Disease Control in the Landscape

Bulletin 614

Chapter 5

Cultural Practices and Considerations that Aid in Disease Management

Awareness of the importance of cultural practices in disease management programs has increased greatly in recent years. Cultural practices serve as important disease control or prevention tactics in several ways. Any practice that helps create an environment within the planting that is less conducive to disease development is beneficial to plant health. Sanitation practices aimed at reducing or eliminating pathogen populations are critical for controlling infectious plant diseases. Reducing plant stress through environmental modification promotes good plant health and aids in reducing damage from some plant diseases.

It is also important to note that the effective use of fungicides within the disease management program is greatly influenced by cultural practices. Fungicides are not an alternative to the use of sound cultural practices. The following practices and considerations should be highly beneficial in preventing plant health problems in "New Landscapes."

1. Choose disease-resistant varieties with good cold hardiness and drought tolerance whenever possible. Many landscape plants have been evaluated for their resistance to specific diseases and pests in addition to their aesthetic qualities. In addition, many cultivars or varieties within a species vary greatly in their cold hardiness.

2. Select healthy nursery stock for planting. Look for any obvious signs of pests and diseases. Avoid plants with galls, torn bark, trunk wounds, and cankers. Look for a slight flare at the base of tree trunks; the trunk flare indicates that the tree was planted and dug at the proper depth in the nursery. Avoid plants with circling roots that may eventually grow into girdling roots.

3. Choose the right plant for the location (or vice-versa!). Among the most important considerations are site exposure, soil moisture, soil drainage capability, pH, and soil texture. Knowledge of individual plant requirements are a must. Some of the very common mistakes seen in Ohio plantings include: understory plants such as the flowering dogwood in hot, dry, and sunny sites; rhododendrons and azaleas in high-pH, poorly drained clay soils; pin oak on a limestone soil; Taxus or rhododendrons on a poorly drained site (Figure 2); or planting anything at all under the eaves of a house where it is always very dry! While it is possible to change soil pH and improve soil texture over time, it requires a commitment to intensive management, and long-term, this will work only for smaller plants in the landscape.

4. Plant properly. Soil that is well-drained and easily worked usually may be used for planting without extensive soil preparation or amendments. However, where the investment is high, a soil test to determine the need for amendments is advised. Compacted, heavy soils will benefit from working organic material into the soil. Good drainage is essential for healthy root system development; plants struggling in poorly drained soils are targets for root-rotting fungi. If water drains from the planting hole in 24 hours, there is fair drainage. Where necessary, install drain tile or regrade the site before planting, as poor drainage is not easily rectified after planting.

   Place plants in the planting hole so the top of the original soil ball before the plant was burlapped is even with the soil grade (Figure 3), or at most 2 to 4 inches above (Figure 4). Enlarging the hole in a horizontal direction outward also will aid in root establishment on compacted sites. A common practice on compacted, poorly drained sites is to place the soil ball with nearly half of the ball above the soil grade; this is not an advisable practice since the portion above the grade often heats up and dries out. A far better practice where drainage is poor is to plant landscape ornamentals in raised beds. Where this is necessary, prepare the soil in the bed, not the hole! When planting balled and burlapped plants, after placing the ball in the hole, pull back the burlap and cut it away as much as possible without disrupting the soil ball.

   Tree trunks are easily damaged at planting, and these wounds can serve as entry points for pathogenic organisms. Remove all trunk wrappiong after planting. Trunk wraps are not recommended after planting. Protect trunks by removing all strings after planting, and where staking is necessary, be certain the stake does not rub against the trunk. Lastly, remember that insufficient irrigation of new ornamental plantings is probably the most common cause of poor establishment. Maintain adequate root-ball moisture after the plants are installed, especially in artificial media. Watering during dry periods is especially important for the first 2 or 3 years after planting.

5. Mulch with caution. Keeping turf away from new plantings and applying a shallow layer (1 to 2 inches) of mulch is helpful in plant establishment. However, too much mulch can keep soils excessively wet and deprive roots of oxygen or prevent water from penetrating the soil resulting in drought stress. These conditions frequently predispose plants to root-rotting diseases. Do not mound mulch around the plant (Figure 5). A 1- to 2-inch, flat layer of mulch around the plant is most desirable.

The following practices and considerations should be highly beneficial in preventing plant health problems in existing landscapes:

1. Practice sanitation in the landscape. Rake and remove fallen leaves from around diseased plants. The fungi that cause scab on crabapple and black spot of rose overwinter in dead leaves on the ground. Removing this source of inoculum helps greatly in disease prevention. Prune out diseased wood containing fungal or bacterial cankers. When pruning diseased material, disinfest pruning tools in 70 percent alcohol between cuts to avoid spreading pathogens. Remove and destroy badly diseased plants. Discard, burn, bury, or compost diseased plant material. Propagate from disease-free plant materials only.

2. Prevent crowding to improve air circulation in the landscape by selective removal or occasional renovation of plantings. Free water on the surface of plant tissues is required by many pathogenic fungi in order to germinate and infect the plant. Any practice that promotes faster drying time of plant tissues greatly reduces the potential for infection.

3. Where foliar diseases are potential problems, do not irrigate late in the day or at night. Water that remains on foliage overnight can promote disease development by allowing plant pathogens to become established.

4. Fertilize to promote plant vigor, but be careful not to over-fertilize. Follow normal recommendations for each plant type; too much fertilizer may be more harmful than too little. Do not fertilize stressed plants simply on the assumption that it might help; fertilize only if it is warranted.

5. Do not make flush pruning cuts or leave large stubs (Figures 6 and 7). Make cuts so that the branch collar is not disturbed (Figure 8 and Figure 9). Do not paint pruning wounds. Whenever possible, remove branches when small (1-inch diameter or less). Do not "top" trees; remove large branches (greater than 4 inches in diameter) only when absolutely necessary. Pruning wounds are common entries for many plant disease-causing organisms.

6. Protect the root environment of woody ornamentals. Construction around trees and shrubs can be very harmful to plant health. Removal or addition of soil (Figure 10) or trenching (Figure 11) close to trees or shrubs cuts major roots (Figure 12) resulting in extreme acute stress to plants. Avoid repeatedly digging and planting of annual flowers under the drip-lines of trees. Perennial ground covers surrounding woody ornamentals are preferred to annual plantings that require digging every season.

7. Protect the trunks of trees and other woody species. "Lawn mower" or "weed eater" blight is a very common cause of plant damage, especially on younger trees and shrubs. Damage from this type of equipment can easily "girdle" a young tree. A weed and grass-free strip around the base of a tree or shrub is very beneficial in this regard. The strip should be wide enough to allow for mowing without touching the trunk. Mulches or other ground cover materials are beneficial in maintaining a weed free strip and can be quite attractive in the landscape. In addition, bark mulches may provide some level of biological control for certain root rotting diseases.

8. Improve the root environment when necessary. Soil compaction frequently becomes a problem in established plantings. Core aeration, sometimes called vertical mulching, is used to help improve drainage and increase the oxygen levels in the soil. The goal is to improve the soil environment for root growth. Core aeration is a labor-intensive practice and is usually done to improve the health of valuable landscape specimens. The size, spacing, and depth of the holes will vary depending on the situation and the condition of the plants. As a general recommendation, auger the holes 1 to 2 inches wide, 12 to 18 inches deep. Place the holes around the drip-line of the tree, usually on 12- to 20-inch centers. Usually the holes are filled with a mixture of peat and a coarse aggregate. If a narrow diameter auger is used, holes may be left empty.

Fertilizers are sometimes placed in the holes, but since core aeration is frequently resorted to when a tree shows symptoms of decline, this should be avoided. One exception would be where iron chlorosis is a problem. Iron sulfate or iron chelate amendments may be beneficial if placed in the holes.