Hazelnut

Three facts govern every hazelnut planting, and the cultivar list is downstream of all three.

It cannot pollinate itself. Hazelnut is monoecious and wind-pollinated, but it is dichogamous and carries sporophytic self-incompatibility controlled by a single S-locus with many alleles — 33 of them known. Both alleles are expressed in the stigma; dominance and codominance can occur in the pollen. Two plants carrying different S-alleles are the minimum, and because the female flowers open over an extended period, orchards are planted with early, mid and late pollinizers rather than one. This is why a database of hazelnuts records pollinizers as cultivars in their own right.

It flowers in midwinter. Anthesis of the pistillate flower is temperature-dependent and can run from late November into early March. A crop set in January is a crop set by whatever the weather did in January.

One fungus wrote the rest of the list. Eastern filbert blight, caused by Anisogramma anomala, kills European hazelnuts by stem canker, and nearly all European cultivars are highly susceptible. It is the reason hazelnut production was for a century a Pacific Northwest crop rather than an eastern one.

The way out came from a single plant. In 1975 an obsolete, late-shedding pollinizer called ‘Gasaway’ was found free of disease in a heavily infected, dying ‘DuChilly’ orchard at Battle Ground, Washington. Its resistance turned out to be a dominant allele at a single locus, carried heterozygous, so its progeny segregate half resistant and half susceptible — which made it trackable with DNA markers and usable at scale. Oregon State University built a generation of cultivars on it, and Oregon acreage roughly doubled behind them.

That is where a grower reading this in 2026 has to slow down. A mutation letting the fungus reproduce on ‘Gasaway’-resistant trees was detected in 2023, and by 2025 three distinct strains had been identified in Pacific Northwest orchards. Cultivars described at release as completely resistant — ‘Jefferson’, ‘McDonald’, ‘Yamhill’ among them — are reported infected. Resistance resting on one gene is a term, not a property. The records below state the resistance as the release sources state it, and the caveat travels with all of them.

The rights are worth reading as carefully as the resistance. Oregon State released ‘Jefferson’ as a public variety with no restrictions on US growers, and patented and licensed ‘Dorris’, ‘Wepster’, ‘McDonald’ and ‘PollyO’. Every Rutgers release carries a US plant patent. Same crop, same decade, opposite answers to what a grower may propagate.