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Phytophthora Diseases

April 17, 2013

Phytophthora is a very common pathogen observed on numerous crops in greenhouses and nurseries. Most growers first consider Phytophthora as a root rot pathogen; however, some of the most problematic Phytophthora infections occur at the base of the stems or on the crown of a plant rather than in the root zone. This pathogen can even cause foliar diseases and branch blights.

Phytophthora is considered a 'water mold' (fairly closely related to Pythium) which thrives in very wet environments.  Once an infection takes hold, it's impact on plants is very dramatic and can be very destructive.

One distinctive characteristic of Phytophthora is its ability to attack plants at the soil line, often causing the plant to collapse.  Infections can be identified by observing a dark lesion or brown canker at or just above the soil surface.  The margin of this canker may appear water-soaked; however, there is often not a distinct margin between healthy and damaged tissue.  As the infection develops, the cankers impede the flow of water and nutrients from the roots to the above ground parts of the plant, causing the plant to wilt readily during sunny conditions.

Phytophthora can occur at any time during production, infecting very young plants to full sized plants in bloom.  Infected plants do not always develop root rots. Plants with mild infections will often develop smaller than normal leaves, have dark streaks going up the stem, and may have dark feeder roots.  In more severe cases, the plants may appear stunted, have reddish-brown root systems, and result in plant mortality.

The occurrence of Phytophthora 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 Phytophthora. It is spread from plant to plant with splashing water from rainfall or overhead irrigation. Phytophthora 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 Phytophthora infections.  Clean and sanitize the production area between crops and avoid reusing containers from previously diseased crops (particularly with crops susceptible to Phytophthora).  Use disease free starting materials and inspect incoming plant materials for any signs of infection. Use a growing mix with good drainage and high aeration porosity to reduce the length of time the root zone remains near saturation. Avoid excessive moisture in the root zone with good irrigation management practices. It is also helpful to reduce the time the leaves and crown have free moisture on them by watering early in the day and avoiding irrigation on cloudy days.

It is recommended to scout crops weekly, inspecting the health of the stems near the soil line, plant crowns and the roots to detect the presence of an infection early. Symptomatic plants can be submitted to a diagnostic clinic (send the entire plant) for proper identification. Once infections are identified, carefully remove infected plants to reduce the spread of the disease to uninfected plants.  With early detection and minimal injury to the crops, management strategies can be implemented to control Phytophthora.

Many growers maintain preventative strategies on crops particularly susceptible to Phytophthora infections. Many growers biological soil amendments such as Streptomyces or Trichoderma to suppress or eliminate Phytophthora infections preventatively. Other growers apply preventative fungicide drench applications approximately once per month to reduce the likelihood of infections from occurring.

Fungicides containing the active ingredients ametoctradin, dimethomorph, cyazofamid, etridiazole, fenamidone, fosetyl-Al, mefenoxam, or pyraclostrobin are effective at controlling Phytophthora on ornamental crops. Be sure to rotate between chemical classes when applying fungicides to reduce the likelihood of this pathogen from developing resistance to these products.

Paul Pilon
Perennial Solutions Consulting
paul@perennialsolutions.com

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