Cause Phytophthora cinnamomi, a water-mold, fungus-like microorganism. The disease is favored in potted plants on plastic sheets, which hold water around the pots. The disease can spread when the sheets are used under other pots. This microorganism survives as oospores or chlamydospores in plant debris such as rotted roots or foliage. Spores may be moved long distances in contaminated soil or through moving water. Spores germinate and produce sporangia and a swimming spore stage called zoospores, which attack fine roots and begin to rot the root system.
Symptoms Fine roots rot away, and a cinnamon-brown color advances up into the root crown. There may be a sharp line between healthy and diseased tissue in the root. Aboveground symptoms vary and include vein reddening, yellow-green leaves, and tan, dry leaves. Wilting is common because of root loss.
Cultural control
- Use well-drained potting media.
- Put pots on sloped gravel beds to avoid puddles beneath pots.
- Do not overwater.
- 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.
Chemical control Use fungicides as preventative treatments. 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.
- Monterey Garden Phos at 2 to 6 teaspoons/gal water as a foliar spray. Group P7 fungicide. H
- Orondis Gold at 28 to 55 fl oz/A as a drench, soil-directed spray or through irrigation water. Do not use within 1 day of harvest. Group 4 + 49 fungicide. 48-hr reentry.
- Subdue and Aliette are not registered on this crop but can legally be used. Test first on a small section before applying on the whole plant to evaluate possible phytotoxicity.
Reference Linderman, R.G. and Zeitoun, F. 1977. Phytophthora cinnamomi causing root rot and wilt of nursery-grown native western azalea and salal. Plant Disease Reporter 61:1045-1048.