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.
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?
- Simple mechanical cultivation that stirs up the soil and increases soil aeration reduces the amount of zoospores. Cultivation also breaks up disease carrying plant debris and removes perennial weeds - some of these can act as alternate hosts.
- Application of manures to hopyards has been shown to reduce downy mildew levels. Apparently manures promote a healthy soil organism complex. Some of these are organisms that destroy zoospores. (Don’t however, apply manure directly over the hop crown – this can cause outbreaks of other pathogens like crown rot and fusarium.)
- Drench application of phosphonic acids (Phostrol, Fosphite, and others). Some of these can be applied through the drip irrigation system; which makes application easy and placement accurate. These products destroy zoospores in the soil AND in the plant tissue. Caution: phosphonic acid is not the same as phosphoric acid!
- Heavy spray applications of Aliette. This fungicide can be applied as a heavy directed basal foliar spray. Aliette also breaks down into phosphonic acid but is unique in that it will translocate downward in the plant, protecting the roots and buds. (Very few systemic fungicides will translocate downward – most fungicides only move upward in plant tissues.) Applications of these other fungicides (applied alone) are quite ineffective against downy mildew in the fall zoospore stage. Save these fungicides for Spring controls when downy is moving upward in new plant growth.
A Suggested Fall Program for Downy Mildew Control
AFTER HARVEST IS COMPLETE:
1. Apply an organic manure such as poultry, cattle, horse, sheep, llama . . . whatever you can source. Additional potassium fertilizer, lime or gypsum is also helpful at this time of the year, too. (Don't over-do the amounts of manure or nitrogen applied, because excessive levels of nitrogen cause softer crown bud growth and root tissues; which fungus can attack more easily.) There is a difference between a healthy plant and an over-fed plant. Use a soil test to help figure out where your soil fertility is and what additional amendments might be needed.
2. Disk/ cultivate the manure in and knock out the weeds at the same time. Remember, you are also trying to remove the seasonal compaction and aerate the soil to kill zoospores; so go deep with the cultivation. Remove any rhizomes you cultivate up. They are possibly already infected. Permanent grass aisle ways may not be an option in yards with heavy Downy mildew pressure.
3. Follow up and Apply a fungicide tank mix combination of Tanos* + Kocide* or Aliette * as a basal heavy spray to remaining post-harvest hop foliage. Curzate / Kocide are both copper products- do not mix these directly with Aliette.
4. Two weeks later apply a phosphonic acid drench to the soil around each crown or inject it through the drip system before it is shut down for the year. Phostrol is simple and effective, but there are other products as well. Repeat this application after 3 to 4 weeks.
* 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