Detection of damping-off of Cape gooseberry caused by Pythium aphanidermatum and its suppression with phosphonate

Cape gooseberry cultivated in commercial greenhouses developed wilt and root rot during the summer growing season of 2013 in Aichi Prefecture, Japan. A Pythium species was isolated from affected roots and identified as P. aphanidermatum on the basis of morphological characteristics and sequence homologies in the rDNA internal transcribed spacer regions. In pathogenicity tests, this isolate caused severe wilt and root rot. P. aphanidermatum, which previously was not known to cause damping-off in Cape gooseberry, was isolated by a bait method from soil samples collected from within and outside of the greenhouses. Pre- and post-inoculation activity of phosphonate and two fungicides, cyazofamid and metalaxyl, was evaluated with in vivo experiments. The three compounds were applied 3 days before inoculation, and phosphonate and cyazofamid also were applied at 24 h after inoculation. All three substances provided similar protectant activity. Phosphonate and cyazofamid also exhibited curative activity. In vitro activity of phosphonate on mycelial growth, sporulation, and zoospore cyst germination was evaluated; the EC50 concentration for the inhibition of mycelial growth was low. Zoospore formation was sensitive to phosphonate, while the germination of encysted zoospores was not.