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

Watermelon, scientifically known as Citrullus lanatus, belongs to the family Cucurbitaceae which includes other plants such as cucumber, squash and pumpkin. The watermelon plant is a vine-like plant that is typically grown in warm, tropical or subtropical climates. The fruit is dominant during summer season especially due to its high-water content and delicious taste. Major watermelon producing states in India includes Uttar Pradesh, Andhra Pradesh, Tamil Nadu, Karnataka and Orissa. Watermelon crops are susceptible to a variety of pests that can cause significant damage to the plants and reduce the overall yield of the crop.  

Common insect pests affecting watermelon crops are discussed here; 

1. Red Pumpkin Beetle

Scientific name: Aulacophora foveicollis 

Pest stage of attack: Larva and adult 

Stage of occurrence: Seedling/Vegetative stage 

Identification of Symptoms

• Complete destruction of seedling due to adult feeding 
• Leaves show riddled holes or notches due to beetle infestation leading to complete defoliation 
• Larvae feed on roots and stems causing rotting and withering  

• The beetles may also feed on the fruits, causing scars or holes, which can lead to rotting or sunburned spots on the fruit 
• The undersurface of the fruits touching the soil surface will have larval tunnels in the surface. 

Management of Red Pumpkin Beetle in Watermelon

2. Fruit fly

Scientific name: Bactrocera cucurbitae 

Pest stage of attack: Maggots 

Stage of occurrence: Fruiting stage 

Identification of Symptoms

• The female fruit fly lays eggs under the skin of the fruit, which causes small puncture marks on the surface of the watermelon 
• After the eggs hatch, the larvae burrow into the fruit and feed on the flesh, contaminating it with frass excreta, thus making it susceptible to secondary infections by fungi and bacteria 
• The damage caused by the fruit flies can cause rotting and premature dropping of fruits 
• Infected fruits become distorted and malformed. 

Management of Fruit fly in Watermelon

3. Thrips

Scientific name: Thrips tabaci 

Pest stage of attack: Larva and adult 

Stage of occurrence: Vegetative, flowering, fruiting stage  

Identification of Symptoms

• Thrips feed on the leaf surface and suck the plant juice causing small, silvery white or stippling spots on the leaves 
• Under severe infestation, the leaves turn yellow, wither and become deformed. This can lead to reduced plant growth and yield 
• They may feed on the flowers of the plant, causing them to drop prematurely, which can reduce the number of fruit that the plant produces 
• Thrips may also feed on the watermelon fruits causing scarred or distorted fruit. 

Management of Thrips in Watermelon

4. Aphids

Scientific name: Peach aphid – Myzus persicae; Melon aphid – Aphis gossypii 

Pest stage of attack: Nymph and adult 

Stage of occurrence: Seedling, Vegetative, flowering  

Vector: Watermelon Mosaic Virus 

Identification of Symptoms

• Small, pear-shaped, soft bodies insects can be found on leaves or stems of the plant 
• Aphids’ feeds on the tender shoots and under surface of the leaves by sucking the leaf sap causing curling and crinkling of the leaves 
• They secrete a sticky, sugary substance called honeydew, which causes black sooty mould development that accumulates on the leaves and attracts other insects, such as ants 
• This also reduces the photosynthetic activity leading to stunted growth. 

Management of Aphids in Watermelon

5. Whitefly

Scientific name: Bemisia tabaci 

Pest stage of attack: Nymph and adult 

Stage of occurrence: Vegetative, flowering stage 

Vector: Watermelon leaf curl virus 

Identification of Symptoms

• Feeding activity of nymph and adult may lead to yellowing, downward curling and drying of leaves 
• They also cause sooty mould development due to honeydew secretion causing restricted photosynthesis 
• It can cause the leaves to turn yellow (chlorotic spots) and fall off prematurely 
• Affected leaves may become deformed.  

Management of Whitefly in Watermelon

6. Leaf eating caterpillar

Scientific name: Diaphania indica 

Pest stage of attack: Larva 

Stage of occurrence: Vegetative stage 

Identification of Symptoms

• Caterpillars feed on the young and tender leaves of watermelon plants, which can result in holes and ragged edges on the leaves 
• They fold the leaves and scrape the epidermal layer of leaves causing skeletonization of the leaves, where only the veins of the leaf remain intact 
• Under severe infestation, it can cause defoliation of leaves, thus reducing plant growth and vigor, resulting in smaller plants and fruit. 

Management of Leaf eating caterpillar in Watermelon

7. Serpentine Leaf Miner

Scientific name: Liriomyza trifolli 

Pest stage of attack: Larva 

Stage of occurrence: Vegetative, flowering and fruiting stage 

Identification of Symptoms

• Serpentine leaf miners create winding, snake-like tunnels on the leaves as they feed on the leaf tissue 
• It can cause the infested leaves to turn yellow or brown 
• Under severe infestation, these leaves dry and drop off from the plant 
• It may lead to stunted growth of the plant and reduce the overall plant vigour 
• The larva sometimes tunnels through the rind of the fruit, causing scars and reducing the fruit’s quality and marketability.  

Management of Serpentine Leaf Miner in Watermelon

8. Red Spider Mite

Scientific name: Tetranychus urticae 

Pest stage of attack: Adult 

Stage of occurrence: Vegetative, flowering and fruiting stage 

Identification of Symptoms

• Red spider mites using their long, needle like mouthpart, suck the chlorophyll content in the leaves and causes chlorosis  
• They feed on the underside of leaves, which can result in stippling or small yellow or white dots on the upper surface of the leaves 
• They often produce fine webbing on the leaf surface 
• Affected leaves completely dry and drop off.  

Management of Red Spider Mite in Watermelon

9. Cutworms

Scientific name: Agrotis spp, Peridroma saucia, Nephelodes minians, Spodoptera litura  

Pest stage of attack: Larva 

Stage of occurrence: Seedling stage  

Identification of Symptoms

• Cutworms often feed on the stems of young watermelon plants, cutting them at or near the soil line. This can cause the plant to wilt or die 
• They feed on the leaves, causing irregular holes or notches in the foliage 
• They are nocturnal and may not be visible during the day, but emerge at night, by burrowing in the soil near the base of the plant. 

Management of Cutworms in Watermelon

10. Cucumber Beetle

Scientific name: Diabrotica spp 

Attacking stage of pest – Larva and adult 

Stage of occurrence – Vegetative, flowering and fruiting stage 

Vector – Bacterial wilt disease 

Identification of Symptoms

• Larvae feed on the roots of the plants, causing wilting and stunting 
• Adult cucumber beetles feed on the foliage of watermelon plants, causing defoliation and thus reduces photosynthesis. 

• They feed on the surface of the watermelon fruit, leaving shallow, irregularly shaped scars on the rind. This type of cosmetic damage may not affect the quality of the fruit, but it can reduce its marketability. 

Management of Cucumber Beetle in Watermelon

NOTE:  

• Refer to the product label to find the right time of application and more details of the product. 
• It is advised to spray Bio-fungicides during early morning or evening hours when the temperature is relatively cool to get best results. 

Conclusion

Insect pests can affect watermelon plants at any stage of growth, from seedlings to mature plants and can cause damage to the leaves, stems and fruit. This can result in reduced plant growth, decreased fruit quality and size and in severe cases, even plant death. Effective pest management strategies, such as regular monitoring, good agronomic practices and the use of chemical and biological control methods, can help in controlling the pest damage, protect the yield and quality of watermelon crops.