Two morphologically distinct formae speciales in Neonectria magnoliae differ in their virulence on Magnolia family hosts Liriodendron tulipifera and Magnolia fraseri

The family Nectriaceae includes numerous phytopathogenic fungal genera that cause canker diseases on both angiosperm and conifer hosts worldwide. Among these, Neonectria species are globally important canker pathogens of numerous plant hosts, but their roles in contributing to forest decline and mortality outside their involvement in beech bark disease and apple canker are largely understudied. In the U.S.A. N. magnoliae causes perennial cankers on two native hosts in central Appalachia: Liriodendron tulipifera (tulip poplar) and Magnolia fraseri (Fraser magnolia) and has been recently confirmed from non-native M. stellata (star magnolia) in West Virginia, U.S.A. Both native hosts occur in the central Appalachian Mountains, but M. fraseri occurs mostly at higher elevations, from 600 to 1700 m. Neonectria magnoliae was first described in 1943 (as Nectria magnoliae), yet its impact across the forested landscape remains unclear. To clarify host-specific differences across the contemporary range of N. magnoliae, we used multilocus phylogenetics, comparative pathogenicity/virulence assays, and morphological analyses to determine whether N. magnoliae represents two cryptic species that specialize on L. tulipifera and Magnolia spp. or whether N. magnoliae has host-specific pathotypes. Our studies revealed two morphologically distinct formae speciales within N. magnoliae: (i) N. magnoliae f. sp. liriodendri—strains originating from L. tulipifera with increased virulence on L. tulipifera and lacking macroconidia production and (ii) N. magnoliae f. sp. magnoliae—strains originating from M. fraseri with increased virulence on M. fraseri and producing macroconidia readily in culture. Overall, the incidence of these two pathotypes indicates that neither pathotype poses serious risks to either plant host but can add to cumulative stresses that both tree species are experiencing in the face of shifting global weather patterns.