Pruning for disease management and yield benefits in hops

Both offer multiple benefits but timing and method are important.

red hop mower going across brown dirt
Figure 1. Hop crowning. | Photo Credit: Dr. Dave Gent, USDA ARS

An important agronomic practice in established hop yards (generally at least two years of age) is spring removal of plant and crown material. This is generically referred to as "pruning", which can be conducted using mechanical or chemical means. Because many of the terms are used interchangeably, it is useful to define the different methods that growers use to remove bines, buds, and new young shoots, according to the Compendium of Hop Diseases and Pests. Mechanical pruning generally refers to removal of shoots from the current or previous season and associated crown buds by physical means. There are multiple ways of mechanical pruning, such as "crowning" or "cutting", or a less aggressive form referred to as “scratching”. Crowning involves removal of the top 0.75-2 inches of the new wood formed in the previous season (i.e., part of the "crown") with an offset implement (See Figures 1-2).  

crowned hops
Figure 2. Recently crowned Oregon hop yard. | Photo credit: Dr. Dave Gent, USDA ARS
crown mower
Figure 3. Hop crown mower. | Photo credit: Sean Trowbridge, Top Hops.

Scratching involves the use of a “harrow-like” implement with spinning discs and tongs to remove material above or just below the soil line (Figure 4).

scratching in WA state
Figure 4. Scratching in the state of Washington. | Source: Dr. Dave Gent, USDA ARS.

Historically, hop producers in regions of Europe and the western U.S. that experience high disease pressure usually “crown” in late winter to early spring to reduce downy and powdery mildew inoculum. Depending on variety and weather, producers may follow up with chemical or mechanical pruning later in spring after the first shoots emerge. Growers who crown typically have “hilled-up” soil on the crowns the previous growing season (for example, in mid- to late June). Less aggressive mechanical pruning methods such as "scratching" do not require annual hilling of soil. While both crowning and other forms of mechanical pruning are useful for reducing mildew disease inoculum, viruses can be spread to adjacent plants through mechanical transmission. Chemical pruning with a desiccant herbicide such as carfentrazone-ethyl (AIM) can help limit the spread of virus, but any green tissue and or infected left behind may shelter inoculum if disease pressure was severe the previous year.  

In 2012, researchers analyzed survey data from 149 hop yards in Oregon and Washington to assess the association between the thoroughness of spring pruning, timing of pruning operations, and the severity of downy and powdery mildews. The most common pruning approach was chemical desiccation (48% of yards), mechanical pruning (23%), or a combination of these practices (15%). Pruning quality was rated as excellent, moderate, or poor based upon the amount foliage remaining post-pruning. Producers who indicated they pruned twice were much more likely to receive an “excellent” rating compared to growers who only pruned once. Downy mildew severity in Oregon increased twofold as pruning quality dropped from “excellent” to “poor”. While pruning quality had no significant effect on powdery mildew in Oregon, increased seasonal severity of powdery mildew on leaves and incidence on cones was greater in yards with “poor” pruning in Washington. Depending upon timing and quality of pruning, growers may be able to eliminate one fungicide application per year. According to the journal article “Association of Spring Pruning Practices with Severity of Powdery Mildew and Downy Mildew on Hop”,  “…early spring sanitation measures are associated with reduced primary inoculum and are critically important for managing both downy mildew and powdery mildew.”  

A recent study on the effect of pruning timing and method on powdery mildew development, yield, and hop quality in a commercial hop yard of Tomahawk in Washington. They found that a 4-week delay in pruning reduced powdery mildew incidence on leaves from 46% to 10% and on cones from 9% to 1%. There was no significant differences in powdery mildew incidence on leaves when pruning method was compared (chemical vs. mechanical). Delayed pruning for multiple years did not negatively affect cone yield, quality, or appearance. 

Another advantage of chemical or mechanical pruning to remove young shoots is uniformity of secondary growth in the run up to training. Because the pruning date affects training date, which is of utmost important for yield, pruning at the appropriate time is critical. To complicate matters, pruning date is cultivar dependent (Table 1).

Variety

Chem. Pruning Dates

Training Dates

Harvest Dates

Willamette

4-12

5-1 / 5-3

8-24 / 8-29

Cascade

4-12

5-4 / 5-7

9-3 / 9-23

Cluster

4-17

5-7 / 5-8

8-29 / 9-10

Millenium

4-18

5/10

9-3 / 9-28

Citra

4-18 / 4-26

5-10 / 5-15

9-3 / 9-18

Equinox

4-26

5-16 / 5-17

9-20 / 9-23

Mosaic

4-26 / 4-28

5-15 / 5-20

9 -5 / 9-23

Zeus

4-28 / 4-30

5-19 / 5-20

9-3 / 9-30

Researchers in the Czech Republic investigated the effect of timing of pruning -early pruning (end of March), mid pruning (first ten days in April), and late pruning (second ten days in April) on yields and alpha acid content for the hop cultivar Agnus. They found that later pruning conferred greater yields and alpha acid content than early or mid-pruning. Timing will likely vary annually based upon the plant phenology and location.

In general, hop plants will regrow and are ready to train approximately 3 to 4 weeks after pruning, depending on how pruning was conducted. Slower growing cultivars and yards with poor vigor may need to be pruned earlier, or not at all, to ensure adequate growth and maximum yield.

Pruning is an important agronomic practice that can have a major impact on hop yield and quality. Methods and timing of pruning will depend upon grower preference, irrigation placement and practice, cultivar, vigor of the yard, yard age, latitude and location, timing, disease severity from the previous season, and seasonal growing conditions after the operations. There is no substitute for local trials and experience to determine the optimum method and timing of pruning operation. When producing a new variety, it is advisable to experiment with varying pruning dates (and by consequence then, training dates) over a period of several weeks to observe how plants respond. 

For more information on hop production practices please sign up for the MSU Hop Bine and Dine webinar series.

 Hop Crown Mower

DK FAB Hop Crowner

Spring Work in Oregon Hop Fields

Scratching in WA

Please continue to visit Michigan State University Extension’s hop webpage or the MSU Hops News Facebook site for up to date information. Thanks to Erin Lizotte and Dave Gent for information and photos included in this article.

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