Kevin Ong, Associate Professor and Extension Plant Pathologist
Phytophthora root rot, also known as crown rot or basal stem rot is one of the most common and severe root-decaying diseases worldwide. It can occur in many types of host plants including trees, shrubs, and roses. A soilborne pathogen, Phytophthora survives in wet or moist soils, waiting for a living host to infect. There are several different species in the genus Phytophthora, and they all produce similar symptoms on diseased hosts. On rose plants, several species of Phytophthora, such as P. megasperma, P. cactorum, and P. citrophthora, are pathogenic and can cause the plant to wilt and die.
Phytophthora root rot can result in leaf chlorosis, wilting,and dieback of canes (Fig. 1). Below the soil, the crown tissue and roots become dark brown and necrotic (Fig. 2). Infected roots often appear water-soaked as they rot away. Larger roots, weakened by rot, can be easy to break off. Typical plant disease symptoms can be mistaken for other abiotic (non-living) ailments and lead to misdiagnosis. For example, chlorosis of the leaves is often confused with nutrient deficiencies. The drought-like appearance on the foliage causes gardeners to compensate by overwatering, resulting in saturated soils—a favorable condition for this pathogen. Because roses that succumb to infection do not usually survive, it is important to recognize Phytophthora root rot symptoms to manage the problem accordingly. Because it is difficult to make an accurate diagnosis based solely on the aboveground symptoms, carefully inspect the crown and roots of potentially infected plants and send a sample to a plant diagnostic clinic for analysis.
Commonly known as water mold, Phytophthora is an oomycete, a fungus-like organism more closely related to algae than to fungi. When conditions are favorable, this pathogen can survive in the soil, infect susceptible plant roots, and spread throughout the root system. This infection destroys the root tissue structure and causes rot. The rot prevents the plant from transporting water and nutrients to its aboveground parts, resulting in drought-like symptoms from water deprivation. Displaced rainwater can carry the organism’s sexual spores (oospores) to neighboring plants or into the surrounding soil, where they await suitable conditions to infect the next host. Zoospores, the asexual spores, have two flagella (appendages) that allow them to “swim” in water for short distances. This ability makes the disease much more prevalent in moist soils with poor drainage where the free water triggers the release of zoospores that swim to nearby roots and infect them. This pathogen also produces chlamydospores that can survive unfavorable conditions by becoming dormant in the soil for long periods, making control much more challenging.
Phytophthora spores can survive in the soil for years without infecting a host plant, but can become infectious in suitable weather and soil conditions when they encounter a susceptible host. Conditions that favor infection are
- Cool, wet weather typical of early spring and late fall
- Flooded or waterlogged soil conditions in or after seasons of heavy rainfall
Because Phytophthora symptoms and damage are most obvious during the hot, dry summer months when plants are already water-stressed, it is important to differentiate between drought symptoms and Phytophthora symptoms.
Unfortunately, no known cure exists for Phytophthora root rot, and once established, it is not easily eradicated from an area. Prevention is the best control method and cultural practices that control disease development and spread include:
- Ensuring good soil drainage and avoiding overwatering or overfertilizing
- Practicing good hygiene and sterilization of garden tools
- Maintaining a good cover of organic matter or composted mulch on the soil
- Using fungicides that are effective at preventing infection, especially those containing phosphorus, which boosts the plant’s natural defenses; as always, read all directions and labels before using any chemical control
- Removing infected roses right away; get as much of the roots and soil surrounding the roots out of the ground as possible
Creswell, T., K. Ivors, and M. Munster. 2011. Suggested Plant Species for Sites with a History of Phytophthora Root or Crown Rot. Publication No. 11-CALS-2604. North Carolina College of Agriculture and Life Sciences, Cooperative Extension. https://www.calsncsu.edu/plantpath/extension/clinic/fact_sheets/index.php?do=disease&id=9.
Moorman, G. 2015. Phytophthora Root Rot on Woody Ornamentals. Publication code: XL0057. Pennsylvania State University Extension. http://extension.psu.edu/pests/plant-diseases/all-fact-sheets/phytophthora-rootrot-on-woody-ornamentals.
Perry, E. 2006. Pest Notes: Phytophthora Root and Crown Rot in the Garden. UC ANR Publication No. 74133 University of California Agriculture and Natural Resources, Statewide Integrated Pest Management Program. http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn74133.html.
Phytophthora root rot. Royal Horticultural Society.Accessed March 30, 2016. https://www.rhs.org.uk/advice/profile?PID=542.
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