Characterization of Phytophthora colocasiae raci. isolates causing blight of colocasia in north-western Himalayas

Leaf blight and corm rot incited by Phytophthora colocasiae is the most devastating disease of colocasia. Twenty isolates of P. colocasiae were isolated from the disease samples collected from five districts of Himachal Pradesh and characterized based on morphocultural variability, pathogenic variability, chlamydospore formation and mating type. Among different media tested carrot agar was best medium for mycelial growth and sporulation of P. colocasiae. On the basis of morpho-cultural characters and pathogenic variability, twenty isolates were categorised into six and five groups, respectively. Only four isolates formed chlamydospores abundantly under dark condition at pH 6.0 in carrot broth incubated at 18°C Out of twenty isolates 18 were of A₁ mating type where as, 2 were of mating type A₂. Maximum oospore formation frequency was on carrot agar media at 25°C and was favored by dark conditions.