Using Ridomil 480EC for early season downy mildew control in hops

It’s already mid-April and the Simcoe Research Station hops are growing.  Many conventional growers have already applied Ridomil to their yards.  If you had planned to apply Ridomil but have not yet done so, the window for applying this product is closing.  The pre-harvest interval for Ridomil 480EC on hops is 135 days – so, for example, if you applied Ridomil yesterday, you could not start harvesting your hops until August 29.

What follows are answers to some of the questions we’ve been receiving lately about Ridomil on hops in Ontario.

How does Ridomil work?

The active ingredient in Ridomil, metalaxyl, 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.

Spreading upwards is a key point – 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.  If it were only applied to plant material above the soil, it would not be moved down to the 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.  Note that Ridomil is labelled only as a soil drench on hops, so it cannot be applied through the irrigation system.

Additionally, 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.

If Ridomil is systemic, will it protect my hops all season?

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.

Isn’t hops downy mildew resistant to Ridomil?

Ridomil resistance has been reported in downy mildew in major hops-producing regions in Oregon and Washington.  However, resistance to Ridomil varies with location and downy mildew strain, so 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.   Although Ridomil is also applied as a foliar spray in the US, this is not permitted on hops in Canada.

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