While Diaporthe humulicola (DIE-a-por-thee hyoo-moo-lick-o-lah) may be fun to say, it is anything but for those growing hops, one of the main ingredients for beer.

D. humulicola causes halo blight of hop. It was first described in 2018 and is a major concern for growers of the crop in the eastern U.S. and Canada. The pathogen grows slowly and can take up to 30 days to develop visual diagnostic features. This doesn’t allow much time to implement halo blight management strategies – and it can lead to a yield loss of up to 60%.

The pathogen is known to colonize hop leaves, cones and bines, but it’s not yet known if D. humulicola can be detected in the crown or rhizome of hop. Luckily for brewers and imbibers, a team of researchers from Michigan State University, the USDA-ARS Forage Seed & Cereal Research Unit, Oregon State University and Virginia Polytechnic Institute & State University are on the case. They presented “Development of integrated qPCR diagnostic tools to determine the lifecycle of Diaporthe humulicola in hop” at the 2023 Great Lakes Expo.

Quantitative polymerase chain reaction (qPCR) is a very cool lab technique that amplifies and measures DNA in real time. A specific qPCR diagnostic tool was developed for D. humulicola to detect blight earlier and to figure out the pathogen’s lifecycle to hopefully stop it from causing issues sooner.

Hop rhizomes (root cuttings of the hop bine) and hop cones were taken from a research hop yard in East Lansing, MI. The rhizomes were cut open for imaging and other sections were reserved for DNA extractions. The cone and rhizome DNA were then tested for the presence of D. humulicola.

The cones were collected from the same plant in the research yard over 10 weeks in summer 2022, and a qPCR assay was run on DNA extracted from those cones. The plant was purposefully inoculated with D. humulicola – and by week 8, 100% of the cones on the plant had it.

In theory, the experiment worked. The researchers were able to develop a tentative lifecycle for D. humulicola. They believe the pathogen overwinters as pycnidia (flask-shaped fruiting bodies) on dried hop bines left after harvest. At high humidity and temperatures around 68º F, those pycnidia will produce conidial ooze. The conidia will then germinate and cause lesions with concentric circles on hop leaves – and the conidia from leaves can infect both cones and leaves. Then, pycnidia can develop on necrotic hop tissue. The pycnidia then produce more conidial ooze. The pathogen is dispersed by rain events.

There’s more good news: The D. humulicola assay the researchers created can detect Diaporthe from a single hop cone and it’s more likely to detect the pathogen than traditional isolation of the pathogen. It can also detect low levels of Diaporthe in asymptomatic leaves.

But, while a disease cycle for D. humulicola has been hypothesized, it is still unknown exactly when the pathogen invades the hop rhizome.

Going forward, the research team will be doing a technology transfer with other collaborators to determine if their assay works on other qPCR machines. Their goal is to deploy the D. humulicola assay to diagnostic clinics across the eastern U.S.

They’re also aiming to use the rhizomes and cones collected in 2022 to investigate the microbiome of a hop yard with differing levels of disease severity.

(It should be noted that funding for this research was provided by MSU Project GREEEN, the Michigan Craft Beverage Council and MDARD.)