Variable weather can lead to plant disease

By Adam Leonberger Published:

The weather this summer has been quite variable. At times, it has been scorching hot and weeks without rain only to switch over to cool and moist conditions.

This creates an odd opportunity where plants may experience diseases associated with both hot, humid, and dry weather as well as those that come with cool, rainy temperatures.

A plant disease is any organism that damages a plant and reduces its productivity or usefulness to man.

Disease Triangle
A convenient way of thinking about plant diseases is by using a concept called the “Disease Triangle.” Diseases represent the interaction between three factors — or three corners of the triangle.

For a plant to get a disease, there must be a susceptible host, a pathogen (disease causing organism), and a favorable environment. If all of these factors are present, disease results. It is especially important to note that all three factors must be present at the same time for infection to take place.

Prudent plant health care would suggest that anything that disrupts this three-sided relationship would prevent a fungal disease from becoming a significant problem.

Homeowners should consider both living and non-living factors in dealing with plant disease.

The living factors include pathogens and pests. Also, consider non-living factors that include mechanical, physical, environmental or chemical actions.

Eliminate a corner
To control diseases in the landscape, you must eliminate one or more of the three factors that produce disease: host, pathogen and environment.

Methods of disease control can be thought of as modifying the disease triangle by reducing or eliminating one of the corners of the triangle. For example, if you use resistant ornamental varieties in your garden (for example a powdery mildew-resistant phlox variety), you eliminate the “susceptible host” and can thus reduce or prevent disease.

Similarly, for some diseases, by removing diseased plant material, you can reduce or eliminate disease because you are eliminating the pathogen.

Finally, you can reduce or eliminate a “favorable environment” for disease by doing something as simple as not over-watering in your garden or increasing air movement within a shrub.

7 steps to fight disease
There are seven steps that can help reduce plant disease in the landscape.

Use host resistance. Choose plants with natural disease resistance. Replace plants that are susceptible. Sometimes, newer cultivars or varieties may have more resistance than the straight species or older varieties.

Choose the right plant for the right place. Don’t make the plant fit the place. Choose plants appropriate for our Kentucky Zone 6 climates. Remember that the zone and climate varies dramatically from eastern to western Kentucky.

Check the soil drainage. Is your soil too wet or too dry? Does water stand in the planting site? Do you have wet weather ponding from downspouts and other sources? Is rain blocked by eaves or overhanging trees?    

Use proper planting techniques. Use correct planting techniques to get the plant off to a healthy start and so there are no problems as the plant grows into the space.  Secondary disease issues can arise from crowded or damaged roots that result from planting improperly (such as girdling roots; strangulation from not removing non-degrading ball wrap).

Water wisely. Don’t overwater, but also don’t underwater. Consider your water source. Is it clean and free from pollutants? You should strive to water more, less often. Instead of watering a new plant a few minutes every day, water for an hour or two every week or every other week.

Keep plants clean and healthy. Pruning of diseased material can help reduce spread of disease. Control insects that may spread disease. Dead plant material can be a hiding place of pathogens and pests so, make sure to dispose of diseased material. In this case, composting is not always an option.

Mulching can be beneficial in preventing splashing of soil that may have pathogens present.

Many problems arise from too much or too little nutrients — over fertilization, or incorrect amounts of certain nutrients.  Mechanical injury, such as lawn mower damage, causes breaks in the tissue where insects and disease can enter and cause secondary infection.

Herbicide damage reduces the health and vigor of the plant, which can cause secondary disease. Chemical injury often mimics disease symptoms, and causes the landscaper to treat inappropriately.

Construction and other injury may occur years before the landscaper or homeowner notices actual problems. Death of plant material can occur 2, 5, or 10 years out from actual damage. Many of these initial mechanical problems are often confused for death by disease organisms.

Use a diversity of plant material and plant landscapes that last. Don’t plant all one species (mono-culture). Use plants from all plant types (evergreen, deciduous trees, shrubs, perennials, annuals, etc.). Plant landscapes that are enduring. If a pest or disease hits one plant species, then you have a good chance of having other plants that are not susceptible. 

By considering the three-part disease triangle and thinking about the seven steps in reducing disease as you plan your home landscape and garden, you can greatly reduce your chance of plant loss by disease.

For more information on plant diseases, visit  Contact the Franklin County Extension Office at 502-695-9035 or email

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