Q: We recently transplanted roses given to us by a neighbor. They seem to be doing fairly well and we have wonderful flowers. We did do some pruning but there are black spots on some leaves. Can you help?
A: Black spot on a rose is inevitable, especially on the thousands of grafted hybrid tea
s, grandifloras or floribundas. There are cultural measures you can provide to reduce black spot.
Prevent irrigation from hitting foliage; make sure that your roses are in full sun; and pick diseased leaves off plants as well as off the ground and discard in the trash.
Begin a pesticide/fungicide spraying treatment program on either a weekly or every-other-week basis. Starting with two of the least-toxic classes would be 70 percent neem oil products like Fertilome Triple Action Plus or Monterey 70 Percent Neem Oil.
A good remedy is 1 heaping tablespoon of baking soda (sodium bicarbonate), 1 tablespoon horticultural or neem oil and 1 teaspoon dish soap in 1 gallon of water. A commercial preparation, Bonide Remedy, adds the nutrient potassium, an immune booster.
Sulfur products like Fertilome Dusting Sulfur or Hi-Yield Dusting Wettable Sulfur or copper products like Fertilome Blackspot Powdery Mildew Control or Hi-Yield Copper Fungicide gives you two more classes of treatment.
Other products are more toxic, but you may try Fertilome Liquid Systemic Fungicide, or Ortho Max Garden Disease Control or Spectracide Immunox Multi-Purpose Fungicide. The goal is to rotate two or three classes of products so the disease does not become resistant, such as rotating 70 percent neem oil, and dusting sulfur or copper and a product containing chlorothalonil, such as Ortho Max Garden Disease Control.
All of these products need to be sprayed on a regular basis because they protect the plants before diseases appear. Always read the label for rates, active ingredients and personal protective equipment needed.