The ornamental plant industry is the most complex production system of any commercial agricultural plant commodity, yet disease control is supported by the least extensive research data base. Research is needed because several recommended controls have been shown to be inadequate when critically tested. My research focus is to develop integrated disease management (IDM) for regionally important diseases that are difficult to control. My approach is to define biological information that substantiates selection of potential control options, critically evaluate efficacy of individual controls and application methods, then determine what combination of controls provides the best balance of control and cost.
An IDM approach is justified when no single control works well, particularly in a complex cropping system. The industry markets an amazing diversity of horticultural plant traits (environmental tolerance, flower color, plant form, etc.), however cultivars of a single plant species can exhibit from high to moderately low tolerance to specific pathogens. Disease tolerance cannot be altered once disease develops. Fungicides are a valuable control tool, but should not be the only line of defense. In an IDM approach, management and sanitation practices can be key in achieving economical and sustainable disease control. However, maximizing efficiency requires dealing with many interactive influences created from different management styles.
RHIZOCTONIA WEB BLIGHT ON AZALEA
Currently, fungicides are the only useful control for web blight. While fungicides successful halt disease progression, sound predictions do not exist for timing the initial application to consistently prevent a rapid appearance of leaf blight. Precise predictions have proven to be difficult, because daily irrigation provides routine high moisture conditions favoring pathogen persistence. We have classified disease progress periods as ‘not rapid’ or ‘not slow’ based on temperature (<68°F and >95°F) and moisture levels being less favorable for pathogen growth. Trials are in progress to combine simple scouting protocols with weather patterns.
We have discovered that Rhizoctonia colonizes the entire azalea ‘Gumpo’ plant, including the living stems as well as fallen dead leaves and the bark media, for 12 months of the year in the Gulf Coast climate. So while symptoms mainly develop in July and August, the pathogen coexists with the plant without causing plant damage for the other 10 months of the year. This in combination with daily irrigation explains why plant spacing will only at best delay but not stop web blight development. Unfortunately, a small percent of the new, healthy appearing stem growth can be infested with Rhizoctonia when cuttings are collected in May and June. The moist, warm conditions used to promote root production from stem cuttings also favor pathogen growth across cell trays. Often cuttings root well and the pathogen kills only some of the lower leaves. The following spring infested liners become infested plants on the nursery.
Based on this information, the only control approach I see as a possibility, other than fungicides, is production of azaleas free of the Rhizoctonia fungi that cause web blight. Controls used in field crops (e.g. crop rotation, deep-plowing, etc.) are impractical in azalea production. The first step of the process to eliminate Rhizoctonia has been completed. Submerging stem cuttings in 122°F (=50°C) water for 21 minutes eliminates Rhizoctonia from stem cuttings while disinfestant chemicals and fungicides did not. Twelve cultivars ['Conleb' (Autumn Embers™), ‘Fashion’, ‘Formosa’, ‘Gumpo White’, ‘Hardy Gardenia’, ‘Hershey Red’, ‘Macrantha Pink’, ‘Midnight Flare’, ‘Red Ruffles’, ‘Renee Michelle’, 'Roblel' (Autumn Debutante™) and ‘Watchet’] rooted well after receiving a 20 minute hot water treatment, but did vary in sensitivity to damage when submerged for extended times from 40 to 80 minutes. So submersion for 21 minutes is safe. The next steps in the elimination concept is to verify that the vast majority of plants can be kept free of the pathogen throughout the entire production cycle; in other words, clean cuttings do not become infested in the propagation house and clean plants do not become infested on the nursery. Risks of pathogen spread will be calculated, to determine which worker activities actually result in significant spread and which ones have little effect. The final step will be to strategically evaluate the minimum combination of sanitation, cultural, and chemical practices that economically and predictably achieve high control impact. These controls are basic management practices, therefore could benefit recommendations of many disease problems.
Disinfestants are commonly recommended and used in a general fashion. My research with disinfestants is designed to improve precision and efficacy when eliminating pathogen propagules from production and plant surfaces and from dispersal in irrigation water. This information also will be critiqued in IDM studies targeting specific pathogens.
Information Developed from My Research Program.
1. Disinfestant rates depend on the type of surface being treated. Rates were calculated based on 100% mortality of Botrytis cinerea conidia, future research will evaluate response of other pathogens. Products must be used according to label restrictions.
a. Bleach (% product in water, 10% is 1 gal bleach per 9 gals water): 7% on metals, plastic and painted wood; 18% on pressure-treated wood; not effective on bare wood.
b. Green Shield (label rate: 1 Tb product per 1 gal water): 5 Tb /gal on galvanized metal, 15 Tb/gal on stainless steel and pressure-treated wood, plastics, and painted wood, not effective on bare wood.
c. ZeroTol (label rate: 2.5 fl. oz. product per 1 gal water): 9 fl. oz./ gall on metals and bare wood, 12 fl oz. on plastics and painted wood, and 14 fl oz. on pressure-treated wood.
2. Foliar disease control was achieved by applying ZeroTol (curative label rate: 1.25 fl. oz. product per 1 gal water) to plants multiple times per week (3–5 times), but not from a weekly application as specified on the label.
3. Chlorine dioxide rates from 2 to 12 ppm were required to eliminate pathogens (Fusarium, Botrytis, Thielaviopsis) in irrigation water depending on the presence of iron or manganese content in the ponds.
DISCLAIMER: Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture.
1. Evaluate whether a single rate of a disinfestant equally eliminates diverse genera of fungal and bacterial pathogens from production surfaces and evaluate application methods.
2. Evaluate use of disinfestants during propagation, e.g. with Rhizoctonia web blight.
3. Determine minimum rates of common disinfestants needed to eliminate Phytophthora in irrigation water based on water flow rates.
4. Research is planned as a collaborator with scientists whose major research efforts are focused on controlling pathogens in irrigation water. (Dependent on successful grant funding.)
CAMELLIA TWIG BLIGHT
Colletotrichum gloeosporioides causes blight and cankers on camellia and many hosts. This project targets timing of diseased plant tissue removal and fungicide applications.
Information Developed from My Research Program.
When camellia twig blight symptoms appear, infection occurred earlier, 2 ½ to 4 ½ weeks earlier in spring, 3 to 4 ½ weeks in summer, 4 to 6 weeks in fall, and 6 to 11 weeks in winter.
The next step is to monitor seasonal spore dispersal, plant susceptibility patterns, and evaluate timing of cultural, sanitation, and fungicide practices.
FUSARIUM WILT OF HOLLY
Fusarium wilt is a systemic disease problem on several woody ornamentals and a very difficult disease to control on susceptible cultivars. A holistic IDM approach is planned to evaluate the role of air- and water-borne spores and use innovative controls by modulating eradication style strategies. This disease will require a very different systems strategy than used for Rhizoctonia web blight.
Research Being Planned.
1. Trap and monitor air and water dispersed spores for 2 years, to determine if ascospores develop, to establish basic environmental factors associated with dispersal of each spore type, and to define a seasonality of when controls may need to be implemented.
2. Other projects will be designed to develop a system whereby pathogen dispersal and infection is severely restricted at multiple stages of crop production.