Phytophthora Leaf Blight of Taro (PDF) - Disease, Symptoms, Management

Phytophthora leaf blight of colocasiabn (Taro) Leaf blight caused by Phytophthora colocasiae Rac. is the most destructive disease of taro causing a 25-50% loss in yield. Besides, the pathogen also causes the serious post-harvest decay of corms. Butler and Kulkarni (1913) reported leaf blight...

Phytophthora leaf blight of colocasiabn (Taro) Leaf blight caused by Phytophthora colocasiae Rac. is the most destructive disease of taro causing a 25-50% loss in yield. Besides, the pathogen also causes the serious post-harvest decay of corms. Butler and Kulkarni (1913) reported leaf blight of taro for the first time in India, which had been observed at various places in India since 1905 causing serious damage. Causal organism/Pathogen Phytophthora blight of colocasia is caused by fungus Phytophthora colocasiae (Kingdom-Oomycota). The fungus produces asexual spores called sporangia and sexual spore as oospore. These sporangia are oval, hyaline, semi-papillate (tip of spore is not pointed), deciduous (spores fall from the colony), and have a short stalk or pedicel attached to the base of the spore. These sporangia release swimming spores called zoospores when water or sufficient moisture is present. The sporangia can germinate directly and form producing germ tubes, but under most conditions’ germination is indirect on the upper surface of leaves and on petioles. Zoospores are able to swim for hours and are attracted to organic matter or host tissue. These spores will stop swimming, encyst (the tails or flagella are lost), and produce a germ tube to penetrate the host. This pathogen has two mating types, A1 and A2. When grown together in culture dishes, sexual spores called oospores are formed. These oospores are thick walled and may serve as survival structures in the absence of the host. However, oospores have not been found in nature. This pathogen grows well between 68 to 77 F. Moisture favors the growth, sporulation, dissemination, and infection processes of this pathogen. Symptoms The early stage of leaf blights is characterized by the formation of small, dark to light brown to olive-green spot often soccur at the tips and margins of upper surface of the leaves where water accumulates. The spots enlarge rapidly, becoming wilted, circular, zonate, and purplish brown to dark brown to black in color. On the lower leaf surface, spots have a water-soaked or dry gray appearance. As spots increase in size they coalesce and quickly destroy the leaf Drops of a clear, amber liquid can be found on the surface of Phytophthora spots. Expansion of leaf spots in moist weather is extremely rapid in susceptible cultivars and large sections of the leaf are rotted in a few days. Favourable conditions This pathogen grows well between 28°C to 30°C The warm, wet Moist climate favours the growth, sporulation, dissemination, and infection processes of this pathogen. Disease cycle Sporangia of this pathogen detach from sporangiophores at the edge of lesions and germinate on leaves and petioles (stalks) or are washed into the soil where they can infect taro corms. These sporangia are readily distributed by splashing water, wind, and rain. On new leaves the fungus rapidly germinates and penetrates the host. The pathogen survives as mycelium in plant tissues or as encysted zoospores in soil. Corms left in the field after harvest can also serve as inoculum sources for newly planted taro plots. If the A1 and A2 mating types or an A1A2 mating type are present, antheridia and oogonia can form and produce oospores. Oospores may act as survival structures in plant tissue or in soil between crops. (But this condition is very rare and reported only in the laboratory not in the field). Although the taro leaf has a waxy surface, minute amounts of water that accumulate on either surface of the leaf is enough for the fungal spore to germinate and penetrate the host. In paddy culture, water movement carries the pathogen throughout the field and other adjacent paddies. The fungus is believed to survive in the corm or in soil during dry periods. Movement of the fungus also occurs when infected corms are transported to new locations or fields. Management Rogue (kill and remove) diseased plants, taking them far from the planting area and destroying them by burying, burning. Avoid close planting Disinfestations of corm with hot water Prepare the soil well and amend it before planting if calcium, magnesium, or phosphorus are needed. Add lime material before planting to raise soil pH to 6.0-6.8. Rotate taro with other crops. Incorporate compost and apply surface mulch.