Control of downy mildew (DM) in the hopyard is an ongoing challenge for growers in many regions; and especially difficult for growers that have susceptible hop varieties. DM takes different forms and produces several different spore types in reaction to environmental conditions - i.e. aerial, oospores, and zoospores. Each spore type has its own set of specific control measures. DM is active whenever temperature and moisture conditions are correct; spring, summer, and fall. In the fall season, downy mildew switches from actively producing airborne spores that mainly affect the bines, foliage, and cones; to forming protective oospores and motile zoospores that can overwinter in the soil and dormant hop crown. The more familiar springtime DM spikes on new shoots are less evident in the fall as hop growth slows and the infected older foliage takes on a mottled appearance; which many growers fail to notice. Once DM has a foothold in a hopyard, outside airborne spores are not necessary to re-start early springtime infections.
Great Lakes Hops has found the following fall practices to be effective in gaining good control over downy mildew and other pests and pathogens in hopyards.
- Do not compost or put harvested bines back into the yard if pathogens were prevalent. Do not spread other hopyards' waste back into your hopyard if you are a processor who handles other grower’s hops. Properly composted harvest waste must reach a temperature of 130 degrees F. to kill pathogens and few growers can accomplish this successfully and instead end up with piles of decayed plant material that continues to be a source of pathogens.
- Change your irrigation frequency post-harvest. DM motile zoospores require wet saturated soil to persist and move in the soil. The quickest way to encourage fall DM crown and bud infection is to leave the irrigation controller running daily going into the fall season. After harvest run the hopyard irrigation on the dry side and only irrigate a couple of times a week; if at all. In most areas, rainfall coupled with cooler temperatures is adequate.
- Limit your fall applications of Nitrogen to 80 pounds per acre or less (banded). Fall fertilization is about replacing some of the nutrients removed by the hops during the growing season and rebuilding the soil microbe population that the high usage rate of synthetic fertilizers destroy. However, high rates of nitrogen applied to fall hopyards can delay natural dormancy and enhance proliferation of pathogens like downy mildew. Many grower use organic light manure applications both spring and fall to rebuild the soil. Additional fall applications of potassium, lime, or gypsum has been shown to be beneficial to increasing next season hops vigor. Weak nutrient-deficient plants are more susceptible to winterkill, diseases and pests.
- Apply fertilizers and manures alongside the rows; NEVER on top of the crowns. High levels of excess nitrogen near the hop crown coupled with wet soil create an ideal condition for DM and crown rots to proliferate.
- Till the hopyard soil. Simple mechanical soil cultivation each fall improves soil aeration and reduces the wet compacted soil conditions that mildews and pests like nematodes favor. Many spores and nematodes are destroyed by this simple fall practice.
- Apply systemic fungicides to the hopyard post-harvest. The idea is to limit fall spore production in infected plant bines and leaves and destroy pathogens that will overwinter in the hopyard. Systemic applications containing phosphonic acids will move downward in both the plant and soil; providing protection against motile zoospores. Tanos or Pristine, combined with Phostrol / Aliette make an effective application for yards that have experienced downy mildew. Usage rates are lower than during the growing season.
- If your hopyard experienced heavy pest and disease pressure during the season, consider a late fall dormant oil spray. Horticultural oils applications will reduce overwintering insects and mites / their eggs and many types of fungal spores.
- Trim bines of infected varieties short in late fall after leaf drop and remove the debris from the yard. Wait 4 to 6 weeks after harvest to cut the bines short to allow the carbohydrates in the remaining bines to translocate down into the crowns and help build strong budding at the soil level. The optimum height to cut is about two inches above the new crown buds. Cutting closer into the crown at this time creates a large open wounds that allows easy access for many types of crown rots. Be sure to cover any visible crown buds with soil or light mulch before the winter sets in.
- Avoid over-application of heavy wet mulches over the crowns. Covering exposed crown buds with field soil to prevent winterkill of buds is probably a better choice in most larger hopyards.
- Get control of those weeds! Hopyards with lots of weeds have higher levels of mildew infections. Use herbicides like Volunteer for grasses and a contact burn-down like Aim for broad-leaf weeds. Organic growers need to cultivate more frequently and can treat weeds with natural products like clove oil. Always destroy weeds before they set seed!
- Remove black plastic or ground fabric used for weed control. Controversial, we know, but plastic mulches hold excess continuous moisture and heat under the cover and result in higher levels of disease and root rots. It is difficult to rebuild the nutrient and microbe depleted soil around the plant and remove underground rhizomes with a permanent plastic mulch or row cover in place. The trade off of fewer weeds by using poly row covers in exchange for for lower yields and higher disease levels is not logical in our humble opinion after working with hundreds of hopyards and comparing grower results.
- Dig up your hops! It is a great idea to dig up a few plants in the yard each fall to inspect the roots and crowns. Are the roots white and healthy in appearance? Any stunting, decay, or lesions? What is the level of soil compaction? This is a great way to find early signs of many pathogens such as crown rots and nematodes. Then simply replant the crowns when you finish these spot checks.