Habitat and variability of Humulus lupulus var. lupuloides in upper midwestern North America: a critical source of American hop germplasm

Well-drained terraces of river basins in Midwestern North America are home to the purest stands of the native hop, Humulus lupulus var. lupuloides. Pl ants of H. lupulus var. lupuloides are of exceptional significance, because they have contributed germplasm to prominent international hop cultivars. The ecology, traits, and survival status of this taxonomic variety are the subjects of this study. Native Humulus populations were typically associate d with only a few tree and shrub species, notably Acer negundo L. [box elde r], Crategus spp. [hawthorn], Fraxinus nigra Marsh. [black ash], F. pennsyl vanica Marsh. [green ash], Populus balsamifora L. [balsam poplar], P. delto ides Marsh. [cottonwood], Prunus americana Marsh. [wild plum], P, pensylvan ica L. [pin cherry], P. virginiana L. [choke cherry], Quercus macrocarpa Mi chx. [bur oak], Ribes spp. [wild currant], Rosa spp. [wild rose], Salix spp . [willow], Sambucus canadensis L. [elder], Symphoricarpos occidentalis Hoo k. [snowberry], and Viburnum opulus var. americanum Ait. [highbush cranberr y]. Cone (strobilus) variations, exceeding those of leaves and vines, inclu ded size (15 mm to 45 mm long), shape (globose to ovoid-conical), and compa ctness (loose vs. imbricated bracts). Trait diversity of seedling progenies exceeded that of each native population from which they originated. Traits of genetically potential usefulness included dwarf plant habit, precocious cone production, early season flowering, pest repellence, frost resistance , potential drought resistance, and commercially desirable cone structure. Both native-plant and progeny populations were generally free of diseases c ommon to commercial hops, thus offering germplasm that is compatible with i nstitutional hop breeding programs. Persistence of native hop populations f rom 1989 to 1999 was problematic, with five of 11 selected study sites sust aining complete loss or serious damage. Protection of sensitive riparian ha bitats occupied by this genetic resource may be warranted, particularly unt il adequate germplasm sources are secured and reposited for further investi gation and use.