Hazelnuts are wind-pollinated, monoecious, mostly dichogamous, and self-incompatible of the sporophytic type. About 90% of the cultivars studied are protandrous. Anthesis of the pistillate flower is temperature-dependent and occurs from December through February, with its peak in January. Stigmatic surfaces may remain receptive for up to 3 months. Four to 5 months separate pollination and fertilization of the ovule, which usually occurs between mid-May and the end of June in Oregon. A 10% pollinizer density has been the standard, with a recommended distance of <20 m between the main cultivar and the nearest pollinizer. Two or three different pollinizer varieties with different times of pollen shed are recommended. The Oregon hazelnut industry is presently combating the fungal disease, Eastern Filbert Blight, Anisogramma anomala. Part of the current management recommendations are to reduce the susceptible pollinizer varieties to a density of around 5%, and then gradually replace those left with immune or more-resistant genotypes. Recent research by S.A. Mehlenbacher refined methods of using fluorescense microscopy to quickly determine genotype compatibility. The self-incompatiblity is controlled by a single gene with multiple alleles. The biochemical, physiological, and molecular aspects of sporophytic self-incompatiblity are being research by A.N. Azarenko.
