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Journal American Rhododendron Society

Current Editor:
Dr. Glen Jamieson ars.editor@gmail.com


Volume 26, Number 2
April 1972

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Control of Phytophthora Root Rot (Wilt) of Rhododendron1
H. A. J. Hoitink and A. F. Schmitthenner2
Reprinted from American Nurseryman, February 1, 1972

1Supported by the Cooperative Agreement No. 12-14-100-10, 640 (34) from the U. S. Dept of Agriculture
2Associate Professor and Professor, respectively, Department of Plant Pathology, Ohio Agricultural Research and Development Center, Wooster, Ohio, and The Ohio State University, Columbus, Ohio

        During the last decade Phytophthora root rot has become the most severe disease of rhododendron and certain other evergreens. The disease, referred to as rhododendron wilt, is present in most commercial growing fields in the United States and other parts of the world. White in New Jersey first described conditions favorable for root rot. These include high soil moisture and temperature. Plants on sandy soils, therefore, are generally not affected by root rot while those on poorly drained soils are. Temperatures are high in containers, therefore, drainage is even more important.
        Several growers have learned to avoid the disease in heavy field soils by growing plants on hills in rows or by improving drainage. A large percentage of plants grown in this fashion appear healthy when shipped, but die from root rot when exposed to high moisture and temperature conditions. We have found that feeder roots of such plants are infected with Phytophthora, but not severe enough to result in obvious foliage symptoms. Rhododendron root rot may have been introduced in this country in this fashion. Phytophthora now has spread across the western world on several plant types. The disease is known in parts of Australia and probably occurs in The Orient as well.

The Causal Organisms:
        Five Phytophthora spp. have been associated with the rhododendron root rot complex. Of these, P. cinnamomi is the most important. From plants grown in container media with an airspace of approximately 20% and excellent drainage (excess moisture drains out in less than an hour) only airborne Phytophthora spp., such as P. citricola and P. cactorum , have been isolated. These can be controlled by regular spray applications with Dithane M-45, Manzate D or Tersan LSR and related compounds. However, P. cinnamomi, is strictly soil-and water-borne and probably over winters in decaying roots in soils from which plants have been harvested. It is spread with infected soil on boots, equipment, and in water. Research by Rattink at Boskoop, in The Netherlands, has shown that zoospores may travel into a drainage tile at three foot depth in less than one hour from which they drain into irrigation water. Our field observations substantiate that much of the spread of Phytophthora results from movement of zoospores in water. Irrigation water, therefore, should not be taken from wells or streams into which run off water collects.
        Phytophthora cinnamomi has a wide host range. It has been isolated from azalea, Pieris, heather, Kalmia, Arctostaphylos sp., Cryptomeria sp. juniper, taxus and over 250 rhododendrons from different geographic locations throughout North America. Crop rotation using woody plants, therefore may not reduce the root rot problem. It is not known, however, whether a rotation with herbaceous crops like corn or rye would be effective.

Chemical Control:
        Chemical control of rhododendron root rot in an established planting is not possible at present. Laboratory and field studies with Dexon and Truban in the United States and The Netherlands failed to demonstrate control of the disease once established. Soil fumigation with methyl bromide does eradicate the fungus.
        It is important that the fumigant penetrates to the water table, which should be below the zone into which roots grow. A wet clay layer under peat beds is not penetrated by methyl bromide and allows spores to survive. After replanting of poorly-drained, fumigated beds Phytophthora zoospores are attracted by root exudates and swim into root zones. Since the natural soil inhibiting soil micro-flora has been removed by fumigation with methyl bromide, the disease causing fungus can spread more rapidly when it re-enters wet fumigated media. Therefore, peat beds should be well drained with a water table at least two feet below the surface. This can be accomplished with a layer of coarse sand and gravel below the peat. Walkways should be below the peat level in beds so that water in the walkways does not drain into the root zone.

Effect of pH:
        In the 1930's, White from New Jersey published that root rot could be reduced by growing rhododendrons in media with a pH of 4.5 - 5.0. Unfortunately at this soil pH, phosphorus is not sufficiently available for maximum plant growth. The optimum pH for growth varies with conditions but generally is from 5.5 - 6.5; a range where Phytophthora is not inhibited. A lower pH, even though it would minimize Phytophthora root rot losses, would not be a practical means of control for commercial operations.

Programmed Disease Prevention
        A critical step in the control of diseases of woody plants, in general, is the production of healthy rooted cuttings. The system described here has worked successfully over the past four years. Canadian or German sphagnum peat has provided excellent drainage and has a pH of 3.5 - 4.1 which checks the development of several pathogens. Aeration was improved by addition of perlite or Styrofoam and coarse sand. Phytophthora has not been isolated from rooted cuttings produced in such mixtures when the procedures outlined below were followed.
        Cuttings were free of soil, insects and disease before they were taken. Stock plants were sprayed routinely with Sevin or Malathion and Fore, Dithane M-45, Tersan LSR, or Manzate D. Once residues were washed off applications were repeated.
        Several cutting dip or soaking procedures are used by growers. Shell soil fungicide 345, Clorox, Captan and LF-10 are examples. Generally, it is not advisable to soak cuttings. Clorox breaks down in a single soak treatment, therefore, a new solution will have to be made for a second treatment. Furthermore, Clorox does not kill plant pathogenic bacteria unless concentrations are used that will kill plant tissue. Soaking of cuttings can lead to spread of bacterial plant diseases; fire blight is an example. Dry cuttings should be sprayed with water so that hormone powders stick. Even dipping cuttings in water or hormone solutions may cause spread of disease organisms. Soiled cuttings should be avoided but if used, they should be washed in a water bath that is overflowing continuously.

Propagation:
        All unpainted woodwork flats, baskets, as well as greenhouse benches were treated with 2 % copper naphthenate. When all the plants were out of the greenhouse the interior was treated with formaldehyde (1 part of 37% formaldehyde solution in 50 parts of water). The entire propagating area was sprayed and the greenhouse was kept closed for 24 hours, then aerated until all odor of formaldehyde was gone.
        The entire head house was sprayed under benches, painted woodwork, and walkways in the propagating house with a solution of 1 part LF-10 in 200 parts of water. This treatment was repeated every two weeks throughout the season. LF-10 is not effective in soil where large quantities or organic matter are present. All propagating tools were soaked (knives, soil levelers, etc.) for 10 minutes in a solution of 1 part of LF-10 in 50 parts of water, (Lehn & Fink Products Corp., 4934 Lewis Avenue, Toledo, Ohio). Propagating beds were filled with a new or sterilized medium. Sphagnum peat with coarse sand and perlite or Styrofoam was used. Treatment with an air steam at 160F is recommended highly, although expensive.
        Experimental research on propagation of hardy rhododendrons under mist showed that the following hormone mixtures when used as a cutting dip resulted in a high percentage of disease-free rooted cuttings:
1) 10% Benlate 50% W. P. or 20% Mertect 60% W. P.
     2% IBA for red-flowered rhododendron or 1 % for pinks 
     88% Talc for reds or 89% for pinks
     50 ppm boric acid
2) 10% Benlate 50% W. P. or 20% Mertect 60% W. P.
     2 % IBA for reds or 15c for pinks
     50% Cut Start #4 (strongest type)*
     38% Talc for reds or 395c talc for pinks
     50 ppm boric acid


* Cut Start is available at Brighton Horticultural Supplies, 
Brighton By-Products Company, Inc., 
P. O. Box 23, New Brighton, Pa. 15066


Hormone mixture (#2) resulted in the formation of a large callus due to the addition of Cut Start, which may or may not be desirable.
        Cuttings were watered after "sticking" to assure contact between cuttings and medium. A combination of wetting agent (Aqua-gro or Tergitol) and Dexon 35% W.P. at 10 oz./100 gals. of water plus either 6 oz. of Benlate 50% W. P. or 6 oz. of Mertect 60 5 W.P. was used at 1 pint per square foot. The new fungicide Truban has been substituted for Dexon on cuttings and rooted cuttings, however its toxicity to un-rooted cuttings is not adequately known. Preliminary data suggests that it is not toxic to cuttings under mist. The wetting agent helps distribute fungicides uniformly in the media. If plants other than rhododendrons are to be grown, experiment with the use of fungicides and wetting agents and treat only a limited number at first to determine whether injury will occur.

Protection During Rooting & Growing Time:
        All cuttings under mist were sprayed with Captan 50% at 2 lbs./100 gal. of water every two weeks and once a month with Sevin and/or Malathion. Misting was controlled so that plants dried after each application.
        After the mist was shut off and cuttings were transplanted, plants were sprayed with Sevin and/or Malathion and a fungicide such as Fore, Tersan LSR, Manzate D or Dithane M-45 (at 2 lbs./100 gals.) every three weeks to keep new growth covered. All wilted or diseased plants and cuttings were removed from the propagating and growing area and placed in a tight container to prevent spread of disease.
        Immediately after each potting or transplanting procedure, the Aqua-gro or Tergitol and Dexton at 10 oz. or Truban 30% W.P. at 6 oz./100 gals. of water plus either 6 oz. of Benlate 50 % W.P. or 6 oz. of Mertect 60% W.P. was repeated. All ingredients were mixed before application. One pint of the solution was used per square surface area. Containers were placed on gravel not plastic, since plastic allows rapid spread of zoospores from infected containers to surrounding healthy plants. 

Discussion
        In the preceding section a combination of fungicides and sanitary procedures is described that prevents introduction of known important disease causing agents of rhododendron. In some parts of the country, such as the Pacific Northwest, relatively few disease problems occur other than Phytophthora root rot. This is largely due to the favorable climate for rhododendrons. In these areas a more relaxed fungicide program probably could be used. However, in the Midwest and New England states, low temperatures result in winter injury which leads to Botryosphaeria and other infections. In the southeast, Phomopsis causes dieback problems. In these areas an integrated fungicide-sanitation program, such as the one presented, is essential.
        The most critical factor for successful production of root rot-free rhododendrons is adequate drainage of the soil mix once the procedures outlined in the text are applied.
        Preliminary data show that the air volume of container media should be higher than 15% and preferably 20%, It is important that the air volume does not decrease in containers during the growing season due to breakdown of organic constituents. Root rot has not been encountered in nurseries where growth media contained more than 60% bark. The highest incidence of root rot occurred in container mixes with sawdust as the major constituent, especially if these were placed on plastic. This probably is due to the rapid breakdown of sawdust, as compared to bark, resulting in a decrease of air volume during plant growth. This in turn leads to higher moisture levels and increased root rot problems. Container mixes in which Michigan peat (muck) and sand are used, promotes root rot development, particularly if a fine sand is used. Addition of coarse sphagnum peat does not easily overcome this property of muck and sand mixes. Air volumes of 6-8% have been encountered in various mixes of these three ingredients. Mixtures of coarse sphagnum peat, coarse sand, and a finer grade sphagnum peat have an air volume of approximately 15%, depending on the proportion of the ingredients.
        Another important property of media is the time in which added water drains out of a saturated mix. Preliminary data show that a saturated bark mix loses added water in less than one hour if containers are on gravel. All bark types encountered on surveys were suitable for growth. It is not known whether some bark types might be better than others.
        Presently rhododendron hybrids and species are being screened for resistance. Differences in resistance have been found. It is expected that detailed data will not be available for two years. In addition, the drainage property of container mixes and its effect on Phytophthora root rot is under investigation.


Volume 26, Number 2
April 1972

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