This is important to consider, because Downy mildew is prevalent everywhere and can destroy your entire hop crop if not managed and controlled. Grape growers face a similar situation and have developed very effective control strategies. Hop growers will need to do the same.
Many hop growers do not understand the importance of controlling downy mildew in their hop yard after harvest is completed. The fungus and the infection are not as visible because there are no new shoots or infected spikes to be seen. Many growers consider the season as finished and leave the yard unattended going into winter. IMHO this is a fundamental mistake in controlling Downy mildew in hopyards. Downy mildew is active whenever conditions permit – it simply doesn't care if it is springtime, summer, or fall. (The 2014 growing season attests to that.) Post-harvest fungicide applications and controls can be the most effective way to manage downy mildew in yards that have had the disease present and reduce the severity of downy mildew spikes during the following spring growing season.
Downy mildew, like most molds and mildews, persists and spreads during the growing season mainly through air-borne spores which infect new leaves and growth whenever environmental conditions are favorable. In the Fall season, however, downy mildew “morphs” into a different creature; producing a specialized spore type called a zoospore. This spore acts much more like a living microscopic worm than a fungal plant-like spore. The zoospore form helps downy mildew complete its annual life cycle by finding a safe resting place for it to overwinter or by forming protective oospores; where it will be protected from the harsh winter that is ahead. This is a high-risk period for the downy mildew organism – it is outside its host and can be attacked most easily at this point.
Since downy mildew cannot continue its normal aerial spore production via the leaves and new growth, it switches it form. As the hop and tree leaves color up and drop into the fall season; an alternate zoospore form develops and actively “swims” through the hop yard soil, very much like a nematode, searching for new hop roots and rhizomes to infect. Upon contact, the zoospore penetrates into the hop root/rhizome and travels upward through the plant tissues and “parks” itself in the newly-formed crown buds. There it will lie safe and dormant. When springtime arrives and the buds sprout and emerge as disease-laden spikes; starting another season of downy mildew infection.
The weakest link and the point where downy mildew is most susceptible to attack is during the time where its zoospores are active in the soil, searching for a new entry point into the hop plant. This phase occurs roughly from after cone harvest until all top foliar growth has fallen off the bines in late Fall. This phase also coincides with the hops forming the new crown buds during September /October. Keeping the developing buds protected during this time period results in much lower levels of emerging infected spikes next spring.
What Controls are Effective?
A Suggested Fall Program for Downy Mildew Control
Timing is important for this to work!! - the longer the hops are exposed in the Fall to wet, saturated / compacted soil conditions the higher the level of disease to expect the following spring. If fields are too wet to work, then at least get the fungicides applied as partial protection.
* Tanos fungicide is a curative SYSTEMIC fungicide that will absorb into the hop plant and kill downy mildew mycelia inside the leaves and stems and stops sporulation. (Pristine or Presido can substitute.)
* Kocide is a copper-based fungicide that destroys already present spore fruiting structures and spores upon CONTACT.
* Aliette fungicide will break down into phosphonic acid and will systemically move downward into the hop roots. (Expect some potential foliar phytotoxicity if copper fungicides are also used in your spray program). Aliette may have to be buffered with potassium carbonate with some tank mixes – always read the label carefully.
Notes: Don’t procrastinate with the fungicide application! If the zoospore is successful in making it to the crown bud... game over. Nothing can touch it until Springtime when it emerges from dormancy. Fungicides have to be applied before the plants are dormant or they will not take up and translocate the chemicals. Also, this is not the only management program you could use – it just happens to be one we at Great Lakes Hops find more effective than most. Universities are working on their own recommendations that should be made available soon.
As you can see, the simple strategy of this management program is to hit Downy mildew from the top, bottom, and sides while it is vulnerable outside of its hosts. Reducing the overwintering inoculum levels results in highly reduced amounts of infected downy mildew spikes the following Spring. Last of all, this technique does not control aerial spores that will float into your yards in early spring. Remain vigilant.
ORGANIC GROWER? Your best controls are the use of clean row cultivation and sanitation, good soil aeration, and drench application of organic bio-controls like Serenade and Sonata. Realize grass aisle ways can increase the severity of downy in your yards by about 30%, according to research done out west. Organic growers should also minimize their risks by avoiding planting hop varieties with known low resistance to downy mildew. Some to possibly avoid would be the Columbus, Zeus, Centennial, Cashmere, Horizon, Sterling, & Glacier. English hops, or crosses with them, seem to have the best natural resistance to downy mildew.
Hops Handbook, pg.10
Control of Downy Mildew of Hops (WSU)
Originally posted 09/15/2013. Updated 09/13/2014
Lynn, the head hop grower at Great Lakes Hops has over 30 years of experience in the horticultural field. Browse the blog articles here to find useful growing information for humulus lupulus, based on personal experience and observations at Great Lakes Hops.