Tips on How to Avoid Plant Diseases

Of the hundreds of tree, shrub and herbaceous plant samples submitted each year to the Plant Clinic, only about 15 percent are actually disease problems. A great percentage are problems that can be traced to poor horticultural choices, such as the wrong plant for the location (pin oak in a high-pH soil, leading to iron chlorosis), lack of soil improvement before planting (rhododendron in a tight soil, predisposing it to Phytophthora root rot), poor planning (five trees in the space needed for one mature tree, setting up a decline syndrome), or similar situations. We tend to go with our hearts in plant selection rather than our brains and don't always follow much-touted advice about plants.

Here is some advice that you should follow now to avoid problems - - and save time and money in the spring. Too busy to bother? Weather too wet to work in the yard? If you choose to ignore this advice, then next spring you can expect to perform extensive pruning of dead canes on your roses (from canker disease), to replace plants that have been frost-heaved out of the soil, to replace turf that has been molding under the snow (Fusarium snow mold) and to deal with plant diseases of stressed plants (including cankers, wood rots, root rots and decline).

Once a pathogen infects a plant, the plant and the homeowner usually have to deal with it for the life of the plant. A good example is Cytospora canker of spruce. The fungus invades trees under stress. Management practices include pruning, watering and fertilizing to establish tree vitality, but the fungus will remain in the tree until its death.

Many disease problems are best controlled with preventive measures. Chemical rescue treatments may act as temporary solutions but are usually not the answer for long-term disease control. These fall lawn and garden cleanup procedures will help prepare plants for winter and discourage development of disease problems.

1. Keep grass mowed until it stops growing. This helps prevent winter injury and damage from fungal snow molds.

2. Prune oak trees in the dormant season so as not to increase the risk of oak wilt. Pruning from September to early March is recommended because pruning during the growing season attracts bark beetles, which transmit the oak wilt fungus. Oak wilt is a potential threat in all of Illinois and can kill mature oaks in one season.

3. Prune trees and shrubs to remove all dead and seriously cankered wood, as well as any crossing and interfering branches. Opening up the center of woody plants helps promote faster drying, lets in more light and reduces foliar and stem diseases. This is a common practice to help prevent fire blight on rosaceous hosts, as well as to prevent bacterial leaf spots of Prunus species.

4. Provide suggested winter protection for roses, evergreens, thin-barked young trees and other sensitive plants. Winter injury causes wounds that become infected with secondary canker fungi. Many of the rose cankers infect such injuries. Plants that have been located out of their natural range are often weakened in this way and predisposed to cankers and insect feeding.

5. Prune tree and bush fruits according to recommendations by Extension horticulturists.

6. Removal and burning (where possible), composting or burying plant debris will help reduce foliar and stem disease next year. It is usually safe to compost any leaf material, but diseased stem and root tissues should be burned or buried, not included in a compost pile.

7. Look over a variety of seed and nursery catalogs. Select resistant varieties (if they are otherwise horticulturally acceptable) and plant them where you've had problems in the past but have no rotation options. Choosing disease-resistant hybrids, varieties and species is usually the least expensive and best long-term method of disease control. If you have had problems with scab on crabapple, consider replacement with a scab-resistant variety showing flower and fruit color that you prefer as well. Try to obtain a variety that is also resistant to powdery mildew and rust.

8. Make a map of your flower and vegetable gardens. Next year, move related plants to another area of the garden to reduce soilborne pathogens that cause Rhizoctonia and Fusarium root rots. Now is also a great time to make soil amendments to improve soil drainage.

9. Divide perennial flowers (where appropriate), remove rotted or diseased parts and replant in a new location. Let the cut edges dry before replanting to avoid soft rot bacteria and other soilborne root rots.

Of course these measures will not guarantee a lack of plant disease in your garden, but they will help reduce disease incidence.

Source: Nancy Pataky, Extension Specialist, Plant Clinic

Autumn 1999
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