Blank fruits in hazelnut (Corylus avellana L) cv'Butler': Characterization and influence of climate

Seedless nuts, or blanks, are a serious problem in many hazelnut (Corylus avellana L.) cultivars and represent an important economic loss to the commercial grower. To evaluate the growth and fruit production of hazelnut in the northern region of Portugal, a trial with II cultivars was established in 1984. 'Butler' was the most productive cultivar and is well-suited to the in-shell market, but in eight years it produced 8.4% blanks. Blanks could not be fully described according to current classifications and so we propose a new one in which ten types of blanks are defined according to the appearance, number, and size of the ovules, aspect of funiculus, existence of internal fibre, and shell appearance. To investigate the causes of blanks, blank fruit rates were correlated with climatic factors (temperature, rainfall, sunshine, wind and relative humidity) in each year from April to July. The increase in solar radiation and decrease of relative humidity in May, and higher minimum temperatures in July, were correlated with a reduction in blank rates in 'Butler'.