You can find specific works about mycorrhizal colonization in Juglans spp. (and other species) looking at this review http://link.springer.com/article/10.1007%2Fs00572-005-0033-6
Based on the published literature, Black walnut has not been found to associate with ectomycorrhizae, but forms associations in greenhouse and field studies with several different species of endo- or arbuscual mycorrhizae. The latter form are not culturable on nutrient media.