Wednesday, December 06, 2006

What can I do about black spot on my roses?

Roses are not the easiest plants to grow in the Washington, D.C. area. Our climate is perfect for the development of black spot, a fungus that can defoliate and weaken plants if not kept in check. Fortunately, antique, heritage, and species roses are generally more resistant to diseases and pests than hybrid tea roses. When control measures are needed, the Arboretum practices Integrated Pest Management (IPM) to control pests and diseases. The practice combines planting disease resistant varieties, promoting proper cultural techniques, and careful monitoring of pests combined with spraying, natural control methods, and tolerance of minor amounts of damage. To minimize black spot problems and limit the amount of spraying you need to do in your own rose garden, follow these tips:

* Promote air circulation and light penetration. Prune trees and hedges surrounding the rose garden low so that sunlight and breezes can quickly dry the foliage after morning dew or rain. Keep companion plantings at a distance to allow maximum exposure of the rose foliage to sunshine and air.
* Eliminate overhead irrigation. Water the soil with a soaker hose and keep the leaves dry. Black spot spores need water on the leaf surface to germinate.
* Prune cankered canes. Cankered stems bear black, dead areas that harbor the fungus over the winter. Prune them out, preserving more vigorous healthy canes, to prevent infection of spring foliage.
* Apply lime sulfur just before bud break in spring. This treatment delays the onset of black spot and powdery mildew infections for several weeks, even if weather is favorable for infection.
* Apply a fungicide based on neem seed extract beginning when leaves are fully expanded. Neem products are not as toxic as conventional fungicides and have the added benefit of controlling many rose insect and mite pests. They only work to prevent new infections, so spraying must be done on a weekly basis as long as weather is humid or wet.
* Apply conventional fungicides as a last resort. If the weather is dry in early summer, and you have applied neem every week, you may not have a black spot problem at all. But if you miss an application, or daily rain and dew create ideal fungal infection and growth conditions, you may need to use a conventional fungicide. Use a fungicide containing chlorthalonil, thiophanate methyl, or propiconazole. It's best to rotate different fungicides, never using the same one two times in a row, to prevent the fungus from developing resistance to the fungicides.
* Tolerate some damage, especially late in the season. Heritage and antique roses generally bloom in late spring and bloom only sporadically throughout the rest of the season. In July and August, with the flowers gone, the plants attract little attention and some black spot can be tolerated without permanent harm to the plants.

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