Controlling Downy Mildew in the Hopyard after Harvest

Downy mildew (DM) of hops is a major disease that affects many susceptible cultivars of hops. It can severely damage both the hop plant as well as making the cones unsuitable for harvest. This discussion pertains to using post harvest DM controls to help hopyards recover from a season of high downy mildew infection.

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 as springtime symptoms. Many growers consider the season as finished at harvest and leave the yard unattended going into winter. This is a fundamental mistake in controlling DM in infected hopyards. Downy mildew is active whenever conditions permit – it simply doesn't care if it is springtime, summer, or fall. If green plant tissue, moisture and the right temperatures are present, DM will remain active. Post-harvest fungicide applications and controls can be an effective way to manage downy mildew in yards that have had the disease present and reduce the severity of downy mildew spikes that emerge during the following spring growing season.

leaf lesions caused by downy mildew. (D. H. Gent)

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 motile spore type called a zoospore.  This spore acts much more like a living microscopic worm than a fungal spore. The motile zoospore form helps downy mildew complete its annual life cycle by finding a safe resting place for it to overwinter or by forming protective dormant oospores. These forms of DM exist outside of the plant in the soil  where it is outside its host and can be interrupted at this point.

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 As the hop and tree leaves color up and drop into the fall season; an alternate motile zoospore form develops and actively “swims” through wet saturated soil, very much like a nematode, searching for new hop roots and rhizomes to infect. Most of this activity occurs in the top 3 or 4 inches of topsoil. Upon contact with hop roots or rhizomes, the zoospore penetrates into the hop tissue and travels upward through the plant tissues to the newly-forming crown buds. There it will persist and go dormant for the winter. When springtime arrives and the buds loosen, DM returns to an active state; emerging with the first flush of hop growth as white spore-laden shoots; starting another seasonal cycle of aerial downy mildew infection.

A weak link and the point where downy mildew is susceptible to Fall controls is when  its zoospores are present and active in the soil. This DM phase occurs roughly from after cone harvest until all top foliage growth has yellowed on the the cut bines into late Fall. This phase also coincides with the hops forming the new crown buds during September  through October. Keeping the hop plant protected from re-infection during this time period results in lower levels of emerging infected spikes next spring.

What Controls are Effective?

• Simple mechanical cultivation that aerates the top 3 or 4 inches of topsoil  reduces the amount of  motile zoospores. This motile form depends on wet saturated soil conditions to survive, so any practice that aids in drying and  reducing excess soil water levels is helpful.

• Application of manures to hopyards has been shown to reduce downy mildew levels. Manures promote a vital healthy soil biome of bacteria, fungi, and microbes. Some of these are organisms that destroy zoospores. (However, do not apply fresh manure directly over the hop crown – this excess nitrogen source can cause outbreaks of other hop pathogens like crown rot and fusarium.)

•  Spot drench application of phosphonic acids (Phostrol, Fosphite, K-Phite and others) directly on infected or susceptible plant crowns.   apply enough solution to soak at least 1 inch into the soil.   Phosphite products destroy zoospores on contact in the soil and are also translocated in the plant tissue. Phosphites are are one of the few chemical classes that translocate downward in plants; protecting roots. Caution: phosphonic acid is not the same as phosphoric acid! Read and follow label instructions. Most phosphites have a long residual effect.  Earlier post-harvest applications are more effective than late fall treatments.  Treatments applied after hops enter dormancy are not effective.

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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 may also be helpful at this time of the year. 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 fungal pathogens can attack more easily. There is a difference between a healthy plant and an over-fed plant. Use a fall soil test to help figure out what the remaining soil fertility is and which additional amendments might be beneficial.

 2. Disk/ cultivate the manure in and knock out the perennial weeds at the same time. The primary goal is  to remove the seasonal soil compaction and re-aerate the soil to inhibit zoospores; so till as close to the hop rows as possible. 

 3. If the symptomatic triangular leaf lesions are present on remaining hop foliage apply an anti-sporulant SYSTEMIC fungicide such as Tanos  as a heavy foliar spray. 

 4. Follow up about two weeks later by applying a phosphite drench to the soil around and over each crown.  

Application timing is critical 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 cultivate, then at the  minimum 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.)

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 fungicide. 

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. Grass aisles 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 cultivars to 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.

Further Reading:

Hops Handbook, pg.10

Control of Downy Mildew of Hops (WSU)

Originally posted 09/15/2013. Updated 09/13/2014