Endophytic-host selectivity of Discula umbrinella on Quercus alba and Quercus rubra characterized by infection, pathogenicity and mycelial compatibility

The fungal endophytic - host relationships of Discula umbrinella and two oak species, Quercus alba and Quercus rubra, were characterized on the basis of endophytic infection, pathogenicity, and mycelial compatibility. Isolates of D. umbrinella were cultured from leaves of Q. alba and Q. rubra collected from a hardwood forest located in Patuxent Wildlife Research Center in Laurel, Maryland, USA. Endophytic infection was observed on sterile leaf discs and living 2-month-old seedlings of Q. alba and Q. rubra. Fungal-host reciprocal inoculations revealed the presence of conidiomata on both hosts but conidiomata production was more abundant on Q. alba. Isolates from Q. rubra produced milder disease symptoms on both oak species. Mycelial compatibility studies identified seven different MCG groups. MCG groups 1 - 3 contained isolates from both oak species whereas MCG groups 4 - 7 contained host specific isolates. Field studies monitored the seasonal appearance of the sexual fruiting structures, perithecia, as a possible source of new genetic variation that might alter host specificity/pathogenicity of the D. umbrinella isolates on Q. alba and Q. rubra hosts. Only 1 - 2% of the leaves contained perithecia throughout the sampling period ( April - September). Isolates collected from Q. alba differed from those collected from Q. rubra in endophytic infection, pathogenic response, and perithecia production. The results of this study suggest that the endophyte - host relationship is one of host selective preference for Q. alba, but that the endophyte has the ability to maintain the endophytic/pathogenic life cycle on the less preferred host species, Q. rubra.