As growers wrap up hop harvest this 2014 season, they can follow these post-harvest tips to help prepare for next year.
For the most part, the work of the 2014 season is complete. However, growers can still consider pest management practices that may impact next year, post-harvest irrigation, compost application and record keeping.
Controlling downy mildew
Many growers struggled to control downy mildew this year due to challenging weather conditions. Growers cultivating particularly susceptible varieties faced a particularly difficult season and may be wondering what can be done post-harvest to combat this important and damaging disease. To properly answer this question, we need to fully understand the disease cycle of Pseudoperonospora humuli, the causal agent of downy mildew of hops (Figure 1) and how growing conditions in Michigan can affect the efficacy of late season treatments.
Mycelium, the vegetative part of the downy mildew pathogen, overwinters in buds and crowns or plant debris (infected leaves, stems) left on the field. As shoots emerge in the spring, they may already be infected with this overwintering mycelium. As the hop bine begins to grow, the mycelium produces a microscopic spore-bearing structure (sporangiophore) on the underside of leaves, giving the underside a gray, fuzzy appearance. These structures give rise to an asexual type of spore called zoospores. Zoospores move via wind and rain and act as the major cause of disease spread during the season, infecting new leaves, shoots and eventually even cones. The reproductive cycle that produces zoospores may repeat multiple times over the season, depending on temperature and moisture availability.
Alternately, mycelium may also yield a resting spore (oospore) that it produces through sexual recombination. Oospores are typically more resistant to environmental changes and are often referred to as resting spores. It is unclear at this time if Michigan’s climate provides environmental conditions conducive to oospore production.
When considering a post-harvest treatment, it is important to remember that the downy mildew fungus will overwinter in the plant itself and is protected by the plant epidermis. While there are no data to suggest that post-harvest treatments are beneficial in terms of a reduction in disease next season, there is a general correlation between disease presence and severity from season to season that warrants further research. If growers want to try an experimental post-harvest application, they should focus on utilizing systemic fungicides that move downward in the plant tissue and might disrupt the mycelium that will be the source of next year’s infection.
Systemic fungicides are typically described as locally systemic, acropetal systemic (moving upward), or basipetal systemic (moving downward). Locally systemic materials are not useful for the treatment of downy mildew at this time because they do not move far from the site of application and don’t reach the sites where the pathogen overwinters. Truly systemic fungicides are taken up by the xylem or phloem tissue of the plant and moved to new tissues. Many of the systemic materials today are only translocated outward via the xylem or water-conducting tissues. Distribution of a systemic fungicide in the phloem or carbohydrate-conducting tissue tissues (basipetal translocation or downward movement) would include translocation into the crown and roots where downy mildew overwinters.
Fungicides labeled for hops that move systemically downward include Aliette and phosphite fungicides. Given the overwintering location of the fungus a systemic fungicide with downward movement would be the best option. That being said, with little remaining leaf area and bines shutting down from shorter day length, there may be limited value to a fungicide application at this time. Based on the lack of supporting data, post-harvest treatments for downy mildew are not recommended as a general practice at this time. Growers with high levels of downy in their hopyards should instead focus on developing an early and aggressive protectant treatment program for next spring. Refer to the article “Battling downy mildew as hop harvest approaches” from Michigan State University Extension for more information.
Post-harvest insect pests
Now let’s consider some of the problematic insect pests still lingering after harvest. Potato leafhoppers, damson hop aphids and two-spotted spider mites were all reported at significant levels in hopyards, and growers are considering what treatment strategies are available post-harvest.
Potato leafhoppers (Photos 1-2) were a real issue for some growers, but a treatment now would not affect populations next season because potato leafhoppers currently in hopyards will not survive the winter. Potato leafhoppers move north on spring storms and reinfest each year. Once potato leafhoppers arrive, they reproduce in the hopyards and can cause significant damage. However, their inability to survive the winter wipes out the entire population in Michigan each year.
Damson hop aphids were observed in higher numbers as harvest approached and some growers had problem infestations at harvest (Photo 3). Again, we must look at the lifecycle of the pest to determine if a post-harvest treatment could help keep numbers down next season. Hop aphids overwinter as eggs on Prunus species, genus of trees and shrubs that includes plums, cherries, peaches, nectarines, apricots and almonds. In early spring, eggs hatch into stem mothers that give birth to wingless females that feed on the Prunus host. In May, winged females are produced and travel to hop plants where additional generations of wingless females are produced. As cold weather approaches, winged females and males are produced, move back onto a Prunus host, mate and lay eggs for before winter. We expect that this migration away from hops and onto plants in the Prunus genus occurs sometime in September.
Growers with particularly high populations could apply a post-harvest insecticide to limit the overwintering populations, but only if they are still present in the hopyard. Growers considering an application should scout their fields and confirm the presence of aphids before applications are made. Refer to the article “Aphids on hop reported in significant numbers” for more information on management.
Finally, two-spotted spider mites (Photo 4) were an issue for some growers this season. Again, if we examine the lifecycle of the pest we can make better decisions about the potential impact of post-harvest management. Hops are a unique situation when it comes to mite management. Many horticultural crops use post-harvest treatments in infested sites to reduce overwintering populations, but hop growers remove the plant itself quite early in the season, likely removing a large portion of the mites with it. Two-spotted spider mites that remain overwinter as mated females on plant debris and trellis structures in the hopyard.
Mites remaining in the hopyard are susceptible to miticide applications, but likely account for a relatively small number in fall when they are beginning to migrate to overwintering sites. Unless the infestations were at an economically significant level, miticide applications should be avoided if possible. Often, one mite treatment leads to continued mite treatments as the natural balance of predators and beneficials is upset. For these reasons, two-spotted spider mite post-harvest treatments are not recommended unless hopyards were left unharvested and experienced extremely high populations. Refer to the article “Michigan hop update – July 31, 2014” for more information on managing two-spotted spider mites.
Sanitize and fertilize in the fall
Growers should also consider the importance of sanitation at this time. Removal of all bines and leaves from the hopyard is recommended after the first hard freeze. Plant tissues can harbor insects and disease and should be removed, buried or burned. Growers who did not harvest this year (as in first year hops) are advised to remove the plants after a hard frost to prevent increased pest and disease pressure next season.
Growers planning to utilize compost fertilizer can apply it this fall. Recommendations from the west suggest applying a couple of shovels full directly onto and around the crown. Conventional wisdom also suggests watering the bines just before shutting down the irrigation for the year, particularly in areas without good snow cover where desiccation might be an issue this winter. Growers are advised to not fully saturate the soils, but keep the final watering moderate, particularly on heavier soils where rot could become an issue. Growers may also consider sub-soiling between rows in areas in need of better drainage, applying herbicides for perennial weed control, and removing unproductive or diseased crowns.
Take time to evaluate the season
Lastly, it is well worth a grower’s time to set aside a moment to reflect on the season. Take note of trouble areas in the hopyard and consider planning how to address pest or nutrient issues in the following season. It is also recommended that growers review their spray records and ensure they are complete.
For more information on record keeping, visit the resources page of the MSU Pesticide Safety program. For more information about growing hops in Michigan and the Great Lakes region, visit hops.msu.edu.