Controlled Heating of Root-Pruned Dormant Prunus spp. Seedlings Before Transplanting to Prevent Crown Gall. Larry W. Moore, Associate Professor of Plant Pathology, Oregon State University, Corvallis 97331. Jack Allen, Former Research Technician, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331. Plant Dis. 70:532-536. Accepted for publication 14 January 1986. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/PD-70-532.

Incidence of crown gall was reduced by heating pruned root systems of dormant seedlings of three Prunus spp. in Styrofoam ice chests at 18–25 C for 1–3 wk before inoculating the seedlings with Agrobacterium tumefaciens. Gall incidence on mahaleb cherry roots was reduced from 100% (unheated control) to 0% after 7 days at about 25 C; the reductions for mazzard cherry and myrobalan plum seedlings were only to 66 and 61% from 100 and 73%, respectively. Later experimentation was conducted only with myrobalan plum seedlings in insulated boxes that provided more uniform and consistent temperature control. In the improved heat boxes, gall incidence dropped from 60% for the unheated myrobalan seedlings to 5% when the seedlings were heated at 24 C for 7 days. At 18 C, 3 wk of heat exposure was required to reduce the incidence of gall to 11%. Gall incidence was reduced further when seedlings were inoculated with the biocontrol agent A. radiobacter K84 before exposure to heat. Incidence of gall was not influenced by treatment with indolebutyric acid (IBA, 500 μg/g in methanol and water) as a possible stimulator of wound callus, but seedling mortality reached 90% when IBA treatments were combined with heat exposure. In the absence of IBA, mortality of myrobalan seedlings appeared random across treatments from 2 to 24 C. Holding heat boxes in a cold room at 2–4 C during temperature treatments kept buds from breaking dormancy. The heating procedure was modified for use in a commercial nursery, and the incidence of naturally occurring crown gall on mazzard cherry seedlings was reduced from 66 (unheated) to 6%.

Keyword(s): biological control.