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      Figure 1.  Pythium root rot of geranium.
      Note yellow, stunted plants and empty
      cells in the carton.       

ROOT ROTTING PATHOGENS

Several fungal pathogens are capable of causing significant losses in the production of flowering potted plants. These fungi may cause death of seedlings before or just after emergence from the potting mix (pre- and post-emergence damping off), or they may cause chronic or acute rotting of roots of mature plants. Unlike the host-specific rust and mildew fungi, the root rot pathogens, in general, have the capacity of attacking a broad range of bedding plants.

There are a host of fungi associated with root rots of flowering plants. Nevertheless, four fungi, Pythium, Phytophthora, Rhizoctonia, and Thielaviopsis, are responsible for most of the damage to potted plants in greenhouse production. It is important for you to determine which of these fungi is responsible for the root rot, since cultural and chemical controls will vary for these organisms.

Pythium and Phytophthora. These fungi (actually pseudo fungi) are present in most non-treated soils. They may also be found in washed sand and in certain peats. Pythium and Phytophthora are sometimes referred to as water-molds because they produce motile spores that disperse or "swim" in water. Although these pathogens are not good competitors with other soil fungi, they can survive for many years in soil by forming spherical, thick-walled resting spores (oospores). The oospores are relatively common in diseased tissue, and their presence is used in diagnosis.

Pythium and Phytophthora root rot often occur in saturated or poorly draining potting material and suboptimal temperatures for plant growth. Disease severity may also be increased by excessive nitrogen fertilization.

Plants affected by Pythium or Phytophthora root rot usually are stunted and have a flaccid or wilted appearance. The foliage may appear dull green or yellow as if suffering from nitrogen deficiency. In almost all cases, the roots of diseased plants will show some type of discoloration. The disease often starts as a discoloration of the small lateral roots and root tips. Small, pale brown lesions girdle the root, causing the outer root layer to slough off, leaving behind a thin tendril of inner root (steele). This rat-tail appearance is common for Pythium root rot. As the disease advances, roots turn dull brown to black and are water-soaked. Diseased roots tend to break apart easily when washed lightly.

Rhizoctonia: Most potted flowering plants are susceptible to root rots caused by Rhizoctonia solani. The fungus is present in most non-steamed or non-fumigated soils. The fungus can be easily identified microscopically by the characteristic right-angle branching of the fungal strands (hyphae). Although it does not produce airborne or motile spores, Rhizoctonia does form hard resting structures called sclerotia that can survive in debris and soil for many years.

Rhizoctonia is easily transmitted from one location to another in soil or contaminated pots or equipment. The fungus is capable of causing a root, crown and even aerial blight. Diseased roots initially develop discrete, light to dark brown lesions. As the disease progresses, lesions may coalesce to kill the root. Roots typically do not develop the water-soaked appearance associated with Pythium and Phytophthora root rots. Root infections may progress up the stem to cause a crown rot. Longitudinal cracks may form in the diseased tissue.

Thielaviopsis: Thielaviopsis basicola is a soilborne fungus that damages a number of bedding plants. The fungus may be introduced into the greenhouse on transplants or contaminated pots or tools or in non-treated field soils and certain peat moss. The fungus produces bead-like chains of spores called endoconidia that are easily seen with a microscope. These spores are easily splashed from one pot to the next and may also be moved by insects including shore flies and fungus gnats. Thick-walled resting structures called chlamydospores allow the fungus to survive for long periods in the soil. Thielaviopsis root rot has been a particularly serious problem in vinca, impatiens and pansy production in Kansas, although many other plants are also susceptible. Affected plants are stunted and yellow. The plants may appear to be suffering from nitrogen deficiency. Initially, roots of diseased plants are not extensively discolored. In fact, it may be difficult to tell if any root rot is present without microscopic examination. Diseased plants eventually develop small black root lesions. The roots may have a "peppered" appearance. As the disease progresses, root blackening increases. However, roots do not become water-soaked as is the case with Pythium root rot. Bacterial blights and soft rots. Losses from bacterial blights have not been extensive in recent years, but these diseases, caused by several species of Erwinia, can be catastrophic when they do occur. Bacterial blights may occur on a number of different ornamental plants, but they are most common on chrysanthemums, poinsettia cuttings, and Christmas cactus. These diseases are favored by high temperatures and humidity and crowded conditions. Symptoms include wiltings and a rapidly developing soft rot of tissues.

Figure 3.  Thielaviopsis root rot of petunia.
 

Figure 4.   Erwinia soft rot of poinsettia cuttings.

CULTURAL AND PHYSICAL METHODS TO ELIMINATE OR REDUCE ROOT PATHOGENS

Disease-free and healthy plants. Root pathogens may be introduced on transplants or on equipment transporting plant material. Carefully inspect all incoming material for evidence of root rots. Pull plants from trays and inspect roots with a hand lens. Look for root discoloration and/or unusual coloring of root tissue. If you see something suspicious, submit a sample to a plant disease diagnostic lab immediately. Try to segregate all new plant shipments from existing plants in the greenhouse for the first several weeks. This helps prevent potential movement of pathogens from diseased to healthy plants. Discard plants with advanced root rot. They cannot be saved and serve as a source of fungal inoculum.

Maintain plants in a healthy state. Plants weakened by sub-optimal growing conditions are predisposed to root rot damage. Carefully monitor the potting mix pH and soluble salts content. Avoid excessive nitrogen fertilization. Recent studies suggest that overfertilization enhances root rot of geranium and other plants.

Pathogen-free Potting Mixes: When possible, use soil-less mixes. Mixes containing soil tend to have problems with compaction, poor root aeration and water movement.

Commercially prepared soil-less mixes also tend to be free of or have low levels of soilborne plant pathogens. However, Pythium and Thielaviopsis have been found in certain, non-steamed sources of peat moss. Therefore, use of commercially prepared potting mixes does not necessarily exclude the possibility of introducing root pathogens.

Any potting mix containing untreated soil or sand should be assumed to be contaminated with root pathogens. Treat all soil-based mixes or potentially contaminated soil-less mixes before use. The following table outlines some of the methods for eliminating pathogens from soil. Do not recycle old potting mixes! You may be tempted to reuse mixes in flats where seedlings failed to germinate, etc., but this mix is likely contaminated with root rot pathogens. Furthermore, the mix may have been treated with fertilizers, plant growth regulators, or other chemicals that may affect the performance of the following crop.

                    Heating

dry heat (180 F for 30 minutes)

aerated steam (140 F for 30 minutes)

The use of contaminated equipment is a major means of introduction of plant pathogens. Keep hose nozzles off the ground. All pots and tools should also be disinfested after use. Those trays or pots from the last crop should be thoroughly washed to remove soil and/or plant debris and should be soaked with a disinfectant before reuse.

Benches should be cleaned and soaked with disinfectant after each crop. Tools should be kept clean and off the ground. Irrigation lines should be thoroughly cleaned.

Avoid using water from holding ponds for irrigation. These ponds may contain root rot pathogens or may be contaminated with chemicals (herbicides) detrimental to bedding plants.

Suppress fungal gnat populations in the greenhouse. Larvae may predispose roots to infection by root rot pathogens. Adult gnats may be contaminated with and move spores of Thielaviopsis and Pythium.

Clorox

Physan 20

PT-2000 Greenshield

Consan 20

Agri-Brom

COMPOSTS AND BIOLOGICAL ORGANISMS TO SUPPRESS ROOT ROT PATHOGENS

The addition of composts can change both the physical and biotic structure of a potting mix that is conducive to root rots to one that is suppressive. Several compost materials, including turkey and chicken litter, composted bark, and brewery and municipal sludge, have been tried. Additions of composted pinewood bark at 20% or more is reported to suppress Pythium blight.

Several fungal and bacterial antagonists, when added to potting mixes, have been reported to reduce the severity of certain root rots. Typically, these products are applied as a drench or are incorporated into the mix before root rot symptoms appear. Several applications during the growing season may be required. These products are less effective when applied after the disease has developed. Some of these products may be used in conjunction with fungicide treatments.

Mycostop

root rot fungi

FUNGICIDES TO SUPPRESS ROOT PATHOGENS

Fungicides are an important tool in managing root rot diseases when used in conjunction with cultural management practices. Fungicides rarely, if ever, eradicate fungal pathogens; they merely suppress disease development for a certain period of time. If conditions for fungal infection are favorable after this period, the disease may once again progress. Therefore, try to maintain environmental conditions that are favorable for plant growth but unfavorable for fungal development.

Fungicides tend to be more effective when applied before or as soon as symptoms are apparent. Certain crops, such as vinca, geranium, and poinsettia, are susceptible to root rots. If you have consistently had problems with root rots on these or other plants, consider a routine preventive fungicide drench program. On the other hand, don't routinely use fungicide drenches on plants that do not have a history of root rot problems. In all cases, rely heavily on cultural management strategies.

Several fungicides are labeled for control of root rot pathogens. Unfortunately, there is no single fungicide product currently labeled that will effectively control all root rot pathogens. Generally, fungicides used to control Pythium and Phytophthora are ineffective against Rhizoctonia and Thielaviopsis. Often, a combination of two products is necessary to give broad spectrum protection against root rot pathogens. Individual fungicides may be mixed before application or purchased commercially premixed. Some fungicides may also be used in conjunction with biocontrol products.

Terrazole 25EC

Terrazole 5G

Truban 30W

Truban 25EC

Phytophthora

(nursery, greenhouse, interiorscapes)

(nursery, greenhouse)

(greenhouse, interiorscape)

Phytophthora

(nursery, greenhouse)

Foliar applications to aglaonema, aphelandra, azalea, begonia, boxwood, cissus, diffenbacchia, geranium, impatiens, hibiscus, leather-leaf fern, marigold, pansy, petunia, pittosporum, pothos, spathiphyllum, vinca, zinnia (REI - 24 hours)

Systemic product. See label for details.

Phytophthora

Subdue 2G

Phytophthora

(nursery, greenhouse, interiorscape?)

Bedding plants and flowers: African violet, ageratum, Algerian ivy, artemisia, aster, baby's breath, begonia, caladium, carnation, chrysanthemum, coleus, columbine, daisy, delphinium, Easter lily, English ivy, foxglove, gaillardia, geranium, gloxinia, impatiens, marigold, pansy, petunia, phlox, pinks, poinsettia, primrose, prostrate rosemary, rose, salvia, snapdragon, verbena, vinca, zinnia (REI - 12 hours)

Defend 75WP

(Nursery, greenhouse)

Bedding plants: abutiolon, alyssum, calceolaria, canna, carnation, chrysanthemum, cineraria, columbine, coral bells, cyclamen, dahlia, delphinium, dianthus, foxglove, fuchsia, gardenia, gazania, geranium, gloxinia, impatiens, lily, lupine, marigold, monarda, petunia, poinsettia, portulaca, salvia, scabiosa, sedum, sempervivum, verbena, vinca, zinnia, similar plants (REI - 12 hours)

Phytophthora

Clearys 3336 50WP

Fungo FLO

Fungo 50 WSB

Systec 1998

Thielaviopsis

(nursery, greenhouse, interiorscape)

Banrot 8G

Phytophthora

Rhizoctonia

Thielaviopsis

Combination of ethazole plus thiophanate effective in suppressing Pythium, Rhizoctonia, and Thielaviopsis. Drenches preferred to granular incorporation. Foliage must be rinsed after drench application (REI - 12 hours).

Thielaviopsis

Web updated 9/01/06
Web comments to jpierz@ksu.edu