Suppression of Phytophthora in Established Plantings with Subdue and Aliette
Rhody Ridge Arboretum Park
1988 was the second year of our grant from the Research Foundation of the ARS and the fifth and final year of testing Subdue 2E/5G and Aliette 80% for suppression of Phytophthora at Rhody Ridge Arboretum Park (an eleven acre Snohomish County arboretum) in established plantings of rhododendrons which had grown well for a number of years (some over twenty years) before symptoms appeared.
The primary goal of testing was to determine if either Subdue or Aliette could accomplish any of the following:
1). Arrest decline and allow recovery.
2). Maintain recovery at an acceptable level over a period of years, including sufficient time to encompass extremes in both winter and summer temperatures as well as drought stress.
3). Protect symptomless rhododendron adjacent to plants in severe decline or areas of widespread earlier loss.
Individual plants were chosen for the test group with the above objectives in mind from various areas of the park: some in considerable decline; others adjacent either to declining plants or to sites of those recently lost to Phytophthora. The purpose has not been to determine susceptibility, but should recovery be possible in permanent plantings of relatively mature rhododendrons, to identify a pattern of responses during that recovery and most importantly to discover if in a diversity of plants the recovery can be maintained. In rhododendrons of some age we have found neither death nor recovery related to Phytophthora is likely to be immediate, i.e. within one year of noticeable symptoms. Although variables of exposure, slope, soil and age, etc., inherent in long-term diversified landscapes even between individuals of the same cultivar make an evaluation of fungicide application imprecise; nevertheless, over a period of years general trends of response become recognizable.
Rhody Ridge Park is an east-facing slope with soil classified as Sandy Loam to Loamy Sand with greater extremes of heat and cold than in many of the milder areas near Puget Sound or Lake Washington. The fundamental stress factor in our particular environment predisposing rhododendron roots to Phytophthora is drought stress rather than the more frequently encountered problems of poor drainage.
During the five years of this test we have experienced a full range of climatic stress from an early November freeze in 1985 to seven degrees F to record setting periods of heat and drought from 1985 to 1988. Though it was not possible to view these temperature extremes in a positive light while laboring to mitigate them, in retrospect, there is considerable value in terms of information gained from the stability of plant recovery so severely tested.
Difficulties at Rhody Ridge became noticeable in 1981 with a decline in numerous rhododendrons. The deterioration intensified through 1984 when a positive identification of Phytophthora was obtained by Dr. McElroy of Peninsu-Lab. By this time nearly fifty rhododendrons of varying maturities had been lost, many more were in serious decline.
At the beginning of testing in 1984 there were sixty rhododendrons including three controls designated for observation. Forty-two plants were treated with Subdue and fifteen with Aliette. By the fifth year we had fifty-two survivors: thirty-eight on Subdue, twelve on Aliette and two controls.
Rates and timing of fungicide application
Rates of application for Subdue are based on 100 square feet as the most convenient measurement corresponding to a typical drip line zone of numerous relatively mature rhododendrons in the park. Foliar application rates for Aliette are based on the 50-gallon tank size of our current equipment. Rates are as follows:
SUBDUE 2E - 1 oz. per 100 square feet with 25 gal. water in gravity drench.
SUBDUE 5G - 5 oz. per 100 square feet in granular application.
ALIETTE 80% - 3½ quarts per 50 gal. in foliar application.
Granular applications of Subdue were found to be much quicker and more convenient than soil drenches. Over the five-year period of the test fungicide applications were made as follows:
SUBDUE 2E - 1984 September; 1985 May.
SUBDUE 5G - 1985 October 10 through November 5; 1986 withheld; 1987 April 1 through 15 (upper area) and October 15 through November 10 (lower area); 1988 withheld.
ALIETTE 80% - 1985 June 9 and August 11; 1986 June 6, September 11 and October 30; 1987 June 23, September 26 and October 17; 1988 June 26, August 19 and September 10. (Combined in tank mix with Orthene 75S beginning in 1986.)
Annual status reviews
An annual status has been assigned each plant based on two measurements of leaf size and stem length and several additional factors found to be of equal or greater importance based on observation over time as follows:
1). Leaf size and color.
2). Length of annual growth.
3). Percentage of plant dieback.
4). Turgidity of new growth.
5). Frequent and uncharacteristic foliar wilt in hot weather on normal watering schedule.
6). Extent of sunscald.
7). Retention of one to two-year leaves (or more) through late summer.
8). Development of new growth low on old wood and/or new shoots from base.
Based on the above each plant was rated yearly in one of the following categories:
The data comparing annual status of test plants in 1988 with 1987 has continued to be heartening. The 38 plants treated with Subdue showed 84% either improved or unchanged-positive and 16% either declined or unchanged-negative. The 12 plants treated with Aliette showed 100% improved or unchanged-positive.
We have used supplementary means to maintain or enhance recovery during these years in two general areas: reduction of moisture stress and soil replacement. The use of anti-transpirants was the most generally successful method used. Of the various products applied over the years, our current choice is Foliocote based on longevity and less clogging of equipment. It is doubtful that recovery could have been sustained, let alone progressed, in the extreme summers of 1985-88 otherwise.
Given the sandy character of our soil, a particularly difficult problem of soil moisture has been drying out of the lower root zone and the soil beneath the dense root masses of older plants. Based on Dr. McElroy's recommendations at the end of 1988, we have begun to use a mix of 50% pumice and 50% superabsorbent (Terra-Sorb AG) in hydrated form for backfilling cores in the root zones. Two-inch cores are dug to a depth of 6 inches below the root zone, beginning approximately fifteen to eighteen inches from the trunk and eighteen inches apart in concentric circles out to drip line. Spacing varies somewhat with plant size.
We have experimented with numerous plants in High to Moderate Stress both within and outside the test group which were small enough in stature to be lifted manually. The original site was completely redug; all original soil removed and replaced by a pasteurized mix and soil from "clean" areas. The plant was then reset in a zone of pure pasteurized mix within the newly prepared site. Regrettably the results were random and inconsistent.
A much more reliably successful method was that of perimeter soil replacement used on a number of large plants. An annual replacement of soil was made along outer edges of the root ball, usually a third of the circumference per year. Existing soil was removed approximately twenty-four to thirty inches outward from viable roots to a depth of eighteen to twenty-four inches and replaced by pasteurized mix, blended with existing soil on the new perimeter to avoid creating a barrier to later root development.
Identification of Phytophthora species
It has not been possible to determine which Phytophthora species has infected rhododendrons at Rhody Ridge park. Given the absence of leaf spots and stem/branch cankers which are characteristic of P. syringae and given that the original positive results were obtained from roots, P. cinnamomi is a possibility as the causal agent, especially because it is frequently associated with root rots of rhododendrons.
Referring back to the original goals set for the project, the results have been better than anticipated with survival of 90% of the original forty-two rhododendrons treated with Subdue and 80% of the fifteen treated with Aliette. Two of three controls survived. Overall survival rate for the total sixty plants in the test group was fifty-two or 87%. All eight plant losses occurred in the first three years, and none thereafter.
Since mere survival is hardly the objective, the final percentages covering the entire five years have greater significance with Subdue rated 89% and Aliette 83% improved or unchanged-positive for the fifty-two survivors. Though Subdue outperformed Aliette, we feel the performance of both is excellent.
In working with relatively mature plants the primary factor on which all else depends is time, and often, a great deal of it. Except in ideal circumstances where the earliest symptoms of Phytophthora are recognized and verified through laboratory testing, there is likely to be some degree of decline including dieback before the problem is correctly identified in landscapes where Phytophthora root rot has not been a problem in the past.
A fairly typical pattern after treatment may follow a course of status quo or continued minor decline for the first or even the second year with improvement beginning in the third year and gaining momentum in the fourth and fifth years. While the original extent of decline obviously affects both the possibility and time required for recovery, it should be understood that some individuals with up to 60% dieback and the most miserable appearance of leaves one-quarter their normal size on minute stems can evolve from this High Stress in 1984 to Normal in 1988. Conversely, with the same amount of care and the same number of years a plant may make only incremental improvement from High through Moderate/High to Moderate Stress.
Another goal, to determine the ability of these fungicides to protect symptomless plants adjacent to those in severe decline or to areas of previous loss, was quite successfully reached with either fungicide. Some rhododendrons in this circumstance maintained Normal status; others had minor decline and returned to Normal.
During the particular five-year period of this work on Rhody Ridge, we have gone through the most alarming extremes of winter and summer temperatures for our location. Even with careful monitoring for watering and supplemental measures to control moisture stress, we feared the early stages of recovery from Phytophthora might be too fragile to endure these additional pressures. To our pleasant surprise we found generally that the status of recovery for most rhododendrons not only held firm but expanded in the last two most difficult years.
Although Phytophthora is now widely dispersed throughout the park, and will presumably be a permanent presence indefinitely requiring careful observation of rhododendron plantings, we feel the outlook is as sparingly as possible in order to preserve its effectiveness.
Fir Butler was the residential caretaker at Rhody Ridge Arboretum when this study was undertaken.
Many thanks to Dr. Russell Gilkey, ARS Research Committee member, for his assistance in preparing this report for publication in the ARS Journal.