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Pythium Root and Stem Rot of Bedding Plants

August 2, 2007

It seems odd to be discussing these wet-season root rots when only a few weeks ago much of the state was under severe drought stress. Nevertheless, the Plant Clinic has received many cases of these root rots in the last 2 weeks, following significant rain events.

Pythium root and stem rot causes the same stressed top growth as seen with other root rots. Plants that are stunted, low in vigor, or slow growing—or those that wilt easily on a warm day—may be infected with a root rot. Diseased roots cannot absorb water and nutrients needed for growth. Pythium root and stem rot occurs in overly wet, poorly drained soils and causes death of root tips. The result is the appearance of brown to black, soft, rotted roots. Be very gentle when washing suspect roots or you will wash off the rotted portion and see only the white inner root. If that happens, the white roots left behind will be much thinner than normal roots—a clue that something is amiss. A more severe infection may include blackened stem tissue moving from the root system up the stem several inches, as seen on these geraniums.

Other stress factors may predispose plants to Pythium root rot. An example is overfertilization. Such injury causes burning of the root tips and provides sites for infection by Pythium. Consider all possible stress factors in the landscape, not just the disease pathogen. Diseases are often only part of the problem.

Pythium root rot in a greenhouse situation is often associated with contaminated soil, plants, or containers. Pythium is a soilborne fungal-like pathogen and becomes a much more complicated problem in the landscape. Control of root rots should be aimed at prevention. Use only healthy transplants. Weak plants may be diseased, and you certainly won’t save time or money in the end if you use weak plants. Because poor drainage usually goes hand in hand with root rot, proper site preparation to provide good water drainage away from roots is imperative. Pythium is a problem on wet sites, requiring moisture to infect. Use a balanced fertilizer if desired, but keep rates low on new transplants. Rotate plantings in the garden every 2 or 3 years with unrelated plants to help prevent the buildup of pathogens in one area. Remove crop residue at the end of the season to help reduce pathogen survival.

Even if all of the practices mentioned are followed, root rot may still occur. Fungicides are available to help control Pythium. Still, fungicides protect plant stems and roots not yet affected but cannot magically revive dead plants. Fungicides are most useful in cases in which a root rot is discovered in a flowerbed and the goal is to preserve remaining healthy plants to the end of the season. Affected plants are removed and nearby plants treated with the appropriate fungicide. Many fungicides are specific to particular pathogens, so treatment relies on accurate diagnosis of the root rot pathogen. Specific chemicals are listed by host crop in the Illinois Commercial Landscape and Turfgrass Pest Management Handbook or the Home, Yard, and Garden Pest Guide. In reality, garden plants are exposed to many different root-rotting fungi. Because these organisms are often difficult to identify and time is usually short, most landscapers use a broad-range fungicide to stop infection, at least until an accurate identification can be made by a diagnostic lab such as the Plant Clinic. Products with the broadest host labels to control a variety of fungi include a combination of mefenoxam (Subdue Maxx) + thiophanate methyl (Cleary 3336, Fungo, Domain) or a combination of metalaxyl (Subdue) + thiophanate methyl (Cleary 3336, Fungo, Domain). Banrot has a more restrictive label but also controls a range of soil fungi. Read all labels carefully before selecting a fungicide.

Consult Report on Plant Disease, no. 615, “Damping-off and Root Rots of House Plants and Garden Flowers,” for more details on root rots. This publication is available in Illinois Extension offices and on the Internet at http://www.ag.uiuc.edu/%7Evista/horticul.htm.