Downy Mildew
Causal Organism:
- Pseudoperonospora cubensis
- Sub-division: Mastigomycotina
Etiology:
- The pathogen is an obligate parasite that survives on collateral hosts or in plant debris.
- Produces sporangia that are dispersed by wind or splashing rainwater.
Symptoms:
- On Leaves:
- Yellow, angular spots on the upper surface.
- Purplish downy growth on the lower surface under moist conditions.
- Leaves turn yellow, dry, and fall off prematurely.
- Stunted plant growth and poor fruit development.
Disease Cycle:
- Primary Infection: Fungus survives on collateral hosts or plant debris.
- Secondary Infection: Spread by wind-borne sporangia or splashing rainwater.
Favorable Conditions:
- Cool (15-25°C) and moist weather.
- High relative humidity (>85%).
Management:
- Cultural Practices:
- Remove and destroy infected plant debris.
- Avoid overhead irrigation to reduce leaf wetness.
- Practice crop rotation with non-cucurbit crops.
- Chemical Control:
- Metalaxyl + Mancozeb @ 0.2% spray at the first sign of disease.
- Repeat applications at 7-10 day intervals during favorable conditions.
Powdery Mildew
Causal Organisms:
- Erysiphe cichoracearum
- Sphaerotheca fuligena
- Sub-division: Ascomycotina
Etiology:
- Obligate parasite producing conidia that are dispersed by wind.
- Can survive in crop debris or as cleistothecia (sexual spores).
Symptoms:
- On Foliage, Stems, and Growing Parts:
- Whitish or dirty grey powdery growth.
- Affected areas turn brown and dry up.
- Severe infection causes premature leaf fall and plant death.
Disease Cycle:
- Primary Infection: Initiated by ascospores from cleistothecia.
- Secondary Infection: Spread by wind-borne conidia.
Favorable Conditions:
- Cool dry weather for conidial germination (20-27°C).
- High Relative Humidity (>50%) favors disease development.
Management:
- Cultural Practices: Use resistant varieties when available. Ensure proper plant spacing for good air circulation.
- Chemical Control:
- Spray Calixin @ 0.1% or Karathane @ 0.2%.
- Sulfur dusting or Neem oil sprays are also effective.
Vascular Wilt
Causal Organism:
- Erwinia tracheiphila
- Type: Bacterial Disease
Etiology:
- Bacterium is transmitted by the Striped Cucumber Beetle (Acalymma vittatum) and Spotted Cucumber Beetle (Diabrotica undecimpunctata).
- Bacteria enter through feeding wounds on the plant.
Symptoms:
- On Leaves and Whole Plant:
- Initial wilting of a single leaf that spreads throughout the plant.
- Leaves become dull green and wilt without yellowing.
- Entire plant collapses and dies.
- Creamy white bacterial ooze seen when infected stem is cut and squeezed.
Disease Cycle:
- Primary Infection: Bacteria survive in the gut of striped cucumber beetles.
- Secondary Infection: Spread occurs as beetles move from plant to plant while feeding.
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Favorable Conditions:
- Monocropping of cucurbits increases beetle populations.
- High beetle populations during warm growing seasons.
Management:
- Cultural Practices:
- Plant resistant cultivars if available.
- Remove and destroy infected plants to prevent spread.
- Insect Control:
- Carbaryl (Sevin), Malathion, or Rotenone sprays to control cucumber beetles.
- Row covers can be used to protect young plants from beetle feeding.
- Preventive Measures:
- Start insect control early, as soon as seedlings emerge.
- Practice crop rotation to break the life cycle of the beetles.
