JARS v36n1 - Integrated Control of Phytophthora Disease of Ericaceae

Integrated Control of Phytophthora Disease of Ericaceae
by H. A. J. Hoitink and C. C. Powell
Professor and Associate Professor Department of Plant Pathology
Ohio Agricultural Research and Development Center and The Ohio State University

Several genera in the family ericaceae are particularly susceptible to Phytophthora diseases. Such diseases of Azalea, Pieris and Rhododendron have been studied in detail during the past decade. The life cycles of some of the diseases, therefore, are now understood adequately to recommend integrated control procedures. Basically, four types of Phytophthora diseases occur on these plants, i.e., 1) root rots, 2) crown rots, 3) stem diebacks and 4) leaf blights.

About a dozen phytophthora species cause root rots on ericaceae. Phytophthora cinnamomì is the most important one of these and may infect Rhododendron, Azalea, Pieris, Erica, Calluna, Kalmia and others. Azaleas generally are more resistant to root rots than rhododendrons (1).

Phytophthora cactorum is the most important dieback species on Rhododendron in Ohio and many other areas (7). A new species, Phytophthora hevaea caused serious dieback losses on Rhododendron in North Carolina recently (3). Several other species may cause rhododendron dieback.

On Pieris japonica , a shoot blight, stem dieback and root rot complex has been reported from several states. The most important pathogen of this disease, Phytophthora citrophthora , has caused severe losses and may also infect Potentilla fruticosa, Rhododendron, and to a lesser extent, Euonymous spp. In southeastern Ohio nurseries this pathogen was frequently recovered from taxus roots in poorly drained soils (5).

Two distinct types of Phytophthora diseases occur on azalea in Ohio. Greenhouse azaleas are often infected with Phytophthora parasitica , a pathogen that has not been isolated from nursery crops in Ohio. This disease on azalea is similar to the disease complex on Pieris. P. cinnamomì , however is the most important pathogen of azalea in Ohio nurseries and causes root rot. Phytophthora dieback and shoot blight diseases are not of economic importance on hardy azaleas in Ohio.

Most Phytophthora species produce four types of spores. Each plays an important role in the life cycle of the pathogen. Chlamydospores and oospores are the resting structures. These produce sporangia which do not survive as well. From the sporangia zoospores are released but only in free water. After a rain or an irrigation zoospores are produced and released in a matter of minutes. These then drain into the soil with the water to the roots. Zoospores then swim to the root tips, infect the plant, and initiate the root rot disease complex. Plants in containers positioned on polyethylene with roots near the drain holes, therefore, may become infected readily if water is allowed to run from infected to healthy plants.

Phytophthora spp. that survive on above ground plant parts or in infested debris produce zoospores in water droplets on lesions or on the debris or in puddles. This inoculum is splashed onto foliage or stems or may be moved on the soil surface in running water or drain into the soil to root tips. Juvenile tissues are most susceptible. Infections then may follow on all of these plant parts. The Pieris dieback complex is a good example of this infection process.

To reduce inoculum buildup of Phytophthora species free water therefore should be in contact with plants or near plants for as short a duration as possible. Containers should be placed on a surface which prevents run off. A rock or gravel base accomplishes this. Furthermore, the air filled pore space in container media at container capacity should be at least 15% for 4" tall pots and 20-25% for 6-8" tall pots (1-2 gal. containers). The percolation rate of these media should be such that free water (1") does not stand on the surface of media for more than 1 minute.

Suppressive Container Media
It is now widely recognized that container media with 50% or more bark (hardwood or softwood) in the organic component suppress Pythium and Phytophthora root rots. However, bark that is mixed with large quantities of wood chips or sawdust does not have those beneficial properties. Peat is conducive to root rot development. Proper management of container media, therefore, may significantly reduce root rots of ericaceae caused by Phytophthora species. These media have no effect on diebacks or shoot and leaf blights.

Due to the decreased occurrence of root rots in bark media, growers have been able to increase fertility levels in recent years during July and August. At this time of year, diebacks are most serious. Furthermore, due to the high fertility levels, plants do not “harden off” in July and August and, therefore, are very susceptible to Phytophthora spp. affecting aerial plant parts (3, 4). This has resulted in a significant increase in these diseases in Ohio nurseries. Therefore, a greater demand for chemical control of these diseases exists presently.

Chemical Control
For control of dieback, shoot and leaf blight, a variety of fungicides have been tested which protect the plant from infection (6). These fungicides are not systemic. A typical example of control on azalea and Pieris is presented in Table 1. Fore, Bravo, and Captan provided excellent control. For control of root rots, Truban is the most effective fungicide in use today in Ohio.

Table 1. Chemical control of Phytophthora shoot blight and dieback on Pieris japonica and 'Roadrunner' azalea.
Disease Rating
Pieris a Pieris a Azalea b
Treatment Rate P. citrophthora P. citricola P. nicotianae var. parasitica
Check -- 2.5 7.0 3.2
Fore 80% WP 2 lb/100 gal 0.0 0.0 0.0
Bravo 6F 2 pt/100 gal 0.0 0.3 0.1
Captan 50 WP 2 lb/100 gal 0.0 0.5 0.5
a Mean number of infected growing points on Pieris japonica plants 2 weeks after inoculation with zoospores. Results based on two replications of four plants/treatment with five growing points/plant for P. citrophthora and 20/ plant for P. citricola .
b Mean number of infected growing points on Roadrunner' azalea plants 2 weeks after inoculation with zoospores of P. nicotianae var. parasitica . Results based on two replications of 12 plants/treatment (approx. 15 growing points/plant).

Several states have special local needs registrations for the use of Manzate 200 on ericaceous plant material. This product is similar to the Fore used in the study mentioned above. Such a registration is expected for Ohio in the near future. Daconil 2787 (similar to the Bravo in table one) has recently been registered for this use on Pieris, Rhododendron, and Azaleas in all states. Captan is not yet registered for use on these crops.

The non-systemic protect.ant fungicides must be applied to the crop in a regular preventive program. Begin sprays in the spring when growth begins. Continue on two week intervals during periods of heavy rainfall or frequent overhead irrigations. The use of a spreader-sticker such as Exhalt 800 or Triton B-1 956 at 4 to 6 ounces per 100 gallons will enhance the protectant ability of Manzate 200, but is not necessary with the Daconil 2787. Spray plants to runoff, making sure that all leaf and shoot surfaces are covered with protectant.

During the past decade a new experimental fungicide was developed with systemic activity against Phytophthora . This fungicide, Subdue 5% WP, may be available in late 1980 or early 1981 to Ohio nurserymen. Research at the OARDC on Phytophthora root and crown rot of greenhouse azalea established that one drench (5 oz/100 gal) provided complete control. The effect lasted for more than 6 weeks. A run-off spray applied to the foliage also controlled the disease. In addition to the root rot phase, leaf and shoot blight were controlled as well. Difolitan 4F (39% EC) applied at 1 pt/100 gal. also controlled the shoot blight phase. Difolitan was less effective than Subdue for control of the crown and root rot phase (Table 2). Similar results were obtained elsewhere (4).

Table 2. Control of Phytophthora crown rot on 'Kingfisher' azaleas with fungicide drenches
Inoculum a Fungicide drench b Dose Mean number of diseased plants c
Uninoculated -- -- 0.0
Inoculated Subdue 5% WP 20 oz/100 gal 0.0
Inoculated Subdue 5% WP 10 oz/100 gal 0.0
Inoculated Subdue 5% WP 5 oz/100 gal 7.5
Inoculated Difolitan 4F 2 pt/100 gal 8.0
Inoculated Difolitan 4F 1 pt/100 gal 9.5
Inoculated Dexon 35 WP 12 oz/100 gal 14.0
Inoculated Truban 25% WP 12 oz/100 gal 15.5
Inoculated -- -- 16.0
a A zoospore suspension of Phytophthora parasitica (10 ml) was poured on the pot mix surrounding the crown of each plant (25 ml/plant in 1 gal pot).
b Drenches were applied 48 hrs. before inoculation (125 ml/1 gal pot).
c Mean number of diseased plants based on two replications of 8 plants/treatment at 6 weeks after inoculation. Mean separated by Duncan's new multiple range test (P = 0.05).

In another study at North Carolina, but with hardy azaleas for control of root rot, one Subdue drench provided control for 28 weeks. High rates were phytotoxic on Hershey Red. Truban (30% WP) was as effective as Subdue, but it needed to be applied at least four times in 28 weeks. Subdue was systemic in azalea and was present in leaves of plants drenched with the fungicide. It, therefore, is effective for control of Rhododendron Phytophthora dieback (2). One drench protected leaves of Roseum Elegans for over 56 days after treatment.

Recently another systemic fungicide, Aliette (Rhone-Poulenc, Inc.) was tested successfully for control of Rhododendron root rot ( P. cinnamomi ) in France. This product was most effective if mixed with the container medium. It also protected leaves of Roseum Elegans against dieback for 28 days after drenching (2).

Several procedures can be used to reduce incidence of Phytophthora diseases on ericaceous plants. Preparation of surfaces on which plants are produced should be such that surface runoff or puddle formation is avoided. Furthermore, whenever possible bark should be used in container media to suppress root rots. Finally, a series of fungicides are available soon for control of all phases of these diseases.

1. Benson, D. M.. 1979. Efficacy and in vitro activity of two systemic acylalanines and Ethazole for control of Phytophthora cinnamomi root rot of azalea. Phytopathology 69: 174-178.
2. Benson, D. M. 1980. Chemical control of rhododendron dieback caused by Phytophthora heveae. Plant Disease 64: (In Press).
3. Benson, D. M., and R. K. Jones. 1980. Etiology of rhododendron dieback caused by four Phytophthora species. Plant Disease 64: (In Press).
4. Ferrin, D. M., and H. C. Melunger. 1977. Control of Phytophthora wilt of azalea with CGA 48988. Proc. Fla. State Hort Soc. 90: 333-336.
5. Gerlach, W. W. P., H. A. J. Hoitink, and A. F. Schmitthenner. 1976. Phytophthora citrophthora on Pieris japonica; infection sporulation and dissemination. Phytopathology 66: 302-308.
6. Hoitink, H. A. J., G. Daft, and W. W. P. Gerlack. 1975. Chemical control of Phytophthora shoot blight and stem dieback of azalea and pieris. Plant Dis. Reptr. 59: 235-237.
7. Hoitink, H. A. J. and A. F. Schmitthenner. 1974. Relative prevalence and virulence of Phytophthora species involved in rhododendron root rot. Phytopathology 64: 1371-1374.
8. Hoitink, H. A. J., and A. F. Schmitthenner. 1975. Comparative efficacy of 2-chloro-6-methoxy-4 (trichloromethyl) pyridine and Ethazole for control of Phytophthora root rot of rhododendron and soybean. Phytopathology 65: 69-73.