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The Journal Gazette

Saturday, May 26, 2018 1:00 am

Several factors in diseases in plants

Ricky Kemery

Question: With all the unusual weather we have seen this year; do you expect to see more plant disease than normal?

Answer: It is difficult to make predictions about plant diseases that may affect plants this season. There are many factors that can influence whether a plant becomes diseased or not. Many plant pathologists and landscape professions reference the disease triangle to help explain why plants become diseased.

The triangle states that for a plant to develop a disease, it must be susceptible to the disease, the pathogen that may cause the disease has to be present, and the environment must be optimal for the disease to flourish.

When people choose plants for their landscapes and gardens, they often make random choices based on what their neighbors plant, or their own preferences. Try to do some research on plants before they go in the ground.

How and where trees and shrubs are planted makes a huge difference. River birch planted on a dry berm will suffer because the tree prefers moist areas. Planting some oaks in wet areas can also be a recipe for disaster because many oaks prefer drier soils. Michael Dirr's book, “Manual of Woody Landscape Plants,” is a good reference for what sites trees and shrubs prefer.

How a tree or shrub is planted also makes a huge difference. Cornell University has a wonderful guide:

The overall cold hardiness of a plant is important. Many folks purchase plants that are not hardy to zone 5. Planting trees and shrubs hardy only to zone 6 and beyond can lead to death in really cold years.

Some plants such as Japanese maple and boxwood are very susceptible to damage from winter winds. Other plants such as big leaf hydrangea and peaches are very susceptible to damage from late frosts and freezes.

Even native plants in a forest can develop disease and have some insect damage.

The issues occur in urban landscapes where plant stress plays a major role. The urban environment, because of its heat and pollution, can cause stress in plants.

Just like with people, stress can play a major role how susceptible a plant can become to disease.

People can also cause lots of stress for plants. We might over-water, over-fertilize, or prune improperly, leading to more problems. Poor sanitation (leaving diseased debris or fruit on the ground) can also help promote disease.

Many diseases do no real harm, while others are real problems. The University of Minnesota has a guide to help identify and control plant disease:

Certified arborists and other landscape professionals can also help identify what diseases are present in your home landscape.

The Plant Medic, written by Ricky Kemery, appears every other Saturday. Kemery retired as the extension educator for horticulture at the Allen County branch of the Purdue Extension Service.