Fungicide sprays usually work in controlling infectious diseases because they act as a chemical barrier on leaf, stem, or flower surfaces. When the pathogen arrives on the plant surface, it encounters this barrier and is unable to infect the plant. Effective fungicide use requires that this barrier be as complete as possible. A spray method must provide the best combination of practical usefulness and good coverage. For many diseases, special attention must be given to undersurfaces of leaves, especially on the lower leaves of the plant.
The completeness of the barrier depends on how well the spray spreads and sticks to the plant surfaces. For this reason, spreader-stickers or spray adjuvants can be added to many sprays. Sometimes the product label alerts the user to these problems. However, observing the spray deposit after you've finished some of the job may be the best way to decide if an adjuvant should be used. Hairy or waxy foliage is especially difficult to cover properly without a spreader-sticker.
In cases of root and crown diseases, a fungicide drench may be more effective than a foliar spray. This method of application should only be used when the plants are small enough to allow complete saturation of the root area. Using a fungicide drench on large shrubs and trees will probably be ineffective. Close attention should be paid to soil type, texture, and pH, as they may influence the efficacy of the fungicide. Label recommendations differ for drench applications as compared to foliar applications, so always read and follow label directions.
Timing refers to when and how often the spray must be applied to effectively control a disease. The first application usually is made at a time close to but before the pathogen arrives on the plant surface. This information is often provided on the pesticide label or is available in extension literature from your local county extension office. In most situations, fungicides are not effective in controlling the disease if the pathogen has already entered (infected) the plant tissues. In many cases, specific information about the disease cycle may be needed to time the first application correctly.
After the first application is made, the pesticide barrier is established on the plant surfaces. Effective use involves keeping this barrier active and complete throughout the time that the pathogen can arrive on and infect the plant. Modern fungicides are developed so that they do not persist in the environment for long periods of time. Rainwater, sunlight, microbial action, and oxidation decrease effectiveness of the fungicide. Reapplication of the spray is needed in many cases to keep the fungicide barrier active.
Plant growth also affects the completeness of the barrier. As new leaves and shoots appear, they are unprotected and may be subject to infection. If so, additional applications will be needed.
The fungicide label gives reapplication guidelines, usually in ranges of 7 to 14 day intervals. If excessive rainfall or rapid growth of the plant occurs, the shorter interval between sprays should be used. If not, use the longer interval.
Due to rapidly changing regulations and fungicide registrations, specific recommendations for fungicides are not provided in this bulletin. General information on materials effective for controlling common diseases of ornamental plants is provided in Table 8, as well as in the sections on common and specific diseases of landscape plants in Ohio. Fungicide recommendations need to be carefully examined and often changed on an annual basis. The following publications are updated annually and should serve as a reliable resource for growers that require specific information and recommendations for fungicide use in the landscape.
New England Management Recommendations for Insects, Diseases and Weeds of Shade Trees and Woody Ornamentals. (Revised annually.) Order from:
University of Massachusetts
Bulletin Center
Draper Hall
Amherst, MA 01003
Phone: (413) 545-2717
Cost: $15.00, includes postage and handling.
Disease Control of Ornamental Trees, Shrubs and Ground Covers, Bulletin BP-33. (Revised annually.) Order from:
Purdue University
Media Distribution Center
301 South Second Street
Lafayette, IN 47905
Phone: (317) 494-6795
Cost: $2.00, includes postage and handling
Pest Management Recommendations for Commercial Production and Maintenance of Trees and Shrubs. (Revised annually.) Order from:
Cornell University
Resource Center
7-BTP
Ithaca, NY 14850
Phone: (607) 255-2080
Cost: $12.75, includes postage and handling.
Control of Diseases on Commercial Outdoor Flowers, North Central Regional Publication 491. Order from:
Michigan State University
Room 10-B Agriculture Hall
East Lansing, MI 48824-1039
Phone: (517) 355-0240
Cost: $10.00
Illinois Urban Pest Management Handbook. Order from:
University of Illinois
Office of Agricultural Communications and Education
69-BK Mumford Hall
1301 West Gregory Drive
Urbana, Illinois 61801
Phone: (217) 333-2007
Cost: $12.00 plus $2.00 postage and handling = $14.00
Pacific Northwest Plant Disease Control Handbook. (Revised annually.) Order from:
Oregon State University
Publications Orders
Agricultural Communications
Administrative Services A422
Corvalis, OR 97331-2119
Phone: (503) 737-2513
Cost: $19.50, includes postage and handling