CORVALLIS - Roses in the Pacific Northwest are especially susceptible to several fungal diseases, including black spot, powdery mildew and rust. To avoid or reduce the use of toxic fungicides, lime sulfurs and copper sulfates, the Oregon State University Extension Service Master Gardener program recommends some less toxic methods to help control fungal diseases:
Rake up and destroy all leaves from infected roses.
Prune out infected wood during the winter/early spring dormant season.
Provide good air circulation through judicious pruning of over-crowded branches.
Choose disease-resistant varieties when buying new rose plants.
Apply registered and labeled fungicides, or try a newer homemade “least-toxic” recipe (see below) to combat fungal infections.
While the makers of baking soda do not support the recommendation of this least-toxic method, many of the rose growers around the nation are using the following least-toxic recipe for rose disease control:
Mix three teaspoons baking soda, 2 1/2 tablespoons horticultural oil and 1/2 teaspoon insecticidal soap into one gallon of water. Store in a large closed jar. Apply with spray bottle to roses after new growth appears in the spring every 10 to 14 days.