A-Z of Hops: M is for Mildew

It could have been Mittlefrau, could have been Magnum. But hops ain't all about aroma and bitterness. Sometimes it's about death.

Like any agricultural crop, hops suffer from disease. Mildew has had a devastating effect on hop production worldwide; crop management and breeding plays a key role in mitigating mildew's adverse effects.

So, what are we dealing with? Powdery mildew (Sphaerotheca macularis) can infest the bud over the winter season and spread into early shoots. Infecting the leaves (and the cone directly in susceptible varieties) the dusty-grey fungus deforms the cone's formation or even prevents it from forming. If not spotted in the early stages, powdery mildew can reduce yields by around 80% even when pesticidal treatment is given.

Downy mildew (pseudoperonspora humuli) thrives in damp, humid environments. Spreading out from infected shoots, it infects the leaves with a blanket-like fungus leading to root rot, stunted leaves, blighted cones and ultimately the death of the bine.

Both kinds of mildew have wrought havoc with hop production. Downy mildew almost destroyed German hop production in the 1920's. An outbreak of powdery mildew in the USA's Yakima Valley in 1997 cost the native hop business around 15% of total crop revenue for several years after. Both kinds contributed to the US hop industry shifting westwards as New York-based production failed comprehensively after mildew took hold in the early 1900's.

The industry's response has been to develop hop varieties that are more resistant to disease and infestation. Cultivars such as Willamette, Cascade and Perle have been developed with resistance to mildew in mind. Given that low levels of leaf disease can still lead to unacceptable reductions in cone quality, plant development and management remain high priorities for growers to secure a disease-free, high-yield, high-quality crop.