Scheduling Disease Control In Woody Ornamentals

The activities of the organisms that cause plant diseases are not governed by the clock or the calendar. Their activity is influenced by temperature, relative humidity, soil moisture, and other highly changeable environmental factors.

Strict scheduling is made even more difficult in the landscape because many different species of plants are being maintained, each with its own, often unique, diseases. As a result, the timing of pesticide application as well as other disease management practices must be tailored to the plant species being grown, the diseases that are present or could pose a serious threat, and the environmental conditions at the site.

Tailoring a very effective disease management plan for a client can be done if notes are taken and records are kept on the plants, planting site, weather conditions, and the diseases that occur. With such records, the plan can be adjusted and improved over the years, increasing its effectiveness and, in some cases, reducing the use of pesticides. Use the following information as an outline for the kinds of useful data to keep.

• Make an inventory of the plants at the site, noting the identity and location of the plants. Mapping and numbering their location on the map will help.
• Note the important characteristics of the site...exposure to wind, proximity to roads, walks, and drainage pattern. Record the date of any site changes such as excavation, paving, and removal of over-story trees.
• Record the general weather conditions at the site during the year including drought, flooding, and wind.
• Record the general appearance and health of each plant being maintained. Especially note unusual characteristics such as smaller than average leaves, unusually light green leaves, smaller than average internode length, and mechanical injuries.
• Each year record for each species the approximate date of leaf bud-break, first flowering, and full leaf and twig expansion. Note the date of the first occurrence of the diseases you most want to control.
• Record the use of insecticides, fungicides, herbicides, fertilizers, or any other chemicals on or near the individual plants or near the general site. Note the chemical, formulation, rate and method of application, and weather conditions at the time of application as well as the time of day the material was applied.

This history of the site and of individual plants will later allow you to accurately identify those plants already diseased, new diseases, general declines in growth, or chemical damage. Knowing what diseases are present and when they occur will greatly improve disease management effectiveness.

The occurrence of key diseases and the timing of important disease management activities can be roughly grouped into four different categories according to the activity of the plant or general weather conditions.

Dormant season - late autumn-winter

• Conduct inventories.
• Run soil tests to check pH and fertilizer status.
• Mulch to protect roots.
• Protect evergreens from drying winds, salt sprays, and ice damage.
• Prune dead, cankered twigs and branches.
• Rake and destroy fallen leaves around trees and shrubs that had leaf spotting diseases, especially rose black spot, apple scab, and anthracnose.
• Examine the plants for galls such as those caused by cedar-apple rust (on juniper), white pine blister rust, pine-pine gall rust, black knot on plum and cherry, and crown gall. Remove infected branches or remove severely affected plants entirely.
• Late in the dormant season at or near the time of bud swell, spray for black knot of plum and cherry, oak leaf blister, peach leaf curl, and fire blight as just one phase of controlling these diseases.

Bud break - spring-early summer

Spray to protect emerging leaves of plants that have a history of severe anthracnose, leaf spots, or twig blights, or are at high risk of these diseases.

Diseases of primary concern include...

• Sphaeropsis tip blight
• Phytophthora dieback
• Juniper twig blight
• Volutella on pachysandra
• Apple scab
• Douglas-fir needlecasts
• Rose black spot
• Cedar-apple rust
• Dogwood anthracnose
• Pine-pine gall rust
• Ovulinia petal blight
• Fire blight

Pick off and destroy any gall or gall-like tissue such as cedar-apple rust galls from junipers and leaf and flower galls from azaleas.

Apply soil drench fungicides to azaleas and rhododendrons which are at risk of Phytophthora root rot.

Summer

Apply fungicides, during wet weather periods, to prevent the further spread of diseases including...

• Apple scab
• Rose black spot
• Volutella on pachysandra

Apply soil drench fungicides to continue the Phytophthora protection. Do this at the recommended interval noted on the product label.

Apply fungicides to control pine and spruce needlecasts.

Cool-weather - late summer-autumn

• Spray to control powdery mildew on highly susceptible plants such as roses, particularly during cool night-warm day periods.
• Spray to protect the new autumn growth on plants such as junipers from twig blight infection.
• Prepare new planting sites that should be fumigated and treat them while soil temperatures remain above 55°F and soil moisture is 50-85% of field capacity. Allow the site to aerate several weeks before planting or cover the treated area with a plastic tarp and plant in the spring.
• Update the site inventory and be certain you have recorded all the diseases that developed on each plant.

In addition to these four key periods of disease control, times during which plants are under stress (such as drought or defoliation) should be shortened as much as possible, through watering, insect control, etc. Stresses weaken plant vigor rendering them more susceptible to weak plant parasites that can cause cankers and root rots.

Abbreviations Of Suggested Control Techniques

* Only if the disease had been severe

• BSp Begin spray schedule-discontinue when the weather dries
• CSp Continue spraying if wet-discontinue when the weather dries
• D Apply soil drench fungicides
• F Fumigate before planting
• I Irrigate to prevent drought stress
• NT No treatment is required
• P Prune
• R Rake and destroy fallen leaves
• X Remove infected plant

Prepared by Gary W. Moorman, Professor of Plant Pathology