ABSTRACT
Dutch elm disease, currently by far the most destructive tree disease in the northern hemisphere, is caused by a combination of the vascular wilt pathogen Ceratocystis ulmi and vector elm bark beetles of the family Scolytidae. The d-factor was first described in a French isolate of the North American (NAN) aggressive subgroup of C. ulmi, and it is with the NAN form of the fungus that most experimental work with d-factors has been carried out. The "d-reaction" is a characteristic though variable pattern of mycelial growth and sometimes of pigmentation that occurs when a diseased or d-infected "donor" isolate is paired with a "healthy" recipient isolate on elm sapwood agar (ESA). The possibility that a d-factor spreads to parts of a recipient mycelium beyond the visibly altered sectors of the d-reaction was investigated in an experiment involving the d'-factor. Indeed, a striking aspect of d-reactions already described is the reduction in growth of the recipient that may follow d-factor transmission.
