Early Season Downy Mildew Management in Hops

Hop shoots are emerging in much of southern Ontario and with the onset of warm, rainy weather it’s time for growers to start thinking about early season downy mildew management. Some of the most common questions we get from hop growers at this time of year center around initial fungicide applications and management of those first shoots emerging in the yard.

Spring pruning:

Pruning is often used to collectively refer to the late winter or early spring removal of crown or plant material in a hopyard is a common early season agronomic practice in hops in many growing regions, particularly the Pacific Northwest. It is used both for disease management to reduce early season disease inoculum from shoots systemically infected with downy and powdery mildew, but also for growth management, as secondary hop growth appearing after the removal of the initial “bull” shoots has more uniform growth, facilitating training.

Methods for early spring plant removal

There are 3 main methods of removing this material, with varying degrees of severity:

Crowning – Removal of the upper 2.5-5 cm of the crown of dormant hops, typically in late winter or very early spring, using an offset implement.

Scratching – Use of a harrow-type implement with spinning discs and tongs to remove plant material just below (within 2-5 cm) the soil surface.

Pruning –  Mowing of the first flush of growth (bull shoots) in early spring. In some regions, this may be done as the only spring removal, or in addition to crowning or scratching.

Pruning in Ontario versus other regions:

Although these methods are commonly used in Pacific Northwest hops, the differing growing conditions in this region need to be considered when deciding how to manage spring growth in Ontario. In Washington and Oregon, growing seasons are long and hot and growers are often trying to slow the growth of their plants and there is more time for plants to recover from the effects of crowning or pruning. 

With Ontario’s shorter growing season, hop growers have less leeway in slowing early season plant growth and many growers have found that crowning and scratching set plant growth back so much that they negatively impact plant growth and yields. In general for Ontario hops, the negative impacts of crowning and scratching outweigh any benefits from disease control.

Pruning/mowing of the initial flush of growth are done in some Ontario hopyards, however this must be done quite early in the season to allow plants time to regrow before training, typically at the shoot stage within the first 15 cm of growth.  For hops in southwestern Ontario, where very warm temperatures have promoted some rapid early season growth, this should be done no later than this week, and may be too late, depending on how tall the plants are. For growers in cooler regions, this should be occurring soon, once shoots emerge. Some Ontario growers have found that even with early pruning plants sometimes fail to reach the top wire.

Simcoe Station Pruning:

For the experimental hop yard at the Simcoe Research Station, we used a hybrid prune/ light scratch very early season. Just as hops were breaking dormancy, the soil was mowed to to about 1 cm below the soil line, and the sides of the rows were disced to cut spreading rhizomes and throw some soil on the crowns (see photos). This removed any old growth remaining from the previous season, any shoots just emerging from the soil, plus some buds in upper rhizomes, but without severely cutting into the crown. Note that this was done several weeks ago and is too late to do this now in Ontario.

Left: Simcoe hop yard before spring mowing in early Arpil. Note young shoots emerging among old plant material remaining from previous season. Right: Mowing of hop yard in progress.

Left: Discing of hop rows. Right: Final result after mowing and discing.

Basal Spike Removal

Regardless of whether you prune your hops or leave them and just try to avoid bull shoots at training, all growers should be scouting their hop yard for downy mildew and removing any basal spikes from the yard.  Basal spikes are hop shoots systemically infected with downy mildew. They tend to be stunted, have shortened internodes, with yellow, brittle, downward curling leaves. 

Basal spike systemically infected with downy mildew. Note yellowed, downward-curling leaves

Because the downy mildew fungus will sporulate on these spikes (look for a purplish-black fuzzy growth on leaf undersides), they serve as important sources of early inoculum and should be removed from the hop yard and destroyed promptly to miminize spores that can initiate foliar infections on healthy bines and leaves.For more details on identifying downy mildew in hops, refer to this post: https://onspecialtycrops.ca/2014/06/03/identifying-downy-mildew-in-hops/

Using Ridomil

What is Ridomil?

Ridomil Gold 480 SL is a fungicide containing the active ingredient metalaxyl (referred to as mefenoxam in the US). 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.Unlike other fungicides registered on Ontario hops, Ridomil is only permitted as an early season drench at the root site. In the US, Ridomil is labelled as a foliar spray on hops, or in irrigation, however this is not the case in Canada, where Ridomil can only be applied to hops once per season, as a soil drench.

How does it work?

Ridomil is also unique amongst hops fungicides registered in Canada in that it is systemic, which means it is taken up by the roots and spreads upwards through plant tissue through the xylem. Once taken up, it interacts and interferes with the downy mildew pathogen within the hops tissue, thereby inhibiting systemic infections. Because it moves through the xylem, Ridomil only goes up in the plant, not down. This is why Ridomil is labelled as a soil drench.

Where should should you apply it?

Ridomil must be applied near 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. However, Ridomil is also highly mobile in the soil, which means heavy rain can wash it down below the root zone, where the plant can no longer take it up. 

Aim to apply Ridomil to the root zone, but not deeper. Avoid applications immediately before very heavy rain is forecast, which can wash product out of reach of the plant.

When should you apply it?

Because Ridomil must be taken up the by plant and moved through the xylem, it should be applied to actively growing shoots which have emerged from the soil, but before rapid growth begins, as the product will be diluted as the hops grow. Ridomil’s pre-harvest interval for hops was recently reduced from 135 days to 90 days, which gives growers more flexibility in application timing, up to early or mid-May, assuming an August harvest. For hops in southern Ontario which have already emerged, applications should be done now if you do not plan to mow your crop. Growers in northern regions where hops are still dormant should wait until they emerge. If you plan to remove your initial flush of hops shoots, you may want to wait to apply Ridomil until regrowth appears, as growing plants with transpiring leaves will have more xylem movement upwards from the roots.

How long will it last?

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 a few weeks of 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 resistance?

Metalaxyl is also prone to resistance, and Ridomil resistance has been reported in hops in Oregon and Washington, although this varies with location and downy mildew strain. Recent studies done in Ontario hops by the University of Guelph in some but not all conventional hop yards (12 out of 40 yards had at least one basal spike positive for Ridomil resistance), which means the product may not always be effective. If you suspect that Ridomil may be resistant in your yard, you may want to consider leaving a section or sections of plants unsprayed and comparing downy mildew incidence to sprayed sections.

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