Leaf stripping (removing lower leaves and laterals; Figure 1) to increase air flow is a common cultural practice to manage hops downy mildew (P. humuli). Leaf stripping is performed earlier in Ontario than in other growing regions due to a shorter growing season and earlier occurrence of downy mildew.





Figure 1: Leaf stripping (removal of lower leaves and laterals) trial on cv. Nugget at the Simcoe Research Station, Norfolk County, in 2015.

With a late start to the 2018 growing season, some cultural practices were either delayed or not performed this spring. However, with above normal temperatures in the latter part of May, many hop cultivars have responded with vigorous growth. Early and mid-season cultivars which were not pruned this spring are now reaching the point for leaf stripping to begin (if you haven’t started already). The preferred time to strip both mechanically or chemically is when the plants have reached the top wire of the trellis, however, if conditions are favourable for downy mildew (i.e. wet, humid weather), mechanical stripping should begin when the plants are at least 3 m (10′) tall. Chemical stripping (basal foliage removal) should always be performed according to the product label, which is when the bine has barked to withstand chemical application, typically once the hops reach the top wire. For more information on products registered in Canada on hops for basal foliage removal, including plant growth stage, application rates and volumes, please refer to the ‘Broadleaf Weeds, Suckers’ section of the Hops Pesticide List, 2017 on this blog.

Limited information is available about how much leaf area can be removed from the hop plant in the early vegetative stage without negatively impacting yield due to decreased photosynthate production. To better understand this impact, OMAFRA staff initiated a field trial in 2015.

In the trial, 5 leaf stripping treatments were replicated on cv. Nugget hops at the research yard located at the Simcoe Research Station. Each plot consisted of three hills (2 strings/hill with 2-3 bines/string). Plants were observed during the growing season for incidences and severity of downy mildew. Bines were harvested at the end of the season and analysed for total fresh yield, 100 cone fresh weight and 100 cone dry weight at 8% moisture level.

The 5 leaf stripping treatments consisted of:

1. Control (no stripping or removal of leaves/laterals).

2. Strip bines of lower leaves and laterals up to 50 cm from ground when plant height has reached 2.5-3 m.

3. Strip bines of lower leaves and laterals up to 1 m from ground when plant height has reached 2.5-3 m.

4. Strip bines of lower leaves and laterals up to 1.5 m from ground when plant height has reached 2.5-3 m.

5. Strip bines of lower leaves and laterals up to 2 m from ground when plant height has reached 2.5-3 m.

Results of the trial (Figure 2) demonstrated no differences between leaf stripping treatments in 2015. These initial results suggest hops can tolerate considerable leaf removal without affecting yield. However, substantial variability was present in the data, most likely due to the occurrence of diseases (powdery mildew, Fusarium canker, Verticillium wilt and Alternaria cone blight), and therefore further investigation is warranted. A second year of data should be collected to better understand the relationship of leaf removal and impact on yield of hops.

Acknowledgments:

Funding provided by Horticulture Crops Ontario (HCO).

Access to hop research yard provided by the University of Guelph.