Occurrence of oosporic stage of Pseudoperonospora cubensis on cucumber, in Punjab, India: A first report

Downy mildew disease in cucurbits is a major constraint to sustainable production of cucurbits in India. The pathogen is known to perpetuate as active mycelium, sporangia and oospores. But in India, the oospores were not reported to play an active role in survival of the pathogen. A little is known about the complete life cycle of P. cubensis, thus, in this study an attempt was made to determine whether the pathogen form oospore under Indian conditions. A survey was undertaken in major cucumber growing regions of Indian Punjab and leaf samples showing symptoms of downy mildew infection were collected. The average size of oospore of P. cubensis ranged from 24 to 28 mu m in diameter. The oospores were yellow to light brown in colour with smooth thick outerwall. Species specific primers corresponding to Cytochrome oxidase subunit 2 (COX-2) gene were further used for molecular identification and confirmation of the fungus. Ludhiana, MW855102 (Ldh10) and Sangrur, MW855103 (SC6) isolates shared maximum nucleotide identity i.e 98.96 per cent (%) with the P. cubensis isolates from Germany and Malaysia. This is the first morphological and molecular evidence of occurrence of oospores of P. cubensis in India. These evidences suggest that, oospores can survive as overwintering stage under Indian conditions and can thus play an important role in causation of the disease.

Automated summary

Downy mildew disease is a major constraint to sustainable cucurbit production in India. This study provides the first morphological and molecular evidence of the occurrence of oospores of P. cubensis in India. These findings suggest that oospores can survive as an overwintering stage under Indian conditions and play an important role in the causation of the disease.