Biological Control Of Phytophthora Root Rot Of Rhododendron
Jack D. Paxton, PhD and Rita Hiajenga
University of Illinois
Phytophthora root rot is a troublesome and sometimes disastrous problem for rhododendron growers. In this study biological control of this disease was explored to determine whether bacteria first found on soybean roots and used to control Phytophthora root rot of soybeans can also be used in the cultivation of rhododendrons.
Rooted cuttings of 'PJM', both Phytophthora root rot susceptible and resistant type that had not been treated with Subdue™, were purchased from the Knight Hollow Nursery in Madison, Wisconsin. These cuttings were held on plastic-lined greenhouse benches. If the plants were carefully watered, the plastic lining allowed judicious flooding of inoculated plants in controlled batches. Wooden dams were formed under the plastic and between rows of rhododendron plants, in order to allow flooding of the plants in groups or individually, for various periods of time. Some plants were watered normally while others were flooded to encourage the growth of disease-producing microorganisms and infection of the experimental plants.
In Table 1 the bio-control agent used is shown not to reduce the survival of 'PJM' rhododendrons, at least as indicated by plants which survived for 95 days after its application, regardless of whether they were flooded or not.
Table 1. Survival of Control Group of Rooted Cuttings of 'PJM' Rhododendron Not Exposed to Phytophthora Root Rot (Percent Survival After 95 Days) Number of Plants Unflooded Flooded 10 Not Treated with Bacterium 100% 100% 10 Treated with Bacterium 100% 100%
Table 2. Effects of Three Treatments to Control Root Rot on Rooted Cuttings of PJM Rhododendron Exposed to Phytophthora Root Rot (Percent Survival) Unflooded Flooded Number of Plants 45 days 95 days 10 Untreated 100% 20% 0% 10 Treated with Broth 100% 40% 0% 10 Treated with Bacteria 100% 80% 60%
Three groups of 'PJM' rooted cuttings were exposed to the Phytophthora root rot pathogen. The pathogen, Phytophthora cinnamomi, had been grown on damp, sterilized oats for 1 month in the laboratory at room temperature. The plants were inoculated by burying one colonized oat kernel 1" deep in each corner of a 4" plastic pot containing a rooted cutting growing in planting mix. The first group of plants was not treated with the bio-control bacterium. The second group of contaminated plants was treated by inoculating each plant in the group with soybean broth (the broth was prepared by adding 1 soybean in 10 ml of water and autoclaving this solution). The third group received bacterial treatment of one ml of a bacterial suspension growing on soybean broth for 1 week. All of these plants were then flooded for 7 days. After the flooding treatment, the plants were returned to normal, once a day watering without standing water present. At various times after inoculation, during a period of roughly 4 months, the plants were evaluated for plant death as shown in Table 2.
The benefit of biological control is shown in the bottom line of Table 2 which shows that 60 percent of the treated plants survived for 95 days. None of the first or second group survived for 95 days. This bacterial treatment at least delayed Phytopthora root rot during this period but did not prevent the disease.
Disease pressure, under conditions used in this test to give rapid disease development in unprotected plants, was rather severe and some plants died that probably would have survived in the field. For example, some 'PJM' resistant plants died which probably would have survived under field conditions with the pathogen present. No attempt was made to compare the above treatment with that of Subdue™ nor were the plants tested in the field.
Under less severe disease pressure and under field conditions this treatment may prove beneficial. Since the treatment is low cost and easy to apply, it is certainly worth further testing. Isolates of the bacteria are available, on a limited basis, from the U.S.D.A. Northern Regional Research Center, in Peoria, Illinois.
Experiments also were performed on rhododendrons in tissue culture. Plantlets of the cultivar 'PJM' were raised on an agar tissue culture medium. After establishment for 3 months, some of the plantlets on the agar medium were inoculated with mycelium of the fungus that had been growing on soybean broth. Some plants were left as un-inoculated controls and other plants were inoculated in addition with 0.05 ml of the bacteria that had been growing on the soybean broth for 1 week. At the end of 95 days all of the plants had succumbed to Phytophthora cinnamomi.
Biological control of plant diseases is an area of active research and is of interest because of the problems associated with the expense of pesticides, the toxicity to humans of various fungicides and pesticides, and partly because of the appearance of new races of pathogens, previously controlled genetically in resistant varieties. The occurrence of new races of pathogens is especially true of various Phytophthora species.
This article is a summary of Dr. Paxton's research, funded in part by the American Rhododendron Society and The Illinois Agricultural Experiment Station. J.D. Paxton is Associate Professor of Plant Pathology at the University of Illinois, Urbana-Champaign; Rita Hiajenga is an undergraduate student in Horticulture at the University of Illinois. The supply, by Drs. Mike Benson and Robert Linderman, of Phytophthora cinnamomi cultures and advice is acknowledged with appreciation.