Shipping of hop plants and planting material to the pacific northwest prohibited by quarantine

As growers and propagators look to source and sell planting materials, it's important to remember quarantine rules.

With spring field activities here, a reminder on important quarantine rules is timely as growers and propagators source and sell planting materials.

Over the past several years, the hop commissions in Washington, Oregon and Idaho have worked together and with their departments of agriculture to strengthen quarantine rules that ensures consistency amongst state regulations in the region. The quarantine rules allow free distribution of hop plants and planting materials within Washington, Oregon and Idaho. Import of these materials from outside the Pacific Northwest, however, is prohibited, with the exception of kiln-dried cones. Details for Oregon, Idaho and Washington are available.

The quarantine rules are science-based and in place to protect the industry against novel strains of the powdery mildew fungus (and potentially other pests and diseases) that do not exist in the Pacific Northwest, but are widely distributed outside of the region.

Powdery mildew has been present on hops in the Northwestern U.S. for over two decades, so why are quarantine measures still needed?

A little background on the disease biology is helpful to understand the rationale and science behind the quarantine rules.

The fungus that causes hop powdery mildew exists in two forms called mating types, somewhat like male and female forms. Research documents that only one of the two mating types of the fungus is present in the Pacific Northwest, but both mating types are widely distributed in all other areas where the disease is known to occur. Therefore, if hop materials or rhizomes are imported and have powdery mildew--a common problem in greenhouses and nurseries--the plants likely will harbor both mating types of the pathogen. This fine point in the pathogen biology could have massive implications for disease management. Because just one mating type of the fungus occurs in the Pacific Northwest at present, the pathogen survives the winter only on live hop plants, only in a handful of yards, and only at a very low level. At present, growers can nearly or entirely eliminate powdery mildew from yards in spring with their pruning practices, greatly limiting the extent and severity of later disease outbreaks. When both mating types are present, however, overwintering mildew becomes impossible to eliminate with cultural practices alone and early season disease outbreaks become more widespread. With only one mating type, infested crop debris isn't a concern for powdery mildew. With both mating types present, shattered cones and infested crop debris from the previous season becomes a new source of mildew in spring, and the disease cycle is kicked off in every yard that had mildew the previous year and not just those few yards with flag shoots. The potential impact is earlier, more widespread, and in general can cause more severe disease outbreaks.

Any individual or firm propagating and/or selling hops nursery stock must obtain a license and be inspected by the Michigan Department of Agriculture and Rural Development (MDARD). Propagators and other individuals selling planting material and other regulated items are encouraged to contact MDARD at (800) 292-3939 for additional guidance on plant quarantine rules.

Please continue to visit Michigan State University Extension’s hop webpage or the MSU Hops News Facebook site for up to date information.

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