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​Answers to the Exercise Questions​

Maria Valeria Avanzato and Craig S. Rothrock
Department of Plant Pathology, University of Arkansas, Fayetteville, AR 72701


1.  Did you find differences among nurseries with respect to the presence of Thielaviopsis basicola?  Should you be concerned about where you buy your pansies?

Answer:  An important control practice for many plant diseases is exclusion, where the introduction of a pathogen to a planting area is prevented.  This is essential for soilborne pathogens because they have a wide host range and persist once introduced.  Soilborne pathogens are not easily disseminated by means other than the movement of soil or diseased plant material.  Once the pathogen is present, excluding additional inoculum is of little value.  Experience has shown that plant material sold at retail outlets differ in the presence of T. basicola; the severity of disease symptoms, and inoculum levels.

2.  Was examining the plants for disease as effective as using soil assays to detect the pathogen?

Answer:  Assays of potting media will detect T. basicolamore frequently than will the examination of roots for symptoms and signs.  Symptoms of black root rot may not be apparent because nurseries may select healthy-appearing plants for sale or shipping, or severely diseased roots are lost as students wash potting medium from the roots.  However, even when plants appear healthy the pathogen may still be present and introduced subsequently to the planting bed.

3.  Can baiting and dilution plating both be used to detect T. basicola?  What are the advantages or disadvantages of each method?  Is the TB-CEN medium a good selective medium?

Answer:  Both techniques should provide similar results.  The advantage of the baiting technique is that it is simple, uses readily available materials, and could be conducted by a knowledgeable plant outlet employee or homeowner.  The disadvantage of this technique is that it is not quantitative (does not measure levels of inoculum).  The advantage of using a selective medium and dilution plating is that this provides a quantitative measure of inoculum, allowing a comparison of pathogen population levels among samples.  Using 5 plates and a 1/10 dilution, the lower limit of detection would be 2 CFU/1 cm3 of soil.  The TB-CEN medium has good specificity for the growth of T. basicola.  In this laboratory exercise, the selectivity of this medium should have been evident when comparing growth on TB-CEN medium to a 1/10 dilution of soil or potting media plated on the non-selective medium (PDA) due to the presence of numerous and diverse fungal and bacterial colonies.  The baiting technique data can be made quantitative by standardizing the amount of potting medium applied to each carrot disk.  By making dilutions of the potting medium with non-infested potting medium, the proportion of baits colonized by the pathogen can be used to estimate population density using the most probable number method (Woomer, 1994).

4.  What are possible sources for the introduction of the pathogen into nursery operations?  How would you suggest managing this problem?

Answer:  At the outset, possible sources of inoculum in the nursery include starting with infested potting medium or purchasing diseased plugs.  However, it is also possible that large numbers of pots can be contaminated during greenhouse production by any contact with infested soil or potting medium or splashing of these materials, infesting previously clean pots.  In addition, inoculum can be present in the facility if diseased plant material was grown previously.  Sanitation between crops is important to reduce carryover of inoculum, which is often difficult with soilborne pathogens.  The first step is to determine the source of the pathogen.  If inoculum is already present in the operation, the development of a good system of sanitation is essential to limit pathogen dissemination.  Old and diseased plant material should be removed and destroyed, and production surfaces should be cleaned and sanitized.  Testing potting medium or plugs of pansies to ensure that the pathogen is not introduced to the operation would be another recommendable measure to avoid introducing the pathogen into new plantings.  Floor production systems are often used for producing bedding plants, but placing plant material at ground level increases the likelihood of infesting pots.  Inoculum is more likely to spread to the subsequent crop with polypropylene ground covers over soil than from concrete pads.  Porous concrete is more likely to retain inoculum than standard concrete.  For bedding plants, production on raised benches would likely decrease disease problems.  The reuse of pots or trays in plug production can also contribute to disease problems.  Disinfestation of growing surfaces and containers is important for managing soilborne pathogens.  Copes and Hendrix (1996) demonstrated that standard greenhouse disinfestants including bromine (Agribrom, Great Lakes Chemical Co.) or quaternary ammonium (Prevent, The Buffalo Co.) had little effect on viability of spores of T. basicola, while solutions of bleach (NaOCl) or the fungicide captan (N-trichloromethylthio-4-cyclohexene-1,2-dicarboximide) were effective at disinfesting materials used as growing surfaces in greenhouses.  Sanitation should be practiced continuously and not when growers have time remaining after other production activities are concluded.  Water recirculation systems also can disseminate many soilborne pathogens by moving spores and infested potting media.  Protection of the plants using fungicides is generally not considered a successful or economical strategy for black root rot.

5.  Was the pathogen detected in landscape settings and if so, was it associated with planting annuals?

Answer:  Yarwood (1974) reported that T. basicola could be found in agricultural sites and in native areas.  The pathogen is most often associated with plants in the families Cucurbitaceae, Leguminosae, Malvaceae, and Solanaceae.  Thus, if you were examining the distribution of T. basicola you would try to isolate from plants in these families from native areas.  The problem with many plants in the landscape is that it is not clear if naturally occurring populations of the pathogen increased with the presence of susceptible plants, or if inoculum was introduced with the plants.  However, unlike soilborne pathogens in other genera such as Rhizoctonia or Pythium, T. basicola is not commonly found in soils.

6.  Were your results similar to the other groups; if not why not?

Answer:  Sources of variability in the results could include technical problems; for example, starting with a poorly mixed soil sample or suspension.  More likely, variability might have come from an uneven distribution of inoculum among pots in a given nursery.  For example, some pots may contain high levels of inoculum if the pathogen sporulated on diseased roots.  Inoculum might not have been detected in other pots if clean medium and plant material had been used and little dissemination of the pathogen had occurred.

Additional questions

7.  Do you think that nurseries are aware of potential problems with black root rot on pansy?

Answer:  One of the challenges with soilborne pathogens is that producers are often unaware of the reasons for problems in production until after a disease is already widespread.  Unlike foliar diseases where individual lesions can be detected, diseases caused by soilborne pathogens are often not recognized until growth is severely reduced, changes in foliage color are seen, or plant death has occurred.  Such problems are often misdiagnosed, and cultural practices such as increasing water or fertilizer are used to minimize symptoms.  Black root rot is difficult to manage because producers don’t recognize the disease, mainly because plant growth reduction rarely causes plant death.

8.  What is the responsibility of the nursery or retail outlet for providing disease-free plants?

Answer:  A nursery should be aware of disease problems that have the potential to occur on the numerous plant species they produce.  Although producers are obviously concerned about producing a healthy vigorous looking plant at the time of sale, they should also be concerned with the distribution and introduction of pathogens to the landscape.  Reasonable production practices to limit the spread of many soilborne diseases would benefit the nursery and the public.

9.  What would be an acceptable cost for providing disease-free plants?

Answer:  Risk is always weighed against cost of production of disease-free plants.  If some of the nurseries in your study sold disease-free plants, then perhaps they follow good sanitation practices and closely monitored crop health, or perhaps the pathogen was not present in the production system.  Conversely, even the best growers can have disease problems, and plants may not develop symptoms until they are shipped.  Techniques to limit disease can be designed by looking at the production system and developing practices to limit sources of inoculum of the pathogen and managing the crop to avoid a favorable environment for disease.  In some cases, these practices may add little to the cost of production and thus not reduce profits.

Literature cited

  1. Copes, W. E., and Hendrix, F. F. 1996. Chemical disinfestation of greenhouse growing surface materials contaminated with Thielaviopsis basicola. Plant Disease 80:885-886.
  2. Woomer, P. L. 1994. Most Probable Number Counts. Pages 59-79 in: Methods of Soil Analysis Part 2 Microbiological and Biochemical Properties. R. W. Weaver, S. Angle, P. Bottomley, D. Bezdicek, S. Smith, A. Tabatabai, A. Woolum, eds. Soil Science Society of America, Inc., Madison.
  3. Yarwood, C.E. 1974. Habitats of Thielaviopsis in California. Plant Disease Reporter 8:54-56.