by Brooke A. Edmunds, Paula H. Flynn, and Mark L. Gleason.
RETURN TO THE EXERCISE
Answers to the Discussion Questions in the Hosta Case Study
- What factors are necessary for a disease to occur?
A susceptible host plant, a pathogen, and the environmental conditions that favor infection (disease triangle)
- What environmental conditions favor the development of crown rot of hosta, caused by the fungus Sclerotium rolfsii var. delphinii?
Prolonged hot [80s-90s°F (26-37°C) days, 60s-70s°F (15-26°C) nights], wet weather
- What are major symptoms of the disease and main signs of the pathogen?
Symptoms: yellowing and wilting of lower leaves, wilted leaves easily removed from crown, petiole bases brown and macerated, stem girdling in species with less succulent stems, plant death sometimes
Signs: mycelium and sclerotia
- Discuss the various methods by which the pathogen can spread from plant to plant or from field to field.
Sclerotia can be moved to healthy plants by rain splash, irrigation, machinery, tools, shoes, etc. Fungal mycelium (produced when the sclerotia germinate) of the crown rot fungus can grow from plant to plant during the growing season when conditions are warm and moist. Long-distance spread of the fungus can occur when infected crowns are shipped to nurseries, retail outlets, or home gardeners.
- What are sclerotia? Why do they make disease control difficult?
Sclerotia are tiny, compact masses of fungal mycelium, usually spherical in shape, with a hard outer shell. Sclerotia are capable of surviving in the soil under unfavorable environmental conditions for several years, then germinating and infecting host plants when the environmental conditions are again favorable.
- What additional information might be helpful to you in developing your disease management plan?
A list of the plants grown at Johnson Nursery, alongside the list of ornamental genera that are susceptible to S. rolfsii var. delphinii, would be helpful in making decisions about potential rotation schemes. More detailed knowledge of the cultivation practices performed at the nursery would help guide cultural management recommendations. Information about the company's finances also would be helpful in determining the feasibility of recommendations, especially those that involve roguing (removing and discarding diseased plants) hundreds or thousands of plants from productions fields.
- How can production methods affect disease management?
The number of passes through a field with equipment, especially tilling and digging machinery that disturbs the soil surface, affects the potential spread of the pathogen. Preparing plants for sale (cleaning and division) also can affect the spread of the pathogen.
- Sclerotium rolfsii var. delphinii has a wide host range. How should this characteristic influence management strategies?
The nursery grower must be aware of the plants grown at the nursery that are susceptible to S. rolfsii var. delphinii. Susceptible species should not be planted in infested fields, and perhaps not even in neighboring fields. Knowledge of which species are susceptible to S. rolfsii var. delphinii also is important when making decisions about cultivation practices.
Decision Forcing Questions
- Using the information from the case and your own personal experience from internships, jobs, or coursework, how would you manage the petiole rot problem in Johnson Nursery? What should be done this year? What management practices do you recommend for a long-term solution?
In order to be effective, several of the following recommendations should be implemented together rather than individually. Different combinations can be used depending on the situation.
- Rogue out any diseased or suspicious-looking plants before the cutting bar passes at harvest. If possible, these plants and the soil down to a 4 inch (10 cm) depth in a 1-foot (0.3 m) radius should be discarded. Diseased plants and surrounding soil should be bagged or otherwise placed in containers to minimize spread of sclerotia during removal. The discarded plants and soil should be either buried or removed from the Johnson Nursery property.
- Treat the infested fields as potential sources for spread of the pathogen to other production fields. Equipment, tools, and shoes should be cleaned after working in these areas and before moving to other parts of the field or other production fields. The goal is to remove as much adhering soil as possible, since it may contain sclerotia.
- Deep-plow infested fields after the diseased crop has been removed. Using a moldboard plow to bury sclerotia at least 6 inches (15 cm) deep has been shown to accelerate their colonization by soil microorganisms, reduce their survival, and reduce their opportunities to infect hostas, since petiole rot infections typically develop at the soil line rather than in the root zone.
- Continue the practice of power-washing crowns after harvest. This will knock off most of the adhering sclerotia. Also, peel off any discolored petiole bases at this time.
- Inspect for the presence of sclerotia and disease symptoms on all hostas to be planted. Discard any suspicious-looking plants.
- Rotate out of hosta and other known host crops of S. rolfsii var. delphinii in the infested fields for at least 4 years.
- Drench with labeled fungicides, such as PCNB or flutolanil, about three times per season when producing any host plant of petiole rot or any herbaceous perennial grown on previously infested ground. The first drench should be timed to precede the onset of the warm weather [80s-90s°F (26-37°C) days, 60s-70s°F (15-26°C) nights] that favor the petiole rot development.
- Switch from mechanical to chemical weed control in and near the infested fields. This will keep the sclerotia from spreading within and between fields.
Additional practices that might be helpful in reducing the long-term risk of petiole rot:
- Switch source material from field-grown crowns to greenhouse-grown plugs. (Plug plants are used extensively in the southeastern U.S. to propagate hostas.) This practice will effectively prevent introducing the pathogen again. However, this could increase the final cost of the plant because plug plants involve a different set of technologies.
- Pre-plant fumigation with methyl bromide was used before 2005, and would kill some, but not all, of the sclerotia. However, virtually all methyl bromide use was banned in the U.S. by 2005, except for critical use exemptions in a few crops and states that have to be approved annually. The EPA website (http://www.epa.gov/Ozone/mbr/) provides information on such exemptions. Other fumigants, such as Vapam, are likely to be less effective.
- What ethical issues are involved in this case?
Several ethical dilemmas can be brought out during classroom discussion. One issue is focused on profits vs. selling potentially diseased plants. Johnson Nursery needs to make a profit in order to stay in business. Sales of its plants are at the heart of making a profit. Repeat business to customers requires customer satisfaction. The nursery manager must struggle with the decision to either play it safe, by not shipping any plants that could harbor the pathogen, or else to ship all the plants that "appear" to be healthy.
A second issue for discussion is the tradeoffs of the different control measures. For example, chemical herbicide control rather than mechanical tillage could reduce both the chance of physical spreading of sclerotia in the field and soil erosion. However, there are risks associated with use of chemical pesticides. Health risks created by worker exposure to these chemicals in concentrated form and environmental pollution should be considered. A class discussion could be helpful to recognize the tradeoffs associated with the various management decisions.