SOIL FUNGICIDE AND FUMIGANT APPLICATION FOR MANAGEMENT OF ONION STUNT CAUSED BY RHIZOCTONIA SOLANI AG 8

Onion stunt caused by Rhizoctonia solani AG 8 is a soil borne disease causing significant economic loss of onions grown in the Mallee region of South Australia. Currently, no effective control strategies have been developed. This work investigated the use of soil fumigants or fungicide application for the control of onion stunt. Fumigation with metham sodium at 350 L per ha significantly reduced soil borne inoculum of R. solani AG 8 by >82% to <3 pg DNA per g soil, a level shown to pose little disease risk. Onion vegetative growth and bulb yield in fumigated plots was significantly increased over non-fumigated plots. In controlled environment trials using an onion seedling bioassay, several fungicides were evaluated as a soil surface spray application. Products containing the active ingredients difenconazole, iprodione, flutolanil and two experimental fungicide formulations increased onion seedling growth under at least one of the rates evaluated. Treatments evaluating boscalid with pyraclostrobin, azoxystrobin with fludioxonil, iprodione, flutolanil and the two experimental compounds reduced pathogen soil infestation. Fungicides azoxystrobin (Amistar®) and penta-chloronitrobenzene (Terraclor^®) applied as a drench to stunted plants on a commercial property 13 weeks after sowing did not increase bulb weight at harvest and a higher rate of azoxystrobin inhibited bulb development. While strobilurins effectively inhibited Rhizoctonia soil activity, they were phytotoxic and the method of application and rate needs to be adjusted to minimise phytotoxicity to onion seedlings.