WL MACDONALD¹, FH TAINTER², J PINON³ and ML DOUBLE¹

¹Division of Plant and Soil Sciences, West Virginia University, Morgantown, WV 26506-6057, USA; ²Department of Forest Resources, Clemson University, Clemson, SC 29634, USA; ³INRA, 54280 Champenoux, France

Background and objectives
Oak wilt, a vascular wilt disease currently known only in the USA, is caused by Ceratocystis fagacearum. It is as potentially destructive as Ophiostoma ulmi, the causal agent of Dutch elm disease, but in the USA lacks an efficient and aggressive insect vector. Oak wilt is a threat to Europe because numerous valuable oak species occur there, as does the European oak bark beetle, Scolytus intricatus, a potentially efficient vector. In the early 1980s, scientists recommended to the European Union that inoculation studies should be conducted to determine the susceptibility of European oaks to C. fagacearum.

Materials and methods
Collections of acorns, made throughout northern and southern Europe during 1982-84, were sent to Clemson, SC and Morgantown, WV for culture. Seedlings grown from these acorns were outplanted and managed until this test in 1996. North American oak species of known susceptibility were included as controls at both locations. In April and June 1996, inoculations were made at Clemson and Morgantown, respectively, by introducing a mixture of spores from three virulent C. fagacearum strains into branches or main stems.

In South Carolina, provenances of the following European white oak species were evaluated: Quercus pedunculata, Q. petrae, Q. pubescens and Q. sessiliflora. North American white oak species included as resistant controls were Q. alba and Q. stellata. Susceptible red oak controls were Q. rubra and Q. falcata. In West Virginia, European white oak provenances evaluated were Q. pedunculata and Q. sessiliflora. Resistant North American white oak controls included Q. alba and Q. prinus. Q. rubra served as the susceptible red oak control.

At each location, symptom ratings began approximately 1 month after inoculation and continued through the 1996 and 1997 growing seasons. Symptom expression was evaluated by rating the portion of the crown of each tree that exhibited wilt and dieback symptoms. A report on the preliminary research associated with this study has been published [1].

Results and conclusions
In the South Carolina test, at the end of the 1996 season, there were no significant differences in symptom expression among the European or North American oaks with regard to species, provenance or inoculation site (branch, main stem). Average symptom expression ratings for the European, North American white, and red oak species were 26.2, 21.6 and 32.9%, respectively. By the end of the 1997 season, symptoms had progressed to 72.8, 46.0 and 83.6%, respectively. Although two-thirds of the inoculated trees had died by the end of the second growing season, the remaining third, while debilitated, appear to have survived infection.

In West Virginia, there was no significant difference in symptom expression between the European and North American red oak species with regard to species, provenance or inoculation site at the final 1996 evaluation. Symptoms for the European white and North American red oak species averaged 96 and 100%, respectively, while American white oak species averaged 20%. After the 1997 growing season, symptoms in European and North American red oak species approached 100% while the North American white oaks averaged 22%. Unlike South Carolina, the only surviving trees were those representing the two North American white oak species. The significant mortality that occurred among the European white oaks indicates that unlike their resistant North American white oak relatives, they could be seriously impacted by the accidental introduction of C. fagacearum into Europe.

Reference
1. Pinon J, MacDonald WL, Tainter FH, 1995. Proceedings of the National Oak Symposium, Austin, Texas, pp. 63-73.