Cause Phytophthora cinnamomi and Phytophthora tropicalis, fungus-like microorganisms. Other species may be involved as Phytophthora citricola and P. citrophthora have been reported in other regions and occur in the PNW. Poorly drained soil or media or flooding encourages disease development. It survives unfavorable periods in soil or media and infected plant debris. Under favorable conditions, spores germinate and infect roots. After infection, the microorganism spreads mainly in the inner bark tissues of the root and stems. They survive as various spores in the soil, container media or infected roots. Movement of infected plants and/or soil can spread the microorganism.
Symptoms The microorganism invades roots, causing a root rot and eventual death or dieback of aboveground portions. Necrotic leaf blights and branch diebacks can also occur in the absence of root rot. Defoliation may result from leaf blights.
Cultural control
- Do not overwater.
- Use sterile well drained soilless media.
- 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.
- 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.
Chemical control Focus on cultural controls. Use these products to prevent, not cure, the disease. The Group 4 and 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. Rotate fungicides that have a different mode of action for resistant management.
- Aliette at 0.4 to 0.8 lb/100 gal water/400 sq ft. Do not use with adjuvants. Group P7 fungicide. 24-hr reentry.
- Areca at 0.4 to 0.8 lb/100 gal water as a drench. Group P7 fungicide. 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. 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. Group 4 fungicide. No restrictions on reentry when used as a soil drench or media incorporation.
- MetaStar 2E at 1 to 4 fl oz/100 gal water 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
- OxiPhos at 1.3 to 4 quarts/100 gal water as a foliar spray. Group P7 fungicide. 4-hr reentry.
- Subdue MAXX at 1 to 2 fl oz/100 gal water. Use 1pint solution/sq ft. Group 4 fungicide. 48-hr reentry.
- Terrazole 35 WP at 3.5 to 10 oz/100 gal water. Group 14 fungicide. 12-hr reentry.
- Truban 25 EC at 4 to 8 oz/100 gal water/400 sq ft. Group 14 fungicide. Group 14 fungicide. 12-hr reentry.
References Gerlach, W.W.P., Hoitink, H.A.J. and Ellett, C.W. 1974. Shoot blight and stem dieback of Pieris japonica caused by Phytophthora citricola, P. citrophthora and Botryosphaeria dothidea. Phytopathology 64:1368-1370.
Redekar, N. R., Eberhart, J. L., Rooney-Latham, S., Blomquist, C. L., and Parke, J. L. 2020. First report of Phytophthora tropicalis causing foliar blight and shoot dieback of Pieris japonica in Oregon. Plant Disease 104:1564-1565.