Inheritance of resistance to Glomerella cingulata in Populus

In natural populations of Populus trichocarpa Torr. & A. Gray, large, concentric leaf lesions caused by Glomerella cingulata (Stonem.) Spaulding & Schrenk only appear in late autumn. In contrast, small, dark lesions associated with crinkling of the leaf lamina have been observed on P. trichocarpa × Populus deltoides Bartr. ex Marsh. hybrid F2 individuals in spring. Inoculation with a monoconidial isolate demonstrated that the two lesion types are not caused by distinct pathotypes of G. cingulata; typical lesions formed on P. trichocarpa and on F2 individuals. Segregation in two related P. trichocarpa × P. deltoides pedigrees demonstrated that resistance is inherited from P. deltoides and conferred by one to several genes. The inoculation study with G. cingulata confirmed that resistance is inherited from P. deltoides. The susceptibility of P. trichocarpa was evident in the study environment of low light intensity and daily misting. The ability of G. cingulata to cause latent infection was tested with forced cuttings from which preformed leaves emerged and displayed diagnostic symptoms. The apparent resistance of P. trichocarpa in nature likely involves such latent infection. It is proposed that G. cingulata can become an early season pathogen of those F2 hybrids deficient in both the polygenic resistance of P. trichocarpa and a major gene for resistance from P. deltoides.