Post-Harvest Hopyard Management

 

 

The big question for many Michigan growers this season has been how to manage the risk of powdery mildew. With harvest underway or completed for many Michigan growers, the focus now shifts to how to best prepare yards with existing powdery mildew infections.

 

Powdery mildew (Podosphaera macularis) is an emerging disease in Michigan hopyards that has serious implications for growers. Hop powdery mildew is completely unrelated to hop downy mildew (Pseudoperonospora humuli) and was first detected in Michigan in 2014.

 

Powdery mildew has been a concern on greenhouse/baby plants, but was not a significant issue in the field until 2017 when severe outbreaks occurred at multiple Michigan farms, likely from introduction on planting material and highly conducive environmental conditions.

 

 

Powdery mildew may overwinter in several ways. The fungus may persist as mycelia (fungal threads) on live tissue of the host, most important in and on crown buds. Shoots emerging from infected buds form “flag shoots,” which are covered with spore masses, appearing white, stunted and with distorted leaves. Flag shoots are rare (less than 1 percent of total shoots) and healthy shoots quickly outgrow infected shoots, making detection difficult. The spore masses produced on flag shoots spread to adjacent healthy tissue causing new infections.

 

 

Another means of overwintering is by sexual spores produced in a survival structured (termed a chasmothecium). However, chasmothecia have not been associated with the outbreaks this season. Ascospores are discharged and land on newly emerged shoots where they germinate, infect the plant and eventually produce a new spore mass of asexual spores (conidia). Conidia are produced in large numbers over multiple cycles and are dispersed via wind, rain splash, insects, tractors, equipment and humans.

 

 

Fungicide applications alone are not sufficient to manage powdery mildew. Complete chemical or mechanical removal of the first flush of growth in spring is an important management strategy. Under high disease pressure, mid-season removal of diseased basal foliage delays disease development on leaves and cones. Desiccant herbicides should not be applied until bines have grown far enough up the string so the growing tip will not be damaged and bark has developed on stem.

 

In trials in Washington, removing basal foliage three times with a desiccant herbicide provided more control of powdery mildew than removing it once or twice, particularly on cones. Established yards can tolerate some removal of basal foliage without reducing yield. This practice is not advisable in baby yards (less than 2 years), and may need to be considered cautiously in some situations with sensitive varieties such as Willamette.

 

Bud infection leading to flag shoots in the following year occurs sometime between mid-July to late September/early October. The earlier disease occurs during this period, the more likely it is that flag shoots will be produced in the following year. In Washington, post-harvest applications of fungicide have not been shown to be effective in reducing disease in the subsequent season.

 

However, there may be some value in post-harvest fungicide applications in hopyards where the disease attacked late in the season (i.e., September rather than July) and mechanical pruning is not possible in spring. In these situations, post-harvest application of products such as horticultural oils and sulfur may reduce the risk of flag shoot development and the number of chasmothecia that may overwinter.

 

Remove all unharvested plants promptly after harvest to minimize the number of chasmothecia in affected yards. Cultivation after harvest will bury infested plant debris bearing chasmothecia that can serve as a source of the pathogen in the following year. Most importantly, implement an appropriate powdery mildew program in spring that starts with the mechanical or chemical removal of the first flush of growth in spring, manage plant growth to limit superfluous growth, and apply well-timed fungicide applications. Weak, stressed or baby plants should not be pruned.

 

Though frequently cited in popular literature, optimal fertilization, soil health and water management alone are not adequate for disease control, although these practices may moderate disease levels. Likewise, biorational compounds, biological controls, manure teas and various botanicals and natural products have shown minimal to no efficacy against powdery mildew under moderate to severe disease pressure. Organic producers should consider a combination of sulfur and oil-based fungicides and, most importantly, select resistant cultivars.

 

For more information on the life cycle, appearance and management of powdery mildew of hop, refer to the Michigan State University Extension article, “Powdery mildew arrives in Michigan hopyards.”

 

Controlling downy mildew

 

Downy mildew was not a major issue for most Michigan hop growers this season. As harvest wraps up, we have received inquiries around the availability and efficacy of post-harvest methods to combat this 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 hop (see image below) and how growing conditions in Michigan can affect the efficacy of late season treatments.