Department of Plant Pathology and Microbiology, Texas A&M University, College Station, Texas 77843-2132, USA

Background and objectives
Oak wilt caused by the fungus Ceratocystis fagacearum causes localized epidemics in oaks (Quercus spp.) throughout 22 states in the midwestern, north-central, and mid-Atlantic USA. This vascular pathogen is particularly devastating in the native live oak (Q. fusiformis) savannahs of central Texas. Although this disease has been studied for 45 years, many aspects of oak wilt epidemiology remain obscure. We have developed DNA markers in the fungus and isozyme markers in the host to elucidate the population biology of this pathogen-host interaction. Our objectives were to: (1) determine the genetic structure of the pathogen population; (2) determine the genetic structure of the host population; and (3) determine how the host population is affected by an oak wilt epidemic.

Materials and methods
For Objective 1, genetic variation in the nuclear and mitochondrial DNA of C. fagacearum was assayed using anonymous nuclear DNA probes and purified mtDNA hybridized to geographically diverse isolates. Polymorphic nuclear probes were used to assay 264 isolates sampled from 48 oak wilt centres across Texas. In one oak wilt centre, 42 isolates were assayed along the perimeter of a large infection focus. To achieve Objectives 2 and 3, 109 surviving oak trees were sampled from a post-epidemic population that had experienced 80% tree mortality and 112 trees were sampled along transects outside the epidemic boundary (called the pre-epidemic population). The percentage crown loss was estimated for all post-epidemic trees. Four allozyme loci were assayed for all 221 trees. Allozyme data were used to compare the genetic structure of pre- and post-epidemic oak populations.

Results and conclusions
Although the mtDNA genome was large (c. 170 kb), no variation was found in any isolate tested. The nuclear DNA also had very little variation across the geographic range of the fungus [1]. The extremely low level of variation in the nuclear and mtDNA genomes of C. fagacearum are consistent with a founder effect or a recent speciation event. We believe that oak wilt is a relatively new disease in North America that has the potential to spread beyond its present range. After screening 437 probe-enzyme combinations, six were chosen for RFLP analysis of the Texas-wide population. Among the 42 isolates collected in one infection focus, two haplotypes were found. Within a live oak centre, the fungus appears to reproduce exclusively asexually. In the Texas-wide population, 22 haplotypes were found. RFLP loci were at gametic equilibrium across Texas, suggesting a random-mating pathogen population statewide. Significant changes were detected for all four allozyme loci between the pre- and post-epidemic oak populations. Allele and genotype frequencies differed, significant multilocus associations developed in the postepidemic population and correlations were detected between allozyme aliele frequencies and crown loss (unpublished data). The results demonstrate that disease can have a significant effect on the genetic structure of natural host populations. We hypothesize that the pathogen caused selection for resistant trees in the oak wilt centre.

1. Kurdyfa TM, Guthrie PAI, McDonald BA, Appel DN, 1995. Current Genetics 27, 373-378.