🌰 The fungus that drew the hazelnut map

Start with the tree's strangest habit: it flowers in winter. USDA describes hazelnuts as monoecious — one tree carries both the male and the female flower parts — and says they bloom in winter, while the branches are still bare.

Basis: USDA Northwest Climate Hub, “Climate-Resilient Hazelnuts in Oregon and Washington”: “Hazelnuts are monoecious (have both female and male parts) and bloom in winter.”

The female flower is easy to miss entirely. In the wild beaked hazelnut, USDA describes it as tiny and inconspicuous — nothing but bright red threads poking out of a bud that otherwise looks like every other bud on the twig.

Basis: USDA NRCS Plant Guide, Corylus cornuta (beaked hazelnut): “female flowers: several in a scaly bud, tiny and inconspicuous with only bright red stigma and styles protruding from the otherwise gray-brown bud.”

A tree that flowers in winter needs a winter it can survive flowering in. USDA says reliable hazelnut crops “can only be produced under moderate climatic conditions,” and names Oregon’s Willamette Valley and western Washington as places that have them.

Basis: USDA Northwest Climate Hub: “reliable hazelnut crops can only be produced under moderate climatic conditions, such as those found in Oregon’s Willamette Valley (which produces 99% of U.S. hazelnuts) and in western Washington.”

Now the fungus. Eastern filbert blight is native to eastern North America. On the wild American hazelnut — its natural host — it does limited damage. On the European hazelnut, the one farmers actually plant, it causes severe cankers, branch dieback and even death. USDA calls it the primary reason commercial hazelnuts have not been grown elsewhere in the U.S.

Basis: USDA Northwest Climate Hub: “Eastern Filbert Blight (EFB), a fungus native to eastern North America, has obstructed hazelnut production throughout the U.S. Its native host is the American hazelnut, in which the fungus causes limited damage. In the European hazelnut, however, EFB causes severe cankering, branch dieback, and even death. EFB is the primary reason commercial hazelnuts have not been grown elsewhere in the U.S.”

The blight reached the Northwest anyway — USDA dates its arrival in Oregon and Washington to the 1960s. Then came plant breeding. USDA says the development of blight-resistant varieties has facilitated the expansion of hazelnut production in Oregon and Washington.

Basis: USDA Northwest Climate Hub: “EFB has also severely impacted hazelnut production in the Northwest since the fungus was introduced to Oregon and Washington in the 1960s”; “The development of EFB-resistant cultivars has facilitated the expansion of hazelnut production in Oregon and Washington.”