Managing the entomopathogenic fungus Conidiobolus thromboides in Russian wheat aphid cultures

Six fungicides were evaluated in vitro for their efficacy in inhibiting vegetative growth and sporulation of the entomopathogenic fungus, Conidiobolus thromboides Drechsler, a contaminant in greenhouse cultures of the Russian wheat aphid, Diuraphis noxia (Kurdjumov). The fungicides copper oxychloride (Cupravit), mancozeb + oxadixyl (Sandofan), captab + metalaxyl (Apron C), bitertanol (Baycor), iprodione (Rovral Dip), and mancozeb (Sancozeb) at a rate of 0.1% active ingredient moderately to strongly inhibited C. thromboides (mean inhibition 81.1%). No true fungicidal activity was observed with any of the fungicides during the test period (7 d at 20 degree C). Amount of sporulation of the fungus varied among treatments, with no spores formed on the mancozeb + oxadixyl-treated medium, versus profuse sporulation on medium treated with bitertanol or iprodione. Because of strong inhibition of C. thromboides and relatively low cost, mancozeb at concentrations of 10.0, 2.0, 1.25, 0.5, 0.08, and 0.016% was further evaluated in vitro. The fungus growth cut-off point, midway between 1.25 and 2.0% mancozeb, was calculated, and a rate of 1.625% active ingredient per liter of water was used to treat greenhouses housing C. thromboides-infected Russian wheat aphid colonies. In addition, sanitary measures were followed diligently, which included fumigation of contaminated greenhouses with potassium permanganate + formaldehyde, construction of new rearing cages, and a change in the watering protocol. These curative and preventative measures were used successfully to decontaminate the Russian wheat aphid colonies.