Black Walnut: A Nut Crop for the Midwestern United States

Abstract

Eastern black walnut (Juglans nigra L.), is a native tree species valued both for its timber and nuts. Individual trees require excellent soils with adequate moisture for maximum wood and nut productivity. The vast majority of black walnut nutmeat production is centered in the western part of the species’ native range and is predominantly derived from wild, unimproved sources. Historically, the size of this crop has ranged from 4.5 to 15.9 million kilograms (fresh weight, without hull) with less than 1% obtained from improved cultivars. Alternate bearing is common in this species, primarily as a result of susceptibility to walnut anthracnose Gnomonia leptostyla (Fr.) Ces. & De Not. Significant reductions in nut productivity caused by walnut curculio (Conotrachelus retentus Say) are common for unimproved, wild trees. Based on a collection of 65 nut cultivars established by the University of Missouri, black walnut exhibits significant genetic variation for a range of commercially important nut traits, including precocity, percent kernel, nut bearing habit, anthracnose tolerance, season length, and yield efficiency. Exploiting this variation through traditional plant breeding techniques will result in new, improved varieties in the future. Defining improved cultural practices that will significantly impact yield and nut quality remains an area of active investigation. Such knowledge will need to be combined with improved cultivars that are well adapted to local growing conditions to ensure the commercial success of this species over the long run.