Author
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Gulya Jr, Thomas |
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Submitted to: Brighton Conference Symposium Proceedings
Publication Type: Proceedings Publication Acceptance Date: 11/1/2002 Publication Date: 12/1/2002 Citation: GULYA JR, T.J. EFFICACY OF SINGLE AND TWO-WAY FUNGICIDE SEED TREATMENTS FOR THE CONTROL OF METALAXYL-RESISTANT STRAINS OF PLASMOPARA HALSTEDII (SUNFLOWER DOWNY MILDEW). BRIGHTON CONFERENCE SYMPOSIUM PROCEEDINGS. 2002. P. 575-580. Interpretive Summary: Downy mildew of sunflower is a systemic disease that attacks young seedlings and generally kills them. For the last 15 years, seed companies have been treating sunflower seed with the fungicide metalaxyl (APRON), which offered good control of the disease. In the last three years, however, the downy mildew fungus has developed tolerance to this fungicide, rendering it completely ineffective. While seed companies hurry to develop hybrids genetically resistant to all races of downy mildew, there is an immediate, short term need for a new fungicide to control this mildew strain which is resistant to metalaxyl. Greenhouse and field trials over the last three years have narrowed down the list of fungicides from 30 to three: azoxystrobin, fenamidone, and zoximide. These fungicides, however, do not completely control downy mildew infection when used alone, and will need to be combined with another fungicide. Two other fungicides, mancozeb and ethaboxam, could also serve as "cofungicides." None of these fungicides are currently labeled for use on sunflower, although azoxystrobin and fenamidone are currently in the registration process. Technical Abstract: Control of the metalaxyl resistant strain of Plasmopara halstedii (causal agent of sunflower downy mildew) was studied in greenhouse and field experiments over a three-year period. Of 30 fungicides investigated, only azoxystrobin and fenamidone showed good efficacy as seed treatments for control of systemic symptoms initiated by root infection. Four fungicides displaying lesser disease control (ethaboxam, fluazinam, mancozeb, and zoximide) were tested as cofungicides with azoxystrobin and fenamidone. None of the fungicides, either used alone or in combination, gave complete control under natural or inoculated field conditions, in contrast to the total control previously experienced with metalaxyl. Disease incidence on all fungicide treatments increased from 2 wk to 7 wk after emergence, suggesting that the fungicide/rates tested were more fungistatic than fungicidal. Fenamidone was the most effective fungicide used singly, and fenamidone combined with either ethaboxam or zoximide was the most effective combination. Greenhouse tests of fungicide seed treatments were not effective at predicting field response of the best treatments, despite several modifications. The greenhouse tests, however, were satisfactory to identify ineffective fungicides. None of the three fungicides likely to be registered as sunflower seed treatments in the United States, namely azoxystrobin, fenamidone, and zoximide, displayed any potential phytotoxicity or seed safety concerns in preliminary laboratory and field emergence tests, intended to run for two years. |
