1. Plant Pathology

Downey Mildew in Cucurbits: Symptoms, Management

Causal Organism: Pseudoperenospora cubensis

Symptoms:
Downey mildew symptoms are variable. Sometimes, the first symptoms on leaves remind one of mosaic; pale green areas are separated by islands of darker green. Soon the pale green areas change yellow angular spots bounded by leaf veins. During moist weather, the corresponding lower surface is covered with a faint purplish fungus-fruiting layer.

Occasionally, the purplish hue lacking and the colour ranges from white to almost black. The entire leaf dries quickly. Usually leaves near the centre of the hill are affected first. Then the symptoms appear progressively on younger leaves until most leaves are killed.

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Etiology:
It is an obligate parasite. The mycelium is coenocytic and intercellular with small, ovate or finger-like haustoria. One to five sporangiophores arise through the stomata. The upper third of the sporangiophore is branched either dichotomously or intermediately between dichotomous and monochotomous branching.

Spore-bearing tips are subacute. Sporangia are greyish to olivaceous purple, ovoid to ellipsoidal, thin-walled with a distal
papilla and 21 to 39 x 14 to 23 um. Zoospores are 10 13 um in diameter. Oospores are not common.

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Perpetuation:
The pathogen survives on the diseased plant debris. In warm and humid climates from old to younger crops takes place all the year round. Where warm and dry summer alternate with cooler and wet winters, year-round survival is possible on summer crops. They may overwinter as thick walled oospores. Sporangia are disseminated by wind.

Epidemiology:
The fungus can infect plants at temperatures from 10 to 27°C with an optimum of 16.1 to 22.2°C. They survive several days when temperatures are over 37°C. lesion development occurs at cycles of about 25°C day time, 15°C night temperature and a photoperiod of 6 h darkness.

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Management:

  • Use of bed system of cultivation, wider spacing and planting sites with good
  • drainage, air movement and exposure to helps to check disease development by promoting good aeration and rapid drying of plant surfaces.
  • Avoiding successive plantings in adjacent fields during a single season reduce spread from early planting to late planting.
  • The removal and destruction of infected vines helps in reducing the of the spread of the disease.
  • Difolatan 0.2 per cent or mancozeb 0.2 per cent or chlorothalonil 0.2 per cent spray has been found to be effective.
  • Seed treatment with Apron at g/kg followed by spraying with 0.3 per cent mancozeb or daconil 2 kg/ha effective in controlling the disease.

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