1. Plant Pathology

Powdery Mildew in Cucurbits: Symptoms, Management

Causal Organism: Erysiphe cichoracearum

Symptoms:
The leaves on the upper surface show small white or grayish superficial spots. These spots enlarge forming large patches of powdery growth on the fungus. Large area, sometimes the entire leaves, are covered by the fungal growth.

Black pinpoint bodies representing the ascigerous stage of the fungus, appear during winter months. When the attack severe defoliation occurs. The fruits from affected plants are very small. Considerable reduction in yield has been noted.

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Etiology:
Erysiphe cichoracearum, the conidia measure 63.8 x 31.9 um, the
elstochecia are globose 80 to 100 um diameter containing 10 to 15 asci, which are 58 to 90 30 to 50 um in size. Each ascus, contains two ascospores and are oval or cylindrical. In S. fuliginea, mycelium is hyaline, occasionally brownish when old.

Conidia are in chains with distinct fibrotic bodies, ellipsoid to barrel-shaped and 25 to 37 x 14 to 25 um. Conidial germ tubes are mostly forked. Perithecia are round, 66 to 98 um in diameter, with various number of tortuous appendages. Asci are broadly elliptic and are 50 to x 12 to 20 um 80 x 30 to 60 pm. In each ascus eight ellipsoid ascospores are found and they are 17 to 22 um.

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Perpetuation:
Perithecia developed on leftover cucurbit crop isolated areas serve as primary inoculum. Wild cucurbits harbour the conidial stage of the fungus and release conidia for primary infection to the spring or summer-sown cucurbits. Conidia are dispersed by wind, thrips and other insects.

Epidemiology:
The disease is favoured by cloudy weather, moderate temperature, reduced light intensity and succulent plant growth. Good conidial germination occurs within a temperature range of 22 to 31°C (with an optimum of 28°C). germination takes place in low relative humidity of 20 per cent less than hr. The incubation period is 3 days.

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Management:

  • Spraying with carbendazim 0.1 per cent or dinocap 0.2 per cent or thiophanate methyl 0.1 per cent or benomyl 0.1 per cent controls powdery mildew effectively.
  • All the affected plant parts should be destroyed.
  • Use tolerant/resistant varieties of cucurbits.

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