Powdery mildew pathogenesis of Arabidopsis thaliana

A powdery mildew (Erysiphales) infection of Arabidopsis thaliana was found in the greenhouse of the Department of Molecular Biology at Massachusetts G eneral Hospital (MGH isolate). We studied the structure of the anamorph, th e sequence and chronology of its development, and its virulence on A. thali ana and other hosts. The teleomorph of the fungus was not found on the mate rial we examined. The MGH isolate caused severe powdery mildew symptoms on some A. thaliana accessions, infecting rosette and cauline leaves, stems an d siliques, but was practically avirulent on others. Previously, two Erysip hales have been reported to infect A. thaliana. These were identified as Er ysiphe cichoracearum (UCSC isolate) and E. cruciferarum (UEA isolate). The MGH isolate was found to be a member of the E. cichoracearum group in the s ection Golovinomyces. It can be distinguished from both of these previously described powdery mildews of A. thaliana based on the morphology, host ran ge and virulence. It might be best referred to E. orontii following the sys tem proposed by Braun.