Heading off the rose diseases

Heading off the rose diseases Even resistant varieties of roses (including those featured on pages 72 and 73) can succumb to disease. Microclimate, cultural practices, and weather contribute to disease development. Best way to avoid problems is to identify the ailment and control the conditions that let it flourish.

Black spot appears most often on upper surfaces of leaves as circular spots with fringed edges, usually circled in yellow. It also infects stems. The plant can lose leaves and gradually weaken. The fungus thrives where overhead sprinkling or rainfall are common. The disease is particularly troublesome in parts of the Pacific Northwest and northern California.

Powdery mildew forms a whitish powder over leaves, stems, and flower buds and often causes leaves to become twisted or distorted. Badly affected flowers may not open properly. Mildew is common in nearly all Western areas. It thrives in warm days and cool nights and spreads on dry foliage (unlike rust and black spot). Many gardeners keep it at bay with a water spray each morning.

Rust usually appears in late spring as yellow to orange pustules on the undersides of older leaves. As the infection progresses, masses of yellow spores cover the bottoms of leaves; upper surfaces display yellow mottling. When severe, leaves drop. Rust, which is common in the Northwest and California, thrives in warm days and cool nights. It spreads quickly on wet leaves.

Controlling disease--naturally

In our survey, discussed on page 72, respondents offered valuable tips on how to keep roses free of these diseases with a minimum of chemical spraying.

Plant in full sun. Growing roses in shady situations increases disease susceptibility. So does poor air circulation among plants set too closely together.

Use drip irrigation. This keeps moisture close to the ground and off foliage.

Water in the morning. This gives plants plent of time to dry off before nighttime, discourage rust and black spot.

Prune. This increases air circulation and helps keep plant foliage dry. It also encourages dormancy in mild-winter climates and removes infected foliage and stems from plants.

Keep your garden clean. Rake up and discard fallen leaves and prunings. Remove diseased leaves from plants whenever you see them.