Ways to manage common diseases in many plants

Part I of “Practical Integrated Pest Management for the Home Gardener” described how to identify pests and diseases, monitoring progress, setting action thresholds, prevention and control. Part II discusses plant disease management.

If insects are not the problem, then diseases caused by bacteria, fungus or viruses may be, and require immediate action. Knowing how the plant should look, then looking for signs and symptoms that can indicate that a disease is present, as well as the type of pathogen causing the disease, is the key to properly treating the plants.

Most of the diseases of plants are caused by fungi. Fungi exhaust the nutrients from the plants and exhibit a wide variety of symptoms on leaves, stem and flower clusters.

Black spot on your roses or hydrangeas is a fungal disease. The best way to manage and control fungus is through maintenance. Keeping the garden areas clear of debris, pruning to improve airflow, avoiding overhead or evening watering, and properly fertilizing are all chemical-free practices to prevent fungal growth. If those fail, then fungicidal applications may be warranted.

Before applying a fungicide, make sure that you know the cause of the disease. Is it a fungal disease? If so, what is causing it? In which season is it occurring? Use the recommended fungicide for the particular problem your plant is facing and pay attention to how often the fungicide should be applied. Follow the label regarding agricultural-use requirements and personal protective equipment.

Bacteria are living organisms that produce their own energy, reproduce themselves and spread in several ways, including by insects, by splashing water, by other diseased plants, or by garden tools. Bacteria enter plants through tiny openings either through damage, or cuts, but also through natural openings in the plant itself. They are typically only diagnosed in a lab via samples.

Preventing the spread of the bacteria is the best option for control. This may include purchasing host-resistant plant varieties; good sanitation, particularly disinfecting pruning tools; and plant rotation. Reduce overwintering of bacteria by cleaning up debris. Finally, prevent surface wounds that permit the entrance of bacteria into the inner tissues. Bacterial diseases in plants are difficult to control. Emphasis is on preventing the spread of the bacteria rather than on curing the plant.

Finally, viruses are parasites and require a wound to gain entrance to a plant cell. They depend primarily on biological agents such as nematodes (microscopic worms that live in the soil and can damage the roots), insects, water and humans (weed trimmer wounds, for example) for their spread.

Viruses cannot be seen but will display symptoms such as mosaic leaf pattern, crinkled leaves, yellowed leaves and plant stunting. There are no effective chemicals to control virus diseases; therefore, sanitation and use of resistant varieties of plants have been the most effective means of controlling plant viruses. In some instances, when the spread of the virus is slow, loss from disease can be reduced by removing diseased plants and replacing them with healthy replants.

Other practices for control of diseases include avoiding extensive pruning during wet or humid weather when the bacteria can easily enter wounds. Mulching helps prevent the spread of bacteria by buffering the soil from extreme moisture loss, reducing the amount of stress during drought conditions. For help in diagnosing diseases, an excellent resource is “What’s Wrong with my Plant?” by David Deardorff and Kathryn Wadsworth, Timber Press, 2009.

Finally, as you plan your integrated pest management program, be realistic. Once you have identified a problem, determine what options you have for managing it. Some pests are more damaging than others. Establish tolerances for pests and pest damage. Don’t expect plants or vegetables to look picture perfect. Some insect damage can be tolerated and will still allow a beautiful garden or good quality vegetable harvest.

While it may seem confusing, knowing the differences will help you determine not only what is causing the problem, but also how to manage it. With a good plan, it is possible to reduce or eliminate pesticide applications while still addressing landscape problems in an environmentally friendly way.

Want to know more? Visit the University of Delaware Master Gardener website at www.udel.edu/canr/cooperative-extension/environmental-stewardship/master-gardeners/.

For questions on this subject or any gardening topic call the Master Gardener Helpline: In New Castle County, 302-831-8862; in Kent County, 302-730-4000; and in Sussex County, 302-831-3389. For information about master gardener workshops, visit: udel.edu/master-gardeners and click on Garden Workshops.