Avanzato, M.V. and C.S. Rothrock. 2010. Use of Selective Media and Baiting to Detect and Quantify the Soilborne Plant Pathogen Thielaviopsis basicola on Pansy. The Plant Health Instructor. DOI: 10.1094/PHI-I-2010-0610-01
The nursery industry plays an important role in the distribution of the soilborne pathogen Thielaviopsis basicola (Berk. & Broome) Ferraris (syn. Chalara elegans Nag Raj & Kendrick), the causal agent of black root rot, by the sale of diseased pansies (Viola × wittrockiana Gams ex Kappert) and associated infested potting media.
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Soilborne pathogens have developed the ability to survive in soil for extended periods after host crop residues have decomposed, a situation where many foliar plant pathogens lose viability. In the absence of a host, soilborne pathogens survive by one or more of the following mechanisms:
A soilborne pathogen maintains itself by one or more of these mechanisms if it is to be successful (Figure 1).
Common symptoms of diseases caused by soilborne pathogens include seed rot, damping-off [defined as the death of developing seedlings before emergence (pre-emergence damping-off) or after emergence from the soil surface (post-emergence damping-off)], root rot, crown or collar rot, vascular wilt, and rots of bulbs, corms, fruit, and tubers. Observation of the initial symptoms of root disease and the detection of associated pathogens is difficult due to the soil matrix around the roots.
In addition to plant pathogens, the soil is home to a vast array of other organisms; algae, bacteria (including actinomycetes), earthworms, fungi, insects, mites, nematodes, oomycetes, and protozoa may influence the activity of the pathogen and complicate its isolation. By far, the most common groups of soilborne pathogens are nematodes and filamentous eukaryotic organisms (oomycetes and fungi) such as Phytophthora, Pythium, Fusarium, Rhizoctonia, Sclerotinia, Sclerotium, Thielaviopsis, and Verticillium. However, bacterial pathogens, such as Agrobacterium tumefaciens, Ralstonia solanacearum, and Streptomyces scabiei are also considered soilborne.
Movement of soilborne pathogens is limited because their inoculum and growth occurs primarily in the soil. Dissemination of soilborne pathogens in the field or landscape or from location to location depends on the movement of infested soil/potting media, plant residue, or colonized plant material. Spread of soilborne pathogens in large part is a result of the activities of man. These activities may include irrigation, tillage practices, movement of equipment among fields, and in many cases, the movement and sale of plants or plant parts, especially vegetatively propagated crops.
To study soilborne pathogens, researchers generally isolate the pathogen from the soil to determine their presence or estimate their population. Pathogens can be recovered from soil by:
Thielaviopsis basicola is a soilborne pathogen that causes disease on over 200 plant species (CAB International; Johnson, 1916; Otani, 1962; Shew and Meyer, 1992). Economically important plant species include field crops, such as cotton and several legumes, and ornamental plants, including pansy, poinsettia, and holly. This pathogen produces two types of asexual spores: clear cylindrical endoconidia and chains of dark, thick-walled chlamydospores, also known as aleuriospores (Figure 3). The pathogen may survive for years as chlamydospores in the absence of a host plant or parasitically on other plants that are hosts. Symptoms on hosts are a black cortical rot of the roots that stunts plant growth, thus delaying development and reducing yield (Figure 4). Generally this disease is a chronic problem, rarely killing the host.
T. basicola is easily baited from soil or plants by placing the material on carrot disks (Yarwood, 1946). This pathogen also may be isolated using the selective medium TB-CEN (recipe below), which contains carrot juice as a nutrient source and ingredients to inhibit the growth of bacteria, oomycetes, and other fungi (Specht and Griffin, 1985).
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In this laboratory exercise we will examine if the sale of pansy plants in the fall or spring is important to the spread of T. basicola on diseased plants or in infested potting media. By observing symptoms and signs on the host and comparing baiting techniques and selective media, the class should be able to prove or reject this hypothesis.
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Each group should select a pot or six-pack of pansies and a bag of soil from a landscape site. Note the source of the pansies (nursery or retail store) and landscape soil (flower bed or turfgrass).
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