Crown And Root Rot In Bedding Plants: Pythium spp.

Popular bedding plants such as geranium, petunia or coleus may develop crown and root rots.  Pythium is one of the pathogens that we have encountered in several plant samples this spring. Symptoms can vary by plant age. For example, young plants may look discolored and die suddenly, a common syndrome known as damping off. This syndrome can also occur in vegetable transplants grown indoors and outdoors.

If plants are infected at a later growth stage, leaves start to discolor and blight (rapid tissue death). Roots are discolored (brown) and water soaked. The root dysfunction is the primary symptom while the foliage symptoms are secondary.

For diagnosis at the clinic, it is important to receive full plants as opposed to leaves and stems. See our examples of submitting whole potted plants at the Clinic website.  

In the Clinic, we observe the roots of the plants and inspect for microscopic pathogen structures (signs). In cases where structures are not readily present, we culture the root in media and incubate for a week or 2 until pathogen structures are developed.

 Lina Rodriguez Salamanca, Plant and Insect Diagnostic Clinic
Note discolored roots (left). Note microscopic pathogen structures (water mold signs) on the root tissue (right). Photo credit: Lina Rodriguez Salamanca, Plant and Insect Diagnostic Clinic

Pythium is a soilborne and waterborne pathogen. Irrigation and soil moisture can exacerbate the problem, mainly because Pythium sp spreads by water, as the zoospores ("seeds" of this pathogen) can swim in water.

To manage these disease, prevention is key. Use a well-drained soilless media and adjust the frequency and amount of irrigation water to reduce the risk of pathogen spread. Exercise good cultural practices that promote vigorous growth; for example, proper light, temperature, and fertilization. When purchasing plants for your garden, when possible, inspect the plant's root. Learning how good roots systems look like can help to avoid introducing soilborne pathogens like Pythium sp in the garden.

Once a problem has arrived, accurate diagnosis will give the steps to manage the disease. When Pythium root rot is confirmed, destroy infected plants, clean propagation tools, use clean pots, and if reusing pots, conduct a thorough disinfection. See important tips for sanitation in this articles:

Prepare for the Growing Season: Think Sanitation!

Sanitation is critical to prevent plant diseases Part 1: Greenhouse sanitation

For commercial production of bedding plants, preventative application of fungicides can help protect neighboring plants from infection. At the Clinic, the diagnosis report will include specifics for commercial production.


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