Pacific Northwest 1998 An Online Guide to Plant Disease Control

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OSU Extension Office
 
Greenhouse Plants, Ornamental -- Black Root Rot
 
Cause: Thielaviopsis basicola is a widely distributed root pathogen reported from at least 30 plant families, including some ornamental plants. Untreated or improperly treated field soil can be a source of T. basicola if used in potted-plant culture. Most soilless media is clean, however T. basicola spores can be present in commercial peat moss and result in disease problems. The pathogen forms thick-walled chlamydospores, which can survive several years in soil. Root exudates stimulate chlamydospores to germinate in soils whose pH ranges from 5 to 8.5, and root rot tends to be more serious in plants grown in neutral and alkaline soils. Root rot also generally is more serious for plants grown at soil temperatures of 55 to 70°F, but can be severe on some plants at as high as 77°F. Disease severity tends to be worse when temperatures are not optimal for the plant and the soil moisture-holding capacities are at or above 70% for extended periods.

Plants that frequently develop black root rot include Begonia sp., Catharanthus spp. (periwinkle), Cyclamen persicum, Euphorbia pulcherrima (poinsettia), Fuchsia X hybrida, Gerbera jamesonii (African daisy), Pelargonium X hortorum (florist's geranium), Pericallis X hybrida (cineraria), Scindapsus aureus, and Viola spp. (pansy).

Symptoms: Above ground symptoms are not unique to T. basicola but are typical of many root diseases or soil conditions that are detrimental to root growth. Plants often are chlorotic and stunted and may wilt if root rot is severe. Initially, roots or portions of roots killed by this pathogen are brown and could be mistaken for Rhizoctonia root rot. The diagnostic flat, black root lesions occur because of the abundant formation of chlamydospores on the root surface and in the root cortex. Multiple root diseases, such as Pythium and Thielaviopsis root rot, can occur coincidentally.

The fungus can be seen in infected tissue with a compound microscope. The fungus can be isolated readily by spreading a thin layer of soil or colonized root tissue over 5-mm-thick carrot root disks in sterile dishes. Sterile water is then atomized over the disks to moisten the soil or roots; after 2 to 4 days at room temperature, the incubated disks are rinsed with water and incubated again. If the fungus is present, endoconidia form in about 6 days followed by chlamydospore formation. Beadlike chains of endoconidia can be observed with a dissecting microscope.

Cultural control: Fungicides do not compensate for poor sanitation or environmental controls.

  1. The disease is greatly reduced at pH levels below 5.5 and adjusting media pH is practiced by some plug producers. However, finished plants are often grown in commercial peat mixes that are buffered at a pH between 6.0 and 6.5.
  2. Dispose of diseased plants and plant material promptly.
  3. If field soil is a component of the growing medium, treat it with steam (122oF for 30 min) or chemical fumigants. Use soilless planting media.
  4. Do not let contaminated tools or hands come in contact with sterilized planting media.
  5. Keep entire propagation and growing areas clean.
  6. Control fungus gnats and shore flies which may serve as vectors. Consult the PNW Insect Control Handbook for currently effective tactics.
  7. After the production cycle, remove any remaining plants and disinfect the growing area.
Chemical control: Fungicides work best when used as a protectant in combination with cultural controls. Chemical control may require multiple applications. If the disease is noticed early, also treat with fungicides. This disease can be difficult to control, and often the higher rate is more effective. Read each label before using and observe all restrictions and regulations.

  1. Banrot 40 WP (etridiozole plus thiophanate-methyl) is registered as a drench for a wide range of crops. For bedding plants, use at 4 to 8 oz/100 gal water applied to 800 sq ft of bed area. 12-hr reentry.
  2. Medallion at 1 to 2 oz/100 gal water. Use as protectant before symptoms occur since it is ineffective once plants are infected. Do not use on impatiens. Use with oils or adjuvants may damage plant. 12-hr reentry.
  3. Thiophanate-methyl is registered as a drench for a wide range of crops and sites. 12-hr reentry.
    1. AllBan Flo at 7.5 to 20 fl oz/100 gal water.
    2. Cleary's 3336 WP at 12 to 16 oz/100 gal water.
    3. Halt (by ferti-lome) at 2.5 tsp/gal water is registered for home use.
    4. OHP 6672 4.5 F at 7.5 to 20 fl oz/100 gal water.
    5. SysTec 1998 WDG at 0.08 lbs/100 gal water
  4. Terraguard SC at 2 to 8 fl oz/100 gal water is registered as a drench. 12-hr reentry.
References:
Daughtrey, M.L., R.L. Wick, and J.L. Peterson. 1995. Compendium of Flowering Potted Plant Diseases. St. Paul, MN: APS Press.
Content edited by: Jay W. Pscheidt on January 1, 2009
 
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