Suppressive effects of low-pH solutions on root rot in hydroponically grown Welsh onion

Abstract

Root rot of hydroponically grown Welsh onion has caused considerable economic losses every year since commercial cultivation began in 1988 in Hiroshima Prefecture, Japan. The primary pathogens, which have not been reported to form sexual structures, are Pythium spp. including Pythium clusters B2a sp., which is most prevalent, and B1d sp. and Pythium clade A sp. These pathogens infect Welsh onion roots by producing sporangia and zoospores. The objective of this study was to elucidate the inhibitory effects of low-pH solutions on root-rot pathogens of hydroponically grown Welsh onion. Naturally occurring disease was significantly lower after treatment with nutrient solutions with a pH maintained at 4.3–4.9 than with conventional unmanaged solutions (pH 5.4–6.4). In a greenhouse after inoculation of infected Welsh onion with Pythium cluster B2a sp., low-pH solutions (pH 4.0 and 4.5) significantly suppressed zoospore production and reduced the percentage of root infections compared to the conventional solution (pH 5.5). In other greenhouse inoculation tests with Pythium clusters B2a sp. and B1d sp. and Pythium clade A sp., the low-pH solution (pH 4.0) substantially decreased zoospore density and root infection compared to the conventional solution (pH 5.5). The low-pH solutions did not negatively impact crop yield in the greenhouse tests and thus can be used to suppress root rot in hydroponically grown Welsh onions.