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Watermelon: Diseases and Symptoms

Downy mildew

Disease symptoms
  • Yellow, angular spots restricted by veins resembling mosaic mottling appear on upper surface of leaves.
  • The corresponding lower surface of these spots shows a purplish downy growth in moist weather.
  • The spots turn necrotic with age. The diseased leaves become yellow and fall down. Diseased plants get stunted and die. Fruits produced may not mature and have a poor taste.

Host range: Musk melon, watermelon, sponge gourd and bitter gourd etc.

Survival and spread
  • The pathogen survives in disease plant debris. Primary spread occurs by means of oospores in soil and sporangia from perennial collateral weed hosts in the vicinity. Secondary spread occurs by wind and rain splashes.
Favourable conditions
  • Relative humidity > 85%, high soil moisture, frequent rains.

Powdery mildew

Disease symptoms
  • Whitish powdery growth on upper foliage, stems and young growing parts. The superficial growth ultimately covers the entire leaf area.
  • The diseased areas turn brown and dry leading to premature defoliation and death. Fruits remain underdeveloped and are deformed.

Host range: Pumpkins, watermelon, bottle gourd, coccinia, cucumber, ridge gourd, bitter gourd.

Survival and spread
  • The fungus overwinters in dormant buds or plant parts. Primary infection occurs by dormant mycelium or cleistothecia in infected plant debris or conidia from collateral hosts. Secondary infection occurs by means of wind borne conidia.
Favourable conditions
  • Morning relative humidity > 50%, cool and dry weather.

Anthracnose

Disease symptoms
  • Water soaked lesions are seen on the leaf which later become yellowish irregular spots.
  • Foliage spots are irregular and turn dark brown or black. Stem lesions can girdle the stem and cause vines to wilt.
  • The most striking diagnostic symptoms are produced on the fruit, where circular, black, sunken cankers appear. The spots may measure 1/4 to 1/2 in (6 to 13 mm) in diameter and up to 1/4 in (6 mm) deep.
  • When moisture is present, the black center of the lesion is covered with a gelatinous mass of salmon colored spores.

Host range: Common on watermelon, muskmelon and cucumber.

Survival and spread
  • Fungus survives in disease plant debris and overwinters on diseased residue from the previous vine crop in the form of resting spore. Secondary infection occurs by means of wind borne conidia.
Favorable conditions
  • 24-30°C and high relative humidity up to 100% for 24 h is optimum for fungal growth and infection with a thin film of water on leaf.

Alternaria leaf spot

Disease symptoms
  • The symptoms are first observed at the top of the portion of the plant.
  • Round to irregular target board spots on older leaves are seen.
Survival and spread
  • Fungus survives in soil, and may persist in plant debris.
  • Primary infection occurs by infected soil or inoculum present in plant debris
  • Secondary spread is through rain or wind borne conidia.
Favourable conditions
  • The disease is favored by continuous wet conditions.

Fusarium wilt

Disease symptoms
  • The first symptom appears as chlorosis of the leaves.
  • Wilting of leaves from bottom to top occur.
  • Brown vascular discolouration inside infected stem or root leads to the death of plants.
Survival and spread
  • Pathogen survives in soil in the form of Chlamydospore (resting spore) for many years and primary infection occurs through inoculum present in the soil.
Favourable conditions
  • Relatively high soil moisture and soil temperature are favourable for the infection.

Bud necrosis disease

Disease symptoms
  • Chlorotic rings, mottling, crinckling on leaves are seen, on stunted plants.
  • These spots turn brownish black, and leaves become brown and distorted.
  • The surface of the fruits having ring spots which later turn to tan, necrotic or scab like lesions.
Transmission and favourable conditions
  • Thrips are the main vector for the transmitting the virus. Dry and hot periods when thrips population increase rapidly favours the spread of the disease.

Cucumber mosaic disease

Disease symptoms
  • Symptoms of mosaic appear on the youngest leaves when infection occurs at 6 – 8 leaves stage
  • Leaves curl downwards and become mottled, distorted, wrinkled and reduced in size
  • Veins appear bunchy because of shortening of internodes
  • Fruit set is very less if infection occurs early in crop growth
  • Fruits are often misshapen, mottled, warty and reduced in size

Wide host range: Cucumber, pumpkin, gourds, cowpea, tomato, chilli, etc. Cucumovirus with spherical particles having ssRNA, banana, clover, corn, passion fruit, safflower, spinach, sugarbeet, wild cucumber, Commelina communis, C. diffusa, C. nudiflora, Solanum elaegnifolium, Phytolacca sp., periwinkle, Gladiolus sp., Impatiens sp. and Phlox.

Survival and spread
  • Primary: Virus particles on collateral and other weeds, ornamentals or crops.
  • Secondary: Virus particles transmitted by aphids (Aphis craccivora, Myzus persicae) and spotted and striped cucumber beetles.

IPM for Watermelon

To know the IPM practices for Watermelon, click here.

Source: NIPHM and Directorate of Plant Protection, Quarantine & Storage



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