Plant Diseases

19 July 2006

Virginia Cooperative Extension, Charlottesville/Albemarle County Office
460 Stagecoach Road, Charlottesville, Virginia 22902
phone: 434.872.4580   fax: 434.872.4578

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In the heat of the summer with rain falling sporadically one of the most common questions I get is What is killing my plant(s)? Plant death is common in landscapes, some seem to die suddenly and some take time. Understanding some basics can help diagnose plant problems as well as prevent them in the future.

Analysis of plant disease samples received from the local landscape industry and homeowners reveals that environmental stresses and/or improper cultural practices are the primary causes of plant failure in the landscape. The majority of plant failures occur soon after planting. The transition from container-growth, under controlled conditions in a nursery, to the landscape can be very difficult. If an appropriate planting site and proper cultural conditions are not provided, the plant will not establish adequately, and is likely to die. Frequently, insufficient irrigation during the establishment period is a major cause of plant failure. However, too much water due to severe weather, watering too frequently, or poor soil drainage may also cause plants to die or become diseased. Salinity of the soil and the quality of well water used for irrigation are other common concerns.

Actual plant pathogens are generally a secondary cause of plant failure or decline in the landscape. Plant pathogens may include fungi, viruses, bacteria or nematodes with most problems caused by fungi. Disease development of this type also is subject to weather conditions and cultural practices. Commercial nurseries maintain healthy plants with regular fungicide programs. However, after transplanting into the landscape, unprotected plants may become diseased. Disease can develop soon after planting or develop after a year or more, depending on the plant, the type of pathogen, and environmental stress.

A less common cause of plant failure is insect damage. Sometimes it is difficult to distinguish disease symptoms from insect damage. It is also difficult to make a disease diagnosis on an unknown or new plant species. Landscape plant species are very diverse, and if the normal appearance of a plant is not known, diagnosis may not be possible. Other signs and symptoms that are often confused with diseases include normal leaf variegation, corky ridges on stems, lack of flowers, and normal leaf senescence and drop, particularly associated with the springtime leaf senescence of broadleaf evergreens or fall needle senescence of conifers.

The first steps in attempting to diagnose a plant problem are to determine (1) what the plant is, (2) what it is supposed to look like, and (3) what environmental conditions it requires.

Keeping this information in mind should help you to learn what to look for in diagnosing plant problems. Characteristics of the plant, environmental conditions, and cultural practices utilized should be the first things to determine, followed by observation of signs and symptoms of plant pathogens.

Plant pathologists usually group causes of plant failure into two categories: infectious or biotic (that is caused by a pathogen(s)), and noninfectious or abiotic (caused by environmental stress or improper cultural practices). Identifying whether a problem is infectious or noninfectious is the first thing to determine when we diagnose a plant disease sample.

Environmental stresses and improper cultural practices that can lead to plant failure are numerous. They include too much or too little water, soil or water that is too salty, pH that is too high or low, use of excess fertilizers or nutrient deficiencies, or chemical injuries (from herbicides, pesticides, runoff or other pollutants). As a general rule, noninfectious disease symptoms are distributed evenly over a large area or over several different plant species. They may also be associated with some specific location(s) or cultural practice, such as where herbicide applications were prepared, or with some environmental events, such as frost or a hail storm. Non-random distribution of symptoms is more likely to be caused by a non-living (abiotic) factor.

In contrast, infectious disease symptoms develop sporadically, are distributed unevenly, and usually are restricted to a particular plant species, or even cultivar. Random distribution of symptoms is more likely to be caused by a living (biotic) organism. Therefore, field distribution of diseased plants and symptoms is an important initial observation for disease diagnosis. It is not possible for professional diagnosticians to examine spatial distribution of every disease sample received. Thus, you are often in a better position to make these observations than distant diagnosticians are.

Which of the four photos shows a biotic/infectious disease? All of them show some kind of yellowing. Knowing the plant species might help you but if you were not sure what clue would you use? Send your answer with your snail mail address to peter@vt.edu and a winner will be selected at random from the correct entries for a special prize.

Which of the four photos shows a biotic/infectious disease?

Next week I will reveal the answer and more on symptoms and signs.

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For more information about these and other landscape gardening topics contact your local Virginia Cooperative Extension office. The local Virginia Cooperative Extension office numbers are Albemarle 872-4580, Fluvanna 591-1950, Greene 985-5236, Louisa 540-967-3422, and Nelson 263-4035.