It’s been a very wet couple of weeks in Eastern Nebraska, with few signs of rain letting up before the end of this week. It’s challenging to harvest, continue management, let alone, get into the fields in many cases, with weather like this. Be aware as the season winds down that downy mildew is still very active for the next couple of months, and the steps we take now can have a big impact on the disease pressure in the 2019 season.
Hop Downy Mildew
Hop Downy Mildew, caused by the organism Pseudoperonospora humuli, is one of the most devastating diseases for hop producers. It is important to manage and control as it has the potential to destroy entire crops. Its survival from season to season, and its ability to infect the crown are why you must be proactive in control measures. In systemically infected plants, plants become weakened by a reduction in carbohydrate reserves. Your best management strategy for controlling downy mildew is through a combination of cultural and chemical management strategies. Another note of caution, do not wait until you see signs and symptoms of disease. Downy mildew effects plants at emergence. Scout as soon as plants emerge in the spring, and continue through dormancy.
I. The life cycle of Pseudoperonospora humuli on hop is pictured below.
- Pseudoperonospora humuli overwinters in the infected, dormant crown and buds, and spreads to infected buds in the spring. The infected buds grow into basal spikes.
- Sporangium release zoospores from the underside of leaves of the basal spikes which can infect leaves, shoots and cones.
- Infection by spores occurs between 60-70 °F when free water is present for at least 1.5 hours, however, leaf infections can occur during temperatures as low as 43 °F and leaf wetness persists for 24+ hours.
- Infections on shoots can become systemic when the mycelia grow throughout the plant, eventually reaching the crown and buds.
High disease pressure, fungicide timing with weather events, challenges with spray coverage and wash-off, and cultivar susceptible to disease are some of the factors that allow downy mildew to infect hop yards even with the best strategies.
Critical times during the season to place focus on management for downy mildew include: before and after training, lateral branch development, bloom, and cone development.
II. Signs & Symptoms
The first signs of downy mildew in the spring appear on emerging shoots from the crown. Rhizomes of infected plants may have reddish-brown to black flecks and streaks. These basal shoots (or “spikes”) are bright, chartreuse green, have shortened nodes, small, curling leaves, and appear stunted. Leaf infections manifest themselves as water-soaked lesions between veins, which become brown and necrotic. Given ideal environmental conditions, asexual spores (sporangia) may develop in a mass underneath infected leaves and spikes with a fuzzy, gray to black appearance. However, as these masses are not always present growers should not rely on them as a sign of the disease.
If the pathogen is not treated, new plant tissue will become infected as it grows up the coir. While new shoots can be trained, yield loss may still occur. In the transition from vegetative to reproductive growth, side arms will emerge, similar in appearance to the spring basal spikes. Infected flowers shrivel and may fall off. Cones become discolored, harden and cease development if infected early in the season; if infection occurs later in the season only the bracts may be discolored.
III. Cultural prevention strategies for Hop Downy Mildew:
- Source clean plant stock
- Adequately space plants ~42” apart
- Prune in the spring when weather permits. Ideally, one mechanical prune and one chemical prune have shown to be most effective for downy mildew control.
- Do not apply overhead irrigation
- Defoliate lower 3’ of plant once plants reach proper maturity (bines become woody) to increase airflow
- Manage weeds in the hop rows to increase air flow
- Cultivate between rows when able to reduce compaction, encourage soil aeration, and to kill zoospores
IV. Chemical Strategies:
Cyazofamid (Ranman, FRAC 21) and Ametoctradin + dimethomorph (Zampro, FRAC 45 plus 40) are products that have shown to be effective fungicides in a rotation for controlling downy mildew. Rotating and mixing these products with cymoxanil (Curzate, FRAC 27), famoxadone + cymoxanil (Tanos, FRAC 11 plus 27, *not registered for hops in South Dakota*) and phosphonate products such as fosetyl-Al (Aliette, FRAC 33). Copper-based fungicides can be rotated in when disease pressure is low.
The fungicide resistance action committee (FRAC) assigns a number and/or letter to group active ingredients that have the same mode of action and target site. Repeated application with the same mode of action and target site greatly increases a pathogen’s ability to develop resistance. Refer to pesticide labels and FRAC codes for assistance in rotating active ingredients. These codes can often be found in the upper corner of a fungicide label. Rotating active ingredients and modes of action, using of tank mixes with multi-site activity, and integrating non-chemical controls assist in managing fungicide resistance.
V. Pre-harvest Management
As harvest quickly approaches, management is key, especially when the bracts begin to form and close into cones, and again during the process of determining maturity.
Protecting cones as they close is important because you will want to keep organisms out of your cones. Products such as famoxadone + cymoxanil (Tanos, FRAC 11 plus 27, *not registered for hops in South Dakota*) & copper hydroxide (Kocide 3000, FRAC M1), or mono- and dibasic sodium, potassium, and ammonium phosphites (Phostrol, FRAC 33) and Reynoutria sachalinensis extract (Regalia, FRAC P5), or Cyazofamid (Ranman, FRAC 21) are great options to protect cones maturing. Ranman and Regalia are especially helpful for late maturing varieties because of low Pre-Harvest Intervals, 3 days and 0 days, respectively.
Another note on downy mildew management around harvest. It is important to monitor disease presence and weather conditions as you approach harvest windows. In some areas, fog, rain events, and high humidity allow for a proliferation of downy mildew around harvest, and some cultivars may need to be harvested earlier to protect from heavy disease infections on cones. If you are in an area that is prone to these type of weather events, this should be taken into consideration.
VI. Post-Harvest Management
Hop Downy Mildew can become increasingly worse in hop yards from season to season if the disease is not managed properly following harvest. Because the disease is not as visible, it is easy to consider the management of the disease to be done after harvest. Downy mildew is active as a zoospore in the fall, searching for a place to overwinter in its protective structure called an oospore. The zoospores actively seek out rhizomes and new hop roots to infect, and travels to the new crown buds to wait for spring. This period is between cone harvest and when the foliar growth has fallen from the bines in late fall, which is also when new crown buds are being formed. Protecting the new buds in this time period reduces disease pressure immensely in the coming spring. With any sprays, read and follow the label instructions very carefully!
Here are some steps to highly consider incorporating into your hop yard after harvest this season:
- Apply an organic manure. In addition, you can use your soil tests to guide you on other additional amendments that might be needed.
- Cultivate/disk. Work in manure, knock out weeds, and aerate the soil to kill zoospores in the soil. Remove in rhizomes you cultivate up. Do not keep for planting as they could be infected.
- Follow up with a fungicide application of Tanos + Kocide 3000 or Aliette as a heavy basal spray. Do not mix Kocide or Curzate with Aliette.
- Apply a phosphonic acid two weeks later as a drench around the crowns or inject through drip system. Repeat 3-4 weeks later.
Tanos – curative systemic fungicide that is absorbed into the hop plant & kills mycelia. *not registered for hops in South Dakota*
Kocide 3000 – copper-based contact fungicide that kills fruiting structures and spores.
Aliette – fungicide that breaks down into phosphonic acid and systemically moves down into the plant.