Stop Ornamental Diseases before they Start

Ornamental plant diseases can significantly affect the aesthetic quality of many plants in the landscape. Not only do some of these diseases impact appearance in this season, but they also impact overall plant health and survival during seasonal weather changes. Managing ornamental diseases begins with the selection of resistant ornamental plants, maintaining adequate nutrition and irrigation, adopting appropriate cultural practices and providing the right environment for plants. To avoid a never-ending disease prone condition, it is critical to adopt a disease management program before getting to the point where fungicide applications are necessary. For some budget-stressed grounds managers, fungicides are not an available option. Consequently, this article focuses on non-fungicidal disease-management programs.

When we consider options for plant disease management it is important to consider how our management plans augment disease development (See Figure 1, page 25).

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  • Pathogen

    The pathogen is the causal agent of plant disease. In many instances, these pathogens are not widely distributed. For example, Sclerotium rolfsii is the causal agent of crown rot of hosta and is widely distributed in the United States but not in all landscapes in the United States. Once you know you have this problem, you should document it and initiate a management program. Management is not necessary if the disease is not present. This pathogen moves with soil and infected plant material and is not air borne. In other instances, the pathogens have wide distributions, such as Venturia Inaequalis, which causes apple scab. This pathogen is widely distributed and is airborne. As such, apple scab is an annual problem in zones where this disease occurs. For a disease such as this, cultivar selection is very important without fungicides.

  • Host

    Cultivar or species selection is the main component to factors in the susceptible host part of the diagram. As you can see in Figure 1 (page 25), the more susceptible the plant material, the more overlap there will be with the pathogen if it is present. Therefore, when planning the landscape, you should consider the prominent diseases in the area. Selecting plant species that are not susceptible or cultivars that are resistant or tolerant to common diseases, will lower their disease potential. Plant development also can affect host susceptibility. For example, many plant species are more susceptible at the seedling stage, resulting in seedling damping off.

  • Environment

    You might expect to have only limited control of the environmental component of the diagram. Assuming we have an active pathogen and a susceptible host, the next thing we need for disease problems to occur is a favorable environment, but at times our cultural practices will impact this component needed for disease development. For example, irrigation timing contributes to disease development. By avoiding irrigating at times, such as evening, where leaves will remain wet for long periods, you can limit the development of many foliar diseases.

Another factor that affects the plant environment is plant spacing. When you space plants further apart, more air movement occurs in the planting, and foliage dries more rapidly. This is particularly important when planting cultivars susceptible to common foliar diseases.

FOLIAR DISEASES

The most common diseases encountered in the landscape are foliar leaf spots and blights. The key to managing these diseases lies in the environmental conditions that favor disease development. Foliar pathogens are typically active with high moisture availability and lengthened periods of leaf wetness. Factors that extend leaf wetness periods are the driving forces of foliar disease.

By irrigating early in the morning, the water will wash off leaf-bound dew that typically is laden with nutrient-rich leaf exudates (a food source for foliar fungal pathogens). You also benefit from the low wind speeds at that time and lack of customers or patrons on the site. On large sites where you cannot irrigate the entire landscape in the early morning hours, consider locating plants that are susceptible to disease in locations that favor rapid drying of the foliage (e.g. eastern side facing sun).

An example of a foliar disease that occurs worldwide in landscapes where roses are cultivated, is black spot. Black spot is a foliar fungal disease, which is favored by lengthened periods of leaf wetness. There is a range of susceptibility among rose cultivars and species. Therefore, the recommendation for managing black spot focuses on cultivar and species selection and positioning susceptible roses in areas with good air circulation, pruning canopies and morning irrigation or use of a drip irrigation system. Most hybrid tea roses are very susceptible to this disease and will require fungicide applications, even when all other recommendations are followed. There are many products registered for managing this disease.

Management of Foliar Diseases:

  • Select resistant cultivars and species not affected by common diseases.

  • Avoid overhead irrigation to susceptible plants or water in the early morning hours.

  • Maintain proper plant spacing, prune canopy and position plants in landscape sites that promote drying of the foliage.

  • Fungicide applications.

POWDERY MILDEWS

Powdery mildew has similar symptoms on all plant species affected and result in a grayish-white powdery coating. This coating is the fungus growing on the leaf surface. While powdery mildew occurs on many plant species in the landscape, most of the powdery mildew fungi have a very narrow host range and attack only closely related plant. Unlike other foliar pathogens, powdery mildew fungi are not favored by long periods of leaf wetness. This pathogen does need high humidity, however, which is one reason it is observed more in shaded areas of the landscape.

Management of Powdery Mildew Fungi:

  • Select resistant cultivars and species.

  • Position susceptible plants in sunny locations in the landscape.

  • Avoid excessive nitrogen fertilization.

  • Fungicide applications (many labeled for this disease).

CROWN AND ROOT ROTS

Virtually all landscape plants have problems with crown and root rot. Crown and root rots are favored by conditions that maintain high soil moisture. This most often is due to poor soil condition (e.g. high clay content when sites are excavated and prepared for the construction of new developments and facilities). Another factor which leads to crown and root rot is over watering or positioning plants in wet zones (e.g. drainage areas or water-pooling locations). Mechanical damage to the crown also predisposes plants to crown and root rot. Crown damage can occur as a result of freezing and thawing and insect injury. Crown and root rot often are caused by fungi that are in the soil and exist in most landscapes, waiting opportunistically for a stressed or damaged plant. In addition, crown and root rot can be introduced with some plant materials. Examples of introduced crown and root rots are hosta crown rot (S. rolfsii) and Phytophthora root rots of many ornamentals.

Management of Crown and Root Rots:

  • Avoid introduced disease problems by inspecting purchased plants.

  • Avoid over-watering plants.

  • Avoid locating plants near drainage areas.

  • Modify soil to achieve adequate drainage.

  • Mulch susceptible plants in cooler climates to avoid crown injury from freezing and thawing.

  • Maintain good control of root and crown feeding insects.

  • Fungicide drenches may be needed (most labeled for Phytophthora and Pythium diseases).

VIRAL AND PHYTOPLASMA DISEASES

Many common landscape ornamentals are vegetatively propagated. Vegetative propagation allows for the maintenance of many of the common systemic infections by viruses and phytoplasmas. Viruses and phytoplasmas are very small and are not visible with the unaided eye. These pathogens move with plant sap and cannot be eliminated from plant materials once they are infected. Even the seeds from infected plants should be avoided because some viruses are highly seed-transmitted. Therefore, viral diseases should be aggressively managed by eliminating the source and spread of the virus when possible. Elimination of the source is achieved by planting virus-free plant material. Insect vectors spread many plant viruses. Therefore, controlling insect vector populations and rouging out infected plants reduces the spread of plant virues.

Aster yellows in coneflower is an example of a common phytoplasma disease in the landscape. As with most of these systemic infections, the terminal growth is altered in some way. For coneflowers infected with a phytoplasma, the flower becomes distorted and develops clusters of small leaves.

Management of Viral and Phytoplasma Diseases:

  • Select resistant cultivars and species not affected by common viral and phytoplasma problems.

  • Utilize plant material from reputable sources that are virus free.

  • Control insect vectors for diseases indicated to have significant insect vector populations present.

  • Rogue out infected plants to avoid spread in landscape.

  • Disinfect pruning equipment between high value plants in the landscape.

Loren J. Giesler is an extension plant pathologist for the University of Nebraska (Lincoln, Neb.).

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