Phytophthora Leaf Blight in Taro Cultivation

### Phytophthora Leaf Blight of Colocasia

• Phytophthora leaf blight, caused by the fungus Phytophthora colocasiae, is a significant threat to taro production, leading to yield losses of 25-50%.
• The pathogen also infects taro corms, causing post-harvest decay.
• The disease was first reported in India in 1905 and has been observed at various locations in the country.
• The fungus produces asexual spores called sporangia and sexual spores called oospores.
• Sporangia are oval, semi-papillate, deciduous, and have a short stalk.
• They release swimming spores called zoospores when water is present.
• Sporangia can germinate directly or indirectly, forming germ tubes.
• Zoospores can swim for hours, attracted to organic matter or host tissue, before encysting and producing a germ tube to penetrate the host.
• The pathogen has two mating types, A1 and A2, which can produce oospores when grown together, serving as survival structures in the absence of a host.
• While oospores have not been found in nature, they have been observed in laboratory settings.
• The pathogen thrives in warm, moist environments with temperatures between 28°C and 30°C.
• Early symptoms include the formation of small, dark to light brown or olive-green spots, often at the tips and margins of leaves where water accumulates.
• Spots enlarge rapidly, becoming wilted, circular, and purplish brown to dark brown to black in color.
• On the lower surface of leaves, spots have a water-soaked or dry gray appearance.
• As spots increase in size, they merge, leading to leaf decay.
• Clear, amber liquid may be observed on the surface of infected spots.
• Infection spreads rapidly in susceptible cultivars, causing significant damage in moist weather.
• Sporangia can detach from sporangiophores at the edge of lesions and infect leaves, petioles, or taro corms in the soil.
• They spread through splashing water, wind, and rain.
• The pathogen survives as mycelium in plant tissues, as encysted zoospores in soil, and in infected corms left in the field after harvest.
• The pathogen can also spread through the movement of infected corms.
• Despite the waxy surface of taro leaves, small amounts of water accumulation are sufficient for fungal spore germination and penetration.
• In paddy culture, water movement facilitates the spread of the pathogen within and between fields.
• The fungus is believed to persist in corms or soil during dry periods.
• Management strategies include:
  • Removing and destroying diseased plants.
  • Avoiding close planting.
  • Disinfecting corms with hot water.
  • Soil preparation and amendment to meet calcium, magnesium, and phosphorus needs.
  • Increasing soil pH to 6.0-6.8 by adding lime material before planting.
  • Crop rotation with non-host plants.
  • Incorporating compost and applying surface mulch.