Extension Communications
Extension 4-H Youth Building
Ames, Iowa 50011-3630
(515) 294-9915
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Extension Communications |
6/14/04
Contacts:
Paula Flynn, Plant Pathology, (515) 294-3494, pflynn@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033,
jmcguire@iastate.edu
Garden Column for the week of June 18, 2004
The Shape of Plant Pathology
Paula Flynn
Extension Plant Pathologist
Iowa State University
The weather this spring has been a nuisance to the health of some of the plants in our yards and gardens. The many days of more-than-plentiful rains and the rapid fluctuations in temperature not only caused direct injury to plants, but also influenced the development of troublesome plant diseases.
A beginning lesson in the field of plant pathology will highlight the fact that weather plays an important role in the occurrence of plant diseases. This concept is called the plant disease triangle. The three sides of the triangle have to come together in order for a disease to develop. One side of the triangle is composed of a susceptible host plant, such as a crabapple or tomato. The second side represents the pathogen, such as a fungus or bacterium. Finally, the third side of the triangle represents weather conditions that allow the pathogen to enter the plant. To prevent problems, we have to keep this triangle from forming.
Many problem-causing organisms, especially fungi, thrive when we have extended days of rainy weather. With saturated soils, the roots of vegetables, garden plants, flowers, trees and shrubs may suffer. Roots need oxygen to grow and thrive. Roots that are subjected to wet soils for many days may turn brown and mushy, and then rot. When this happens, the aboveground part of the plant often wilts, appearing as if it's short of moisture. An investigative look at the roots will show they are in poor health and unable to sustain the plant.
Too much rain can also be a problem for the leaves of plants. Leaf disease pathogens often need leaves to be wet for a certain length of time before they can sneak their way into the plant. This spring, brown or black spots on leaves have been observed on shade trees such as ash, maple, oak and sycamore. This disease is referred to as anthracnose. Sick leaves often fall to the ground, causing owners to worry. Fortunately, leaf diseases are usually a minor annoyance on established trees. Warmer and drier conditions of summer limit the advance of these fungi.
By removing one side of the disease triangle, we can work to avoid plant disease problems. We may not be able to change the weather, but we can help leaves dry more quickly by properly spacing the plants to allow good air circulation. Proper pruning can also help to improve the airflow and sunlight penetration to leaves. These practices help to reduce environmental conditions that favor infection by the pathogen.
We can deter a pathogen from getting into a plant by buying a disease-resistant variety. Do a little research before selecting a plant. It's much simpler, for example, to buy a crabapple that is resistant to the leaf disease scab, than to apply a fungicide to protect the leaves of a susceptible crabapple. This practice eliminates the susceptible host plant side of the triangle.
Finally, sometimes we can get rid of the pathogen side of the triangle. Good sanitation practices such as removing diseased leaves, branches or even plants will help prevent the spread of problems in your yard and garden. Since many plant disease-causing organisms are fairly specific to a plant group, it's helpful to grow a wide assortment of plants. That way, when a plant disease problem occurs, you won't be at risk to lose a great number of your plants. (Remember the devastating effect of Dutch elm disease? Some cities lost a majority of their street trees.)
In addition to helping certain plant pathogens gain access to our plants, the weather this spring directly caused damage. The cold temperatures that occurred after tender new growth emerged caused some leaves to turn black and shrivel. Other leaves showed a ragged, tattered appearance. Some trees, such as hackberry and oak, dropped green healthy leaves prematurely. Although this was an alarming sight, most plants put out a flush of healthy leaves once temperatures returned to normal.
Unfortunately, plant enthusiasts are challenged by the unpredictable Iowa weather. But with foresight when purchasing and installing plants, proper care and flexible expectations, gardeners can discourage the three sides of the disease triangle from coming together to create a nuisance.
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Editors: No photos are available for this site.
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