By Paula Flynn
Extension Plant Pathologist
Iowa State University Extension
The easiest way to deal with plant disease problems is to keep them out of your yard and garden in the first place. Prevention is the key. Many plant diseases can be avoided if a few simple preventive practices are followed.
Match the plant species to the site. Matching the plant to the site requires some homework. Consider the characteristics of the planting site. Are the conditions suited for the plant? For example, is there adequate space for the plant to grow over time? Is the lighting sufficient? Are the soil characteristics suitable for the plant? A little investigative work beforehand will allow you to avoid bad decisions that have long-term consequences, like planting a pin oak tree in a high pH soil or planting a shade-loving hemlock in an exposed, sunny location. If a plant is struggling to grow, it won't be able to protect itself from disease agents and environmental stresses.
Start with healthy plants. This is an easy way to start the growing season right. To determine if a plant is healthy, take a close look at the leaves, stems and roots if possible. Be wary if you notice spots on the leaves or stems, wilting, stunting or soft, discolored roots. You do need to know what's normal for a particular plant before you can determine that it's sick.
Another way to make sure your yard and garden will not be overrun with diseases is to use a number of different plant species. Many of the problem-causing pathogens attack groups of related plants. For example, if you plant only spruce trees, a needle disease fungus could trouble your entire planting. However, if you include some pine and fir trees, you will lessen your chance of a widespread disease problem.
Look for disease-resistant cultivars. A disease resistant cultivar has the ability to resist certain diseases. If you have had a problem with a particular disease, investigate whether resistant cultivars are available. It's easier than using a fungicide and more friendly to the environment. For example, you can find roses with resistance to powdery mildew, sweet corn that resists smut and tomato varieties that are resistant to the wilt fungus Fusarium.
Give your plants enough space. Allowing good airflow between plants will help reduce the risk of diseases on leaves. Many of the fungi that cause leaf spot problems on plants look forward to conditions of high humidity and long periods of leaf wetness. So when doing your homework, figure out proper plant spacing. The label or seed packet often gives spacing information. As the season progresses, keep the air moving by controlling weeds.
Watch your watering practices. You can spoil the progress of many troublesome fungi by keeping water off the leaves of plants. To reduce the amount of time water is on the leaves, water in the morning. The water will evaporate off the leaves more quickly than if you water late in the day. If possible, water at ground level. Also consider other aspects of your watering practices. Are you watering frequently and lightly, possible discouraging deep root growth? This is a common mistake made on home lawns. Frequent and light water, especially late in the day, can encourage leaf and root diseases. As a general rule, deep and infrequent watering is best for established plants.
Clean-up is a good way to remove problems. Removing diseased leaves, stems, branches, roots and fruit is an important way to stop problems from spreading to neighboring plants. Wind and rain can help move diseases around, but pathogens also can be spread as you work in the garden. Keep gardening tools clean. It's also a good idea to remove diseased plant tissue in dry weather only. When cutting out a diseased portion of a plant make, cut back into the healthy tissue to make sure you remove the entire problem area.
Following these prevention practices will help reduce the occurrence of troublesome diseases. Combat problems that do sneak in with early detection. Scout frequently. A final homework assignment is to learn about the most common problems of your favorite plants and how to control them. That way you will be prepared for the home garden battles that may lie ahead. If you find yourself stumped, contact your county Iowa State University Extension office or the Iowa State University Plant Disease Clinic (www.extension.iastate.edu/Pages/plantpath/pdcintro.html) for assistance.