Integrated Biological and Chemical Control Of Phytophthora Root Rot Of Azaleas And Rhododendrons
W. H. Wills
Virginia Polytechnic Institute & State University
In the early years of the 1980's experiments conducted at Virginia Polytechnic Institute and State University in Blacksburg, Virginia had given promise of a possible biological control for Phytophthora root rot of woody plants grown in containers under nursery conditions. The hope was centered on the proposed use of a common soil-borne fungus, Mortierella alpina, which had shown antagonism to Phytophthora isolates in vitro and to some extent in pine bark potting media (1, 2, 3, 4, 5).
As part of an ongoing research project and with some additional support from an American Rhododendron Society research grant, further studies were conducted by Mr. R. S. Kularatne, at the time a graduate student in the Department of Plant Pathology, Physiology and Weed Science at Virginia Tech, under the direction of his advisor, W. H. Wills. The portion of the research which was to be aided by the grant was in two parts; to study survival of pathogen and antagonist in pine bark potting media and to compare root rot development of azaleas grown in pine bark media in which certain fungicides and the antagonist had been introduced (6).
It was found that steam sterilization under pressure for as much as 15 minutes (normal practice for sterilization) rendered pine bark toxic in some way to both pathogen and antagonist whereas steaming without increasing pressure did not. In un-amended pine bark both pathogen and antagonist survived for several months even in the absence of any host plant. The pathogen spread quickly through the bark from point of infestation and the antagonist remained static at the point of infestation. This fact alone militates against use of the antagonist by any simple method of application such as drenching.
However, we had found earlier that when bark was amended with as little as 10% peat plus lime the antagonist spread while the pathogen was restricted in movement, indicating that restriction of movement of the two organisms might be controlled by varying the composition of the potting medium (5). All of these studies were done in the laboratory in glass containers in the absence of the host plant.
The second part of the study which was supported in part by the grant was to develop methodology for studying the effects of chemical and biological control agents on development of Phytophthora root rot of azaleas in several potting media treated with the control agents, as well as effects on survival and spread of the fungus by the same treatments. After preliminary studies four media were selected for evaluation: pine bark alone; pine bark and peat; pine bark, peat and expanded shale; pine bark and soil. Small nylon mesh screens colonized by the pathogen were placed at specified points in each pot of medium which had been treated by drenching with the selected control agents.
Chemical control agents used included two systemic fungicides and a contact fungicide. The biocontrol agent was Mortierella alpina. Mortierella was cultured in broth and a suspension of the vegetative growth form of the fungus was applied directly to the roots of azalea plants growing in the several treated media. The chemical agents studied were ethazole, a contact fungicide, and metalaxyl and fosetyl al, systemic fungicides, all known to be active against Phytophthora. The treatments were replicated and randomized in a manner necessary for statistical analysis of the results. Two distinct experiments were run; one to study the treatment effects on the pathogen in the absence of host plants and another to study effects of the treatments on disease development.
In the disease control experiment all three fungicides gave some measure of protection, the best by metalaxyl and fosetyl al, the least by ethazole. The treatment of the roots with Mortierella provided almost no protection.
Less disease developed in the pine bark-only medium than in any of the amended media. In this same experiment the pathogen was recovered from individual treatments at termination of the experiment. Recovery ranged from 0 to 93% of the plants treated with metalaxyl. It may thus be concluded that none of the treatments used will assure the elimination of the fungus from infested media, although fungicide treatment, especially that with metalaxyl, definitely lowered the level of root infection. Apparently there are some complex interactions among the control treatments and potting media and (1) these results should be used as a departure for further experiments.
Although none of the treatments used succeeded in eliminating the pathogen under all conditions, there were some significant differences seen in the experiments conducted to study the effects of the control agents on survival and reproduction of the pathogen in the absence of a host plant. All control agents reduced the reproduction of the pathogen, and two fungicides, metalaxyl and ethazole, did so significantly.
Regardless of the effects of the control agents, numbers of reproductive units of the pathogen followed a pattern of initial large increases in the first 48 hours followed by a gradual reduction to very low numbers over the next two weeks. In addition, numbers of sporangia (the reproductive body responsible for rapid spread of the pathogen), were significantly different in each of the media, from bark alone to bark-peat-shale mixture, to bark-peat and then to bark-soil in descending order, the greatest numbers formed in bark alone. It should be noted that Phytophthora spread through plain pine bark but not through a mixture of bark, peat and shale in earlier laboratory studies.
The results of these studies seem to indicate, unfortunately, that the hoped for biocontrol is not likely with Mortierella, because the effects of Mortierella on both the pathogen and the disease process were minimal. Although minimal, there were some detectable influences of Mortierella leaving some hope that this particular biocontrol agent may yet be useful.
The justification of the support of the project by the Society was to develop methods for this type study. The results reported here go beyond methodology to the substance of the problem, but useful methodology was developed and the results seem to justify further pursuit of the investigation of the possible role of Mortierella in consort with reduced chemical usage to control Phytophthora root rot in azaleas and other ornamentals.
1. Kularatne, R.S. and W.H. Wills. 1984. Effects of fungal control agents and media amendments on formation and survival of Phytophthora cinnamomi propagules in pine bark. Phytopathology 74:757 (Abstr.).
2. Lambe, R.C. and W.H. Wills. 1983. Fungicides and Mortierella for the protection of azaleas from Phytophthora root rot. Phytopathology 73:503 (Abstr.).
3. Wills, W.H. and R.C. Lambe. 1980. Mortierella antagonism to oomycetes. Phytopathology 70:694 (Abstr.).
4. Wills, W.H. and R.C. Lambe. 1981. Spread of Phytophthora cinnamomi (PC) and Mortierella alpina (M-7) on two pine bark potting media (A & B) in vitro. Phytopathology 71:772 (Abstr.).
5. Wills, W.H. and R.C. Lambe. 1982. Spread of Phytophthora cinnamomi and Mortierella alpina in modified pine bark media. Phytopathology 72:712 (Abstr.).
6. Wills, W.H. and R.C. Lambe. 1982. Integrated biological and chemical control of Phytophthora root rot of azaleas and rhododendrons. A research proposal to the American Rhododendron Society.
Dr. Wills, Professor of Plant Pathology at VPI & SU, reports on research done on Phytophthora root rot which was partially funded by the American Rhododendron Society Research Foundation.