GROW SMART, GROW SAFE

   Diseases

There are easy ways to prevent diseases such as black spot and powdery mildew without hazardous chemicals. Damp Northwest weather is hard on some varieties of plants. Fortunately, there are far more plants that are naturally resistant.

You can prevent plant disease problems without toxic chemicals using the research-based strategies described here. You can also find least-toxic pesticides and learn about the hazards of particular products using the Grow Smart, Grow Safe search tool.

Prevent problems before they appear

Build healthy soil. Healthy soil grows healthy plants.

• Compost and organic fertilizer will enrich your soil with nutrients, help hold water, loosen clay and feed beneficial soil life.
• Healthy plants are less likely to be harmed by diseases.

Choose disease-resistant plants. Local nurseries can help you choose plants suited to where we live.

• Opt for native plants, or non-natives adapted to wet-winter, dry-summer climates.
• Avoid the few natives prone to problems in garden settings. Substitute a dogwood resistant to anthracnose or a maple resistant to verticillium in the place of susceptible native varieties.
• Some roses are susceptible to diseases such as black spot or powdery mildew, but there are varieties that fare better in this climate.
• Great plant picks provide information on a wide range of attractive, disease-resistant flowering plants.

Don't overfertilize with nitrogen. Most diseases settle first on tender new growth stimulated by nitrogen fertilizer.

Space plants for good air circulation. Prune or thin annual plants once or twice in summer and fall, especially if foliage has become crowded.

Give roses lots of sun and air. Keeping the leaves dry discourages most diseases.

• Plant roses where they will get six hours of sun daily.
• Leave space between plants.
• Prune in early spring to keep the middle of the plant open.
• Remove leaves close to the ground and clean up diseased leaves.

Use a drip watering system. Drip irrigation and soaker hoses keep water on the ground and off the leaves.

• Water regularly to prevent plant stress.
• Sprinklers splash water and soil, which can spread plant diseases.
• Avoid overwatering, which can foster disease.

Mulch. Mulch helps prevent the spread of disease spores.

• Mulch between two and four inches deep provides many benefits.
• Be sure to keep a few inches of bare space around plant stems and crowns.

Rotate annual crops every year if you have the space. You’ll reduce the spread of soilborne diseases by planting flowers and vegetables in different areas from year to year. For example, switch tomatoes with cucumbers or petunias with zinnias.