The Morton Arboretum, Illinois Route 53, Lisle, Illinois, 60432, USA

Background and objectives

The gibberelin inhibitor paclobutrazol (PBZ) is taken up by roots and translocated to aerial plant parts in the xylem. PBZ results in an increased root:shoot ratio and is becoming widely tested in urban trees as a method to slow branch elongation and control tree size. Recently, mature oaks treated with PBZ exhibited improved vigor and showed less leaf scorch in years following the treatment. Similarly, oak decline and dieback symptoms appeared to diminish in treated trees compared to controls [1]. Here we examine the possibility that pathogens might be directly inhibited by PBZ.

Materials and methods
Seven fungi that cause a variety of diseases were grown in culture: Armillaria mellea, Botryosphaeria dohthidea, Ceratocystis fagaceraum, Fusarium roseum, Sphaeropsis sapinea, Verticillium dahliae, and Sirococcus clavigignenti-juglandacearum. Mycelial plugs were taken from colonies of each fungus and transferred to the center of a quadrant in partitioned Petri plates containing agar media. Four, 5-mm wells were made opposite the mycelial plugs at the edge of each quadrant and each well was filled with one of 5 solutions: sterile, distilled water (control), PBZ at the label rate (297-743 g a.i./tree), and PBZ dilutions of 1:100, 1:200 or 1:400 of the label rate. Plates were randomly positioned in a 25C incubator with 12 hour daylength, and each treatment combination was replicated 6 times. Mycelial growth was monitored until control colonies reached the outer well containing water. At that time, inherent growth rate (mm/day) for the species was calculated and colony diameters in the PBZ treatments were measured for that species. The growth rate and zone of inhibiiton (mean colony diameter in control plate - mean colony diameter in treated plate) were calculated for each treatment combination. Treatment effects were compared using the Analysis of Variance procedure for a nested design.

Results and Conclusions
Two of three trials have been completed to date. The seven species grew at widely different rates ranging from 0.7-6.5 mm/day. Regardless, the PBZ treatments caused a significant reduction in growth rate in all species. Colony diameters in control plates were at least 2-fold larger (and sometimes much more) than in treatment plates. The three PBZ dilutions were as effective in inhibiting the fungi as PBZ used at the label rate. Direct inhibition of fungal growth by PBZ suggests that the observed improvement in tree vigor in the field may be related to disease suppression. Studies to evaluate effects of PBZ on conidial germination and disease development in vivo are underway.

1. Watson GW, 1996. Journal of Arboriculture 22, 211-17.