Landscape - Plant Disease Free Material

One of the basic rules of disease control in the landscape is to plant disease free material. The following is an article on the subject.

This is the time of year when Delaware landscapers will be planting trees, shrubs, perennials, and annuals for color. Landscape introductions carry the risk of introducing new disease-causing pathogens. Landscapers, knowing that plant disease will not occur if the plant and the pathogen can be kept apart will want to consider using the principle of exclusion for disease control.

Normally, when exclusion for plant disease control is considered, we think of quarantines and inspections to keep new diseases from entering a state or nation. Currently, Delaware and other states in the eastern U.S. are excluding Phytophthora ramorum, cause of sudden oak death, by inspections of nursery plants moving from the West Coast where the disease occurs, to the East. Similarly, through exclusion, Ralstonia solanacearum race 3 biovar 2, a devastating disease of potato and other crops, is prevented from entering the U.S. on geraniums or other transplants. We can only imagine what our Eastern U.S. forests or urban streets would look like if chestnut blight or Dutch elm disease had been excluded.

On a much smaller scale, although a landscape disease may be widespread in Delaware, it doesn't mean that every site harbors the pathogen. In order to keep the site free of some plant pathogens, it becomes the responsibility of the landscaper to exclude diseases that might be damaging to plants. This means landscapers need to undertake careful inspections of incoming plants and reject those that don't meet good health standards.

Some examples might include:

Black root rot, caused by the fungus Thielaviopsis basicola, may be present on roots of holly, petunia, pansy, impatiens, geranium and other transplants. The fungus is not already present in most soils, but once introduced, will persist in the soil for many years. The tops of the plants may not show symptoms in the nursery or garden center so in order to find the disease, the plants must be knocked out of the pots and the roots examined. Inspect plants carefully and reject those with dark root lesions or blackened root tips. Once planted in the garden, these plants will decline and grow poorly.

Geranium bacterial blight, caused by the bacterium Xanthomonas campestris pv. pelargonii can devaste geraniums grown in outdoor beds. This disease is sometimes introduced into the landscape via infected transplants. On leaves, look for brown spots, often with yellow margins or check for sunken cankers on the stems. If these symptoms are observed, reject the plants because once the pathogen is introduced into the landscape, that bed can become contaminated and provide a source of infection for future geranium plants grown in the same bed. Examine geranium plants carefully and exclude any that have symptoms of leaf spot or stem cankers.

Viruses such as Impatiens Necrotic Spot Virus and many others can be carried on transplants. Most commercial transplants are free of this disease, however, we do see it from time to time on greenhouse and nursery grown annuals and perennials. Virus symptoms on transplants include deformed leaves, spotting, and yellow mosaic patterns on the leaves. Be wary of transplants with virus symptoms; exclude them from the landscape.

For a healthier landscape, remember to use the principle of exclusion to reduce the chances of introducing a pathogen that causes a plant disease that will be regretted later in the season or in the coming years.

Modified from AN EXCLUSIONARY PRINCIPLE FOR THE GARDEN By John Hartman in the March 24, 2008 edition of the Kentucky Pest News from the University of Kentucky, College of Agriculture.