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Managing Pythium Root Rot by Paul Pilon

October 26, 2010

Many crops are susceptible to Pythium root rot. Under optimal circumstances, Pythium can occur on nearly every life stage throughout the entire production cycle. Many growers do not monitor the root systems often enough during production and the detection of this disease and other root rot pathogens often doesn’t occur until significant injury and/or death has occurred.

There are several symptoms growers can look for to determine if their crops are infected with Pythium. Chlorosis of the lower leaves, plants take on a gray/green appearance with leaves that flag slightly during the heat of the day, and stunting are the most recognizable above ground symptoms that plants exhibit when they are being attacked by this pathogen. The extent of these symptoms varies largely by the severity of the infection. In many instances, particularly in the early stages, symptoms on the top growth are not observed. 

Pythium usually attacks the root tips first and then works its way upward in the root system. Roots infected by Pythium are generally soft, mushy, and appear various shades of brown. The outer covering of the root, the cortex, is usually rotted and slides off easily when pulled, leaving the string-like vascular bundles behind. This is commonly referred to as sloughing of the roots. 

Pythium is an opportunistic pathogen; requiring damaged or stressed tissues to gain entry into the plant. Plants under stress caused by high soluble salt concentrations, poor drainage of the growing medium, and over-watering are particularly susceptible to Pythium infestations. Root injury from insects, such as feeding from fungus gnat larvae, can also provide an entry site for the pathogens through the feeding wounds.

Pythium is often referred to as a water mold; water molds prefer wet conditions for them to infect and spread. In many instances, the injury necessary for root infections (such as insect feeding, extremely dry conditions, and high salt levels) occurs prior to the moist/wet conditions more favorable for this pathogen.

Managing Pythium Root Rots

There are many steps growers can take to reduce the likelihood of Pythium infections.  Maintaining good cultural practices throughout production goes a long way towards preventing this pathogen from becoming problematic. General recommendations for preventing Pythium infections include using a porous, well-drained sterile potting medium, maintaining proper fertility levels, avoiding excess water and extreme drought, controlling fungus gnat and shore fly populations, and thoroughly cleaning and sanitizing the production area between crops. 

The incidence of this disease can largely be reduced using proper water and fertility management. There is often a fine line between proper and improper irrigation practices.  The longer a growing medium remains wet; there is a greater chance for Pythium to develop. Plants should be watered only when they need it and when conditions are conducive for irrigation to be applied. 

Whenever crops appear chlorotic, stunted, or wilted, be sure to check the root system for the presence of root rots; this step could be the difference between saving the crop and losing it. Growers should remove the plant from the container and look for any signs of decay, brown-mushy roots, lesions, or discoloration. In many cases, Pythium can be detected early, and with the appropriate culture practices and/or chemical treatments, it can successfully be controlled.

Fungicides can be applied to prevent Pythium from occurring or to control it, provided the infections have not reached epidemic proportions. Most growers apply fungicides as media drenches where the active ingredients can reach the infected roots. Fungicides are most effective when the pathogen levels are low and conditions are favorable for the disease to develop. 

Several fungicides including products containing etridiazole, fenamidone, fosetyl-Al, mefenoxam, and pyraclostrobin are effective at controlling Pythium on ornamentals. Many growers implement preventative fungicide programs, drenching susceptible crops every 30 days using the active ingredients mentioned above.

It is easier to control Pythium using good cultural practices, on a preventative basis, or when infection rates are low then it is to control existing populations that have already caused significant injury to plant roots.

Paul Pilon
Perennial Solutions Consulting
paul@perennial-solutions.com

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