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Phytopthora Crown and Root Rot by Paul Pilon

September 14, 2011

Although Pythium is the most common root rot pathogen, Phytopthora (a close cousin to Pythium) can be a very destructive and difficult to control disease.  Phytopthora infections occur less frequently than Pythium diseases, but when Phytopthora takes hold, its impact on plants is very dramatic.  One distinctive characteristic of Phytopthora is its ability to attack plants at the soil line, often causing the plant to collapse.  

Infections can be identified by observing a brown canker at or just above the soil surface.  The margin of this canker appears water-soaked; however, there is often not a distinct margin between healthy and damaged tissue.  As the infection develops, it often girdles and kills the plant.  Infected plants usually have nutrient deficiency symptoms and wilt readily on sunny days.

The roots of infected plants often appear reddish-brown; however, root rots do not always develop on infected plants.  When infections are mild, plants often have dead feeder roots, dark streaks up the stem, and/or smaller than normal foliage.  More severe infections are characterized by smaller than normal leaf expansion, plants with a stunted appearance, wilting on sunny days, reddish-brown root systems, and plant death.

The occurrence of Phytopthora is most prevalent during extended periods of wet soil conditions.  However, it only takes 4 to 8 hours for sporangia (thin-walled bags of zoospores) to develop and the zoospores can be released 10 to 60 minutes later.  Water is key to the spread of Phytopthora. It is spread from plant to plant with splashing water from rainfall or overhead irrigation; Phytopthora is also transferred in runoff water and in subirrigation systems. Infections are most prevalent during mild to warm growing temperatures of 59 to 82° F.  

There are several cultural factors that growers can manage to reduce the likelihood of Phytopthora infections.  Use a growing mix that drains quickly to reduce the length of time the root zone remains near saturation.  Avoid excessive moisture in the root zone with good irrigation management practices. Inspect incoming plants for any signs of infection and use disease free starting materials. Clean and sanitize the production area between crops and avoid reusing containers from previously diseased crops (particularly with susceptible crops).  Once infections are identified, carefully remove infected plants to reduce the spread of the disease to uninfected plants.

Susceptible crops could be treated monthly with a preventative fungicide drench application. Several growers use biological soil amendments such as Streptomyces or Trichoderma to suppress or eliminate Phytopthora infections preventatively. It is recommended to scout crops weekly, inspecting the health of the crowns and roots to detect crown and root rot pathogens early.  Growers not familiar with Phytopthora symptoms and identification should submit symptomatic plants (send the entire plant) to a diagnostic clinic for proper diagnosis.

Several fungicides including products containing dimethomorph, etridiazole, fenamidone, fosetyl-Al, mefenoxam, and pyraclostrobin are effective at controlling Phytopthora on ornamentals. Be sure to rotate between chemical classes when applying fungicides to reduce the likelihood of this pathogen from developing resistance to these fungicides.

Paul Pilon
Perennial Solutions Consulting

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