Rhododendron-Phytophthora Root Rot

Cause Phytophthora root rot is caused by a number of Phytophthora species, including P. cactorum, P. cambivora, P. cinnamomi, P. citricola, P. cryptogea, P. plurivora, and P. pini. These pathogens are fungus-like microorganisms and are also known as water-molds. These species have a wide host range and can infect other plants, including apple, azalea, beech, blueberry, Chamaecyparis, heather, mugo pine, Pieris, Prunus, Taxus (yew), and Viburnum. Phytophthora cinnamomi and P. plurivora are the most commonly isolated species from rhododendron nursery plants in the PNW and can quickly cause severe root rot symptoms when soil moisture is high. Rhododendron plants in both field and container production are at risk for this disease especially after propagation.

Poorly drained, waterlogged soil or media, plastic ground covers, and excess irrigation favors these water-mold organisms. High soil moisture alone does not lead to disease, but water plays an important role in the spread of the disease. Phytophthora can survive unfavorable periods for many years as long-lived oospores or chlamydospores in plant debris or soil. The pathogen can be moved long distances on infected plants, as well as contaminated plant debris, potting media, pots, or water. Spores germinate and produce sporangia and a swimming spore stage called zoospores. Zoospores are produced in the greatest numbers under saturated conditions and are attracted to the fine roots where they attack and begin to rot the root system. After infection, the microorganism spreads, mainly in the inner bark tissues of the root and stems. Warm temperatures, as well as high salinity or pH in the root environment and root damage can favor disease development.

Hybrid cultivars that are at least moderately resistant include 'Anna H Hall', 'Bali', 'Brittany', 'Crete, 'Ginny Gee', 'Hawaii', 'Ingrid Mehlquist', 'Martha Isaacson', 'Normandy', 'Peter Tigerstedt', 'Pink Trumpet', 'Professor Hugo de Vries', 'Red Head', 'Rocket', 'Serenade', and 'Vernus'. Cultivar 'Yaku Princess' appears to be somewhat resistant to both P. cinnamomi and P. plurivora, with symptoms developing more slowly under high disease pressure than on the susceptible cultivar 'Cunningham's White'. Rhododendron species that are resistant include R. davidsonianum, R. delavayi, R. glomerulatum (R. yungningense), R. hyperythrum, R. keiskei, R. lapponicum, R. occidentale, R. quinquefolium, R. sanctum, R. pseudochrysanthum, R. simsii, and R. websterianum. Some cultivars are considered tolerant to Phytophthora root rot except when exposed to excess soil moisture (e.g., 'English Roseum' and 'Caroline'). Most Rhododendron species and cultivars have only been tested for resistance against P. cinnamomi. Data are mostly lacking for resistance against other soilborne Phytophthora species.

Symptoms Fibrous roots rot first, turning brown or black, followed by the rest of the root system. Larger roots are present, but firm, with lesions in various shades from cinnamon-brown to dark black. The edge of the advancing lesion is distinct from adjacent, unaffected tissues which appear moist and light in coloration (white to green). Plants with severe root rot are weakly rooted, resulting in wobbly plants that are relatively easy to pull from the ground compared to adjacent healthy and well-rooted plants. Infected plants may develop a canker at the base of the stem as the pathogen moves from the roots up into the stem.

Once a sufficient amount of the root system or stem is rotted, symptoms of nutrient deficiency and drought stress appear aboveground. Leaves may become yellowish- or reddish-green and eventually curl or wilt. Wilted leaves do not recover even when irrigated sufficiently. Finally, the plant dies with dead leaves remaining attached to the plant. Sometimes new roots may be observed to regenerate near the root crown of the plant.

Cultural control Use many different tactics to manage root rots but focus efforts on water management.

• Quarantine new plants at least 1 year before setting them out in the production area. Examine and destroy any plants that show disease.
• Grow resistant cultivars and types.
• Use pathogen-free cuttings.
• Provide good drainage for plants in beds, fields, or containers. Place containers on gravel beds (4 inches or more deep) to allow drainage. Do not place containers on poly sheets; they can prevent containers from draining into soil and allow contaminated drainage water to spread from the base of one container to another.
• Avoid areas of the field where drainage is poor or increase drainage with tiling.
• Rapid drainage of container media throughout the production cycle of the crop is critical. Using growing media with air-filled pore spaces of 15% for 4 inch-high media (1 gal pots) and 20% to 35% for 7-inch high media (2.5 gal pots) and low matric potentials (-5.0 to -10.0 kPa) is associated with fewer losses.
• Use raised beds in landscape plantings.
• Use care in watering. If using a hose, take care not to splash, and keep the nozzle and hose off the ground, walkways, or other surfaces that may be contaminated.
• Keep benches and premises clean. Do not use sides of low benches as a footrest; soil on shoes may be contaminated. Keep walkways and areas beneath benches clean and free of plant debris.
• Be careful with the placement of cull or dump piles. Spores produced on contaminated plant material may be carried into production fields by rain or irrigation water.
• Clean all propagating tools with disinfectants.
• Use composted hardwood or conifer bark. The suppressive effect should last about 1 year at which time potted plants may be repotted into fresh media. Keep media on clean concrete pads and keep away from contaminated equipment or water.
• Use clean irrigation water. Common chemicals used to disinfect water include chlorine, ozone, peroxide and copper. Irrigation water can also be disinfected using ultraviolet light.
• Reduce salinity of irrigation water and build-up of salts in root environment.
• Avoid reusing pots from a previous crop for propagation. If pots must be reused then wash off all debris and soak in a sanitizing solution or treat with aerated steam for 30 min.
• Prevent potted plants in nurseries from falling over allowing foliage to touch the ground.
• Low pH of the media (4 or lower) can suppress sporulation when propagating cuttings.
• Do not use excess applications of nitrogen fertilizer as it can increase root rot caused by some Phytophthora species.

Chemical control Use fungicides as preventative treatments. Multiple applications may be needed in container production when media temperature is above 45°F. Resistance to FRAC group 4 and P7 fungicides have been found in PNW nurseries. The Group 4 and Group P7 fungicides used to manage Phytophthora do not kill this organism. They can only prevent establishment of the organism before it gets into the plant. They can also prevent continued growth if the organism is already inside the plant thereby delaying symptoms that might have developed. Once chemical activity has subsided with time, the organism can resume growth within infected plants. Alternate or tank-mix products from different groups that have different modes of action. Limit the use of any one group during crop production.

• Fumigate beds, containers, and planting media (including field soil). Keep treated soil and rooting media in new or treated containers.
• Soil drenches to help prevent root infection.
  • Adorn at 1 to 4 fl oz/100 gal water plus another fungicide. Group 43 fungicide. 12-hr reentry.
  • Banol at 2 to 3 oz/10 gal water. Registered for azalea only. Group 28 fungicide. 24-hr reentry.
  • Banrot 40 WP at 6 to 12 oz/100 gal water. Group 1 + 14 fungicide. 12-hr reentry.
  • Empress at 1 to 3 fl oz/100 gal water can be used for cuttings or seedlings. Group 11 fungicide. 12-hr reentry.
  • Fenstop at 7 to 14 fl oz/50 to 100 gal water. Use 1 to 2 pints/sq ft. For greenhouse use only. Group 11 fungicide. 12-hr reentry.
  • Fosphite at 3 quarts/100 gal water. Do not use copper products within 20 days of treatment. Group P7 fungicide. 4-hr reentry.
  • Mefenoxam 2 AQ at 0.98 to 1.96 fl oz/100 gal water as a soil drench or at 1.23 to 2.45 fl oz/1,000 sq ft followed by at least 0.5 inch rain or irrigation. Reduce rates for all Azaleas, especially the cultivar Coral Bell. Group 4 fungicide. No restrictions on eentry when used as a soil drench or media incorporation.
  • MetaStar 2E at 1 to 2.5 fl oz/100 gal water and apply as a drench. Group 4 fungicide. No restrictions on reentry when used as a soil drench or media incorporation.
  • Monterey Garden Phos at 2 to 4 teaspoons/gal water as a foliar spray. Also labeled for soil drench, see label for details. Can be used in landscape sites. Group P7 fungicide. H
  • Stature SC at 6.12 to 12.25 fl oz/50 to 100 gal water. Use as a drench. Group 40 fungicide. 12-hr reentry.
  • Subdue MAXX at 1.25 to 2.5 fl oz/1,000 sq ft, irrigated in with 0.5 inch water within 24 hours. Group 4 fungicide. No restrictions on reentry when used as a soil drench.
  • Terrazole 35 WP at 3.5 to 10 oz/100 gal water. Group 14 fungicide. 12-hr reentry.
  • Truban 30 WP at 3 to 10 oz/100 gal water. Group 14 fungicide. 12-hr reentry.
• Foliage sprays of group P7 fungicides will be translocated down to the roots. Foliar sprays may not be as effective as soil drenches for some root Phytophthora root rot pathogens.
  • Agri-Fos at 1 to 2 quarts/100 gal water. 4-hr reentry. WA only.
  • Aliette at 2.5 to 5 lb/100 gal water. Do not use with adjuvants. 24-hr reentry.
  • Areca at 2.5 to 5 lb/100 gal water as a foliar application. 12-hr reentry.
  • Fosphite at 1 to 2 quarts/100 gal water. Do not use copper products within 20 days of treatment and do not use spray adjuvants. 4-hr reentry.
  • OxiPhos at 1.3 to 4 quarts/100 gal water as a foliar spray. 4-hr reentry.
  • Phospho-Jet is registered at 1 to 2 quarts/100 gal water as a foliar spray. Group P7 fungicide. 4-hr reentry.
  • Sparra at 1 to 2 qt/100 gal water as a foliar spray. Group P7 fungicide. 4-hr reentry.

Biological control Use in combination with other tactics. Will not be effective under high-disease pressure. Products usually contain specific species of organisms antagonistic to Phytophthora, including Trichoderma, Pseudomonas, and Bacillus species.

• RootShield Plus Granules (Trichoderma harzianum Rifai strain T-22 and T. virens strain G-41) at 1 to 3 lb/cubic yard of soil mix or 0.5 to 7.5 lb/1,000 sq ft. No restrictions on reentry required when soil incorporated. O
• Bio-Tam 2.0, Tenet WP, or Obtego (Trichoderma asperellum and T. gamsii) is registered and may be effective. Use 0.5 to 1.5 lb/cubic yard of substrate. See label for details. No restrictions on reentry when soil incorporated. O

References Sacher, G. O., Scagel, C. F., Davis, E. A., Beck, B. R. and Weiland, J. E., 2021. Virulence of five Phytophthora species causing rhododendron root rot in Oregon. Plant Disease, 105:2494-2502.

Scagel, C. F., Weiland, J. E., Beck, B. R. and Mitchell, J. N. 2023. Temperature and fungicide sensitivity in three prevalent Phytophthora species causing Phytophthora root rot in rhododendron. Plant Disease, 107:3014-3025.