Overall Program Direction

            My research objective is to develop or improve cultural and chemical control strategies based on understanding the behavior of a plant pathogen in the field as well the environmental factors that affect it. Due to the detailed nature of scientific work, a scientist can work on only a few problems at a time. I select projects based on the following criteria: the pathogen inflicts damage to a significant portion of the ornamental crop in a greenhouse or nursery production system, the disease problem occurs annually or frequently in the gulf coast region of the southern United States, and I believe the research effort has a good potential to develop economically practical controls. Comments from producers are a valuable consideration when making decisions for future projects. Please feel free to contact me.

 Insight into Plant Pathology

            Although fungi, bacteria and viruses are different organisms than lions and gazelles, alligators and frogs, sharks and mackerels, hawks and sparrows, and spiders and flies, the same basic information about territorial ranges during different seasons, dietary patterns, and the environmental conditions that are favorable or unfavorable for specific activities is needed. As an example, if you were studying sharks you would need to study more than just the shark attacking the prey, such as reproductive rates, seasonal movement, and predictability of behavior. Likewise with plant pathogens, studies can’t be limited just to the phase when plants express disease symptoms. The type of control strategy being proposed will influence the type of information needed, such as where the plant pathogen population is located or what it is doing at a specific time of the year. As an added challenge, plant pathogens are microscopic in size. They can be difficult to monitor even when they reside within a one hundred foot radius, yet pathogen populations can range miles and even across continents.

Difficulties Related to the Ornamental Plant Commodity

            Many of the disease control practices used in ornamental plant production were originally developed in food- and fiber-based plant production systems. The principles of these controls are sound. However, the production practices, growing environment, and field diversity of plant species of ornamental crops are so different from traditional agriculture that many of the control practices may not be effective when adopted directly in an ornamental production system. Because less in-depth research has been done for ornamental plant production compared to traditional row and field crop production, control practices need to be critically evaluated for their suitability and cost-effectiveness specifically in greenhouse and nursery settings.

            A second problem is the ornamental industry encompasses a vastly diverse mixture of plant genera, species, and cultivars, so only a limited number of producers are helped from any one project. There really is a lot of work to do.

Scientific Approach

            It would be great to know exactly what to do and test it. However, our intuition is often not correct, partly due to the complexity of nature. While producers can use a practice because they believe it works, the purpose and structure of scientific research is to develop information based on measurable and repeatable events that have to be critically evaluated by other scientists before it can be published. Because many ideas don't result in measurable differences when critically tested, it is important to understand the pathogen's field habits (its biology) as a means to select and develop practical control strategies and effective application knowledge. Typically, a specific aspect of a problem is tested to properly identify main and interactive effects. In some cases, several control approaches must be evaluated individually before testing them in an integrated system. In other cases, a highly controlled laboratory study must be done so we know how to properly approach a more variable field study. The purpose of USDA research is to develop usable technology, even new techniques that are currently unknown to the industry. However, annually publishing scientifically peer-reviewed journal articles is a mandatory part of our job and the main means of evaluating a USDA-ARS scientists' productivity. As I develop practical controls from my research within each project area, I will publish useful results in association newsletters and trade magazines and give short presentations at meetings.

Current Projects as the Principal Investigator

Efficacy of disinfestants on production surfaces.

Rhizoctonia web blight of azalea (Rhizoctonia spp.)

            Population diversity of Rhizoctonia species causing web blight.

            Environmental conditions favorable for disease development.

            Seasonal survivability and residence of the pathogen.

            Means of spread.

Camellia twig blight (Colletotrichum gloeosporioides)

            Seasonal differences in the time between infection and symptom expression.

            Seasonal patterns for spore production.

Current Projects as a Cooperative Investigator

Holly black root rot (Thielaviopsis basicola)

            Principal investigator: Colleen Warfield (NC State Univ.). Other scientists include Mark

            Windham (Univ. of TN), Bob Trigiano (Univ. of TN), and Cecil Pounders (USDA-ARS).

Daylily rust (Puccinia hemerocallidis)

            Principal investigators: Younghao Li, Mark Windham, and Bob Trigiano (Univ. of TN)

            Principal investigator: Hamidou Sakhanokou

Dogwood powdery mildew (Erysiphe pulchra) -

            Principal investigators: Younghao Li, Mark Windham, and Bob Trigiano (Univ. of TN)

Projects Being Considered in the Future as the Principal Investigator

Rhizoctonia web blight of azalea (Rhizoctonia spp.)

            Improve fungicide usage on outdoor pads.

            Evaluate control practices in propagation houses.

Camellia twig blight (Colletotrichum gloeosporioides)

            Considerations to improve fungicide usage and sanitation (such as pruning efforts).

Integrated disease management of pathogens that cause root disease in mist propagation houses.

Practical rate considerations for treating irrigation water with disinfestants.

INFORMATION GENERATED FROM MY RESEARCH PROGRAM.

• Problem. The main cultural recommendation given for control of Rhizoctonia web blight was to increase spacing between container-grown azaleas in the nursery so evaporation would limit the build-up of moisture which is favorable for the development of web blight. However in the nursery disease appeared to develop in tightly-packed and widely-spaced plants.

  Objective. Verify the effectiveness of the control method.

  Result. With increased spacing from 0 to 10 inches between outer branch tips of containerized plants, evaporation increased and temperature decreased in response to evaporation, but the response was not enough to negatively impact disease development. In the widest spacing, leaves were wet for an average of 7.5 hours per day and a relative humidity of 95% or higher existed in the plant canopy for 93% of the days from July to September. Both moisture conditions are favorable for Rhizoctonia web blight. This information means that more plants can be grown per square foot area without increasing the risk of web blight in the southern U.S. Spacing should still be done to achieve quality plant form. Additionally, the Rhizoctonia fungus is known to spread from plant-to-plant when plants touch, so plants should not be tightly packed together.

  Next Steps. Two approaches: 1. understand weather conditions in the nursery so fungicides can be applied at the most effective time, and 2. obtain information about survival and spread of the fungus to develop a practical sanitation approach.

   

   

  Problem. While disinfestants are relatively safe compounds that are currently used at standardized rates to kill plant pathogen propagules (e.g. spores, mycelia) on production surfaces, such as ground covers, benches, pots, and tools, the scientific literature in other areas of study suggest standard rates may not always be effective.

  Objective. Establish formal dose curve responses of disinfestants applied to different surfaces in a laboratory setting.

  Result. We demonstrated that EPA labeled standard rates of disinfestants are not equally effective on all production surfaces and established rates for six disinfestants to control Botrytis cinerea. The research shows that standard rates are effective in some cases but not in all cases. For example, 10% bleach was effective on galvanized metal, stainless steel, pot plastic, and ground fabric, but 17%  bleach was required on pressure-treated pine and 100% bleach was not effective on raw pine wood. Label rates of a brand of quaternary ammonium was effective on galvanized metal, but 2 to 3 times the rate was required on most surfaces and the highest rate was ineffective on raw pine wood. A brand of hydrogen dioxide was effective at 2 to 3 times the label rate on most surfaces, including raw pine. Note: Botrytis cinerea may be more tolerant than some pathogens to these chemicals, therefore lower rates than established in this research project may be effective against some pathogens. 

  Next Step. To test how much rate response varies against different types of plant pathogens (e.g. other fungi, bacteria, water molds) and under conditions (eg. surfaces soiled with organic media and algae) commonly found in greenhouse and nursery production systems.

   

   

  Problem. Due to increasingly severe water-use and run-off regulations, the need for greenhouse and nursery operations to disinfest irrigation water pumped from catchment ponds that may contain pathogens has increased.

  Objective. Establish formal dose curve responses of chlorine dioxide as affected by different types of water in a laboratory setting.

  Result. We established rates of chlorine dioxide to control several fungal pathogens with adjustments for common variants in water properties (pH, water hardness, nutrient leachates). Three ppm ClO² killed fungal spores (conidia) of Fusarium oxysporum and Thielaviopsis basicola under most water conditions. However, 12 ppm ClO² was needed to kill fungal spores when water at pH 8 contained 5 ppm manganese and iron. The research established that chlorine dioxide is commercially viable for disinfesting irrigation water, although water with a high level of micronutrient leachates could require higher rates.

  Next Step. Determine additional rate issues of several disinfestants when applied through an actual irrigation system.

   

   

  Problem. Disinfestants used to treat irrigation water may damage some of the many ornamental plant genera produced in greenhouse and nursery systems.

  Objective. Test the potential of recommended and excessive rates of chorine and hydrogen dioxides to damage herbaceous and woody plants.

  Result. We determined that most plant species were tolerant to chlorine dioxide and hydrogen dioxide, the only disinfestant presently labeled on ornamental plants. A few plant species were sensitive to each disinfestant. Our research demonstrated that no reductions in plant growth and quality were evident from rates of chlorine dioxide needed to disinfest irrigation water. While most plants tested were not injured by EPA approved rates of hydrogen dioxide, two plant species (Coleus and Rhododendron) sustained injury at label rates.  

  Next Step. None planned.