North Central Forest Experiment Station, USDA Forest Service, St. Paul, MN, 55108, USA

Background and objectives
Oak wilt mats formed on recently killed oaks are the inoculum source for the primary insect vectors of the pathogen, Ceratocystis fagacearum, in the North Central states of the USA. These insects (Coleoptera: Nitidulidae) are attracted by the sweet odor of the fungal mats. After traversing and feeding on the mats, the beetles leave with pathogen spores on and in their bodies and may transmit C. fagacearum to fresh wounds on healthy oaks. The fungal mats and the xylem tissue adjacent to the mats are often colonized by other organisms. The Graphium anamorph of Ophiostoma quercus is a common colonizer of spring mats in Minnesota and West Virginia, and is a putative biocontrol agent of overland transmission of the pathogen [1]. Other associated fungi have been shown to affect pathogen viability in laboratory studies. For example, Gliocladium roseum and Trichothecium roseum prevented growth or caused mycelial death of C. fagacearum [2]. Laboratory and field studies commenced in 1995 to determine if overgrowth of oak wilt mats by putative biocontrol fungi negatively affect (i) frequency and number of viable pathogen propagules present on mats, and (ii) frequency and pathogen propagule numbers on nitidulids visiting such mats.

Results and conclusions
O. quercus overgrowth of C. fagacearum colonies in petri dish culture did not reduce the number of viable C. fagacearum propagules recovered through serial dilution plating of subsamples removed from the cultures. In two field experiments, augmentation sprays with O. quercus were used to establish different levels of oak wilt mat colonization by that fungus. The extent of O. quercus colonization was highest for sprayed, postmature mats with open cracks compared to non-sprayed mats naturally colonized by O. quercus (P<0.02). However, no differences were found for both extent of C. fagacearum recovery and number of viable propagules per cm2 of mat surface on the same treated and non-treated mats(P<0.05). The frequency of nitidulids with O. quercus was highest (83%) for those captured from O. quercus treated, postmature mats open on the spray date (P<0.05). The frequency of C. fagacearum recovery from these same beetles was also highest (88%) for post-mature mats. In conclusion, O. quercus colonization of mats does not significantly affect C. fagacearum viability or vector acquisition of the pathogen from oak wilt mats.

Preliminary biocontrol screening trials were conducted in the laboratory with four isolates of G. roseum and two isolates of T. roseum. Aqueous suspensions (0.2 ml) of the biocontrol candidates were pipetted across one diameter of established C. fagacearum colonies (av. 4.2 cm) in petri dish cultures. Following 7 days incubation (24C, dark conditions), mycelial plugs were removed from overgrowth areas of C. fagacearum colonies and subjected to serial dilution plating. Isolates within each biocontrol candidate species varied in their effect on recovery of viable C. fagacearum propagules. Overgrowth by one isolate of each species resulted in no recovery of C. fagacearum from any treated plates. Other isolates resulted in slight to significant reductions in frequency of pathogen recovery and number of viable propagules obtained per cm2 of colony surface. All subsamples from water-treated colonies (controls) yielded C. fagacearum propagules with a mean of 3.9 x 106 propagules per cm2. Based on these preliminary results, colonization of oak wilt cultures by certain isolates of G. roseum and T. roseum appears to drastically affect availability of viable C. fagacearum propagules. Further in vitro screening of additional isolates is under way and experimental field trials are planned.

1. Reutze M, Parameswaran N, 1984. European Journal of Forest Pathology 14,326-333.
2. Shigo SL,1958. Mycologia 50,757-769.