Using Ridomil for early season hop downy mildew control

With recent warm weather, hops are starting to emerge in many parts of the province, and growers are thinking about early season downy mildew control.  What follows are some answers to questions we often receive about using Ridomil in Ontario hops.

  • What is it? Ridomil Gold 480 SL is a fungicide containing the active ingredient metalaxy.  Unlike other fungicides registered on Ontario hops, Ridomil is only permitted as an early season soil drench at the root site.  This means there is a fairly narrow window of application, right around the period the hops plants first become active.  It is currently the only Group 4 fungicide registered for control of hop downy mildew in Canada, making it potentially useful in helping to manage fungicide resistance.

  • Don’t US sites suggest Ridomil for use on hops later in the season?  In the US, Ridomil is labelled as a foliar spray on hops, or in irrigation, however this is not the case here.  In Canada, Ridomil can only be applied to hops once per season, as a soil drench (not through the irrigation system), and it cannot be applied less than 135 days before harvest.
  • How does it work? Ridomil is a systemic fungicide that is taken up by the roots and spreads upwards into plant tissue through the sap.  Once taken up, it interacts and interferes with the downy mildew pathogen within the hops tissue, thereby inhibiting systemic infections.  Interestingly, Ridomil only goes up in the plant, not down.  This is why Ridomil is labelled as a soil drench.  The goal is to get the Ridomil down to the roots so it can be taken up by the plant and moved up in the rhizome.  If it were only applied to plant material above the soil, it would not be moved down to underground plant parts, even though it is a systemic product.  Aim to get the Ridomil into the root zone, but not deeper.  Avoid applications immediately before very heavy rain is forecast, or you risk the fungicide being washed below the root zone, where the plant cannot take it up.
  • When should I apply it?  Because Ridomil is spread upward through the sap (the plant xylem), it should be applied to actively growing  plants.  If applied when the plant is dormant, it will not be taken up and moved through the plant tissues.  However, as mentioned earlier, the Pre-Harvest Interval for Ridomil is 135 days.  This means that if you applied Ridomil this Saturday, the earliest that you could harvest your hops is August 28.
  • How long will Ridomil protect my plants?  While Ridomil is systemic, its protective effect is diluted as the plant grows, and it may also be metabolized over time.  Ridomil should help reduce systemic infections already present in the overwintering hop roots, and may provide some protection from very early season foliar infections in the plant tissue present at the time of the application.  However, because hops grow so rapidly, this effect will eventually be diluted and subsequent sprays of fungicides with a different mode of action will be required to protect above-ground hops parts from foliar infections.  Use of Ridomil complements, but does not eliminate, early season cultural practices to manage downy mildew in hops, such as pruning and removal of basal spikes.
  • What about Ridomil resistance? Ridomil resistance has been reported in hops in Oregon and Washington, however, resistance to Ridomil varies with location and downy mildew strain.  The presence of resistance in the Pacific Northwest does not necessarily mean that is also true for strains in Ontario, where this product has not been applied as extensively against hop downy mildew.   This also does not mean that resistance will not occur, so growers should follow all resistance management guidelines on the product label carefully.  In particular, Ridomil 480 EC can only be applied once per season, and only as a soil drench.

If you suspect you are seeing resistance to Ridomil in Ontario hops, we would be interested in hearing about it.  Please contact Melanie Filotas at melanie.filotas@ontario.ca.

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