLICK HERE